Abiotic stresses as tools for metabolites in microalgae

Accepted Manuscript Review Abiotic stresses as tools for metabolites in microalgae Chetan Paliwal, Madhusree Mitra, Khushbu Bhayani, Vamsi S.V. Bharadwaj, Tonmoy Ghosh, Sonam Dubey, Sandhya Mishra PII: DOI: Reference:

S0960-8524(17)30712-5 http://dx.doi.org/10.1016/j.biortech.2017.05.058 BITE 18084

To appear in:

Bioresource Technology

Received Date: Revised Date: Accepted Date:

31 March 2017 8 May 2017 10 May 2017

Please cite this article as: Paliwal, C., Mitra, M., Bhayani, K., Bharadwaj, V.S.V., Ghosh, T., Dubey, S., Mishra, S., Abiotic stresses as tools for metabolites in microalgae, Bioresource Technology (2017), doi: http://dx.doi.org/ 10.1016/j.biortech.2017.05.058

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Abiotic stresses as tools for metabolites in microalgae Chetan Paliwala,b, Madhusree Mitraa,b, Khushbu Bhayania, Vamsi Bharadwaj SVa,b, Tonmoy Ghosha,b, Sonam Dubeya, Sandhya Mishraa,b,* a

Salt and Marine Chemicals Division

CSIR-Central Salt & Marine Chemicals Research Institute Gijubhai Badheka Marg, Bhavnagar 364002, Gujarat, India

b

Academy of Scientific and Innovative Research

AcSIR-CSMCRI Gijubhai Badheka Marg, Bhavnagar 364002, Gujarat, India

*Corresponding author: Dr. Sandhya Mishra Principal Scientist, Salt and Marine Chemicals Division CSIR-Central Salt & Marine Chemicals Research Institute Gijubhai Badheka Marg, Bhavnagar 364002, Gujarat, India Email: [email protected]

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Abiotic stresses as tool for metabolites in microalgae 1. Introduction 2. Effect of different abiotic factors on metabolites 2.1 Lipids 2.1.1. Light irradiation 2.1.2. Temperature stress 2.1.3. Salinity stress 2.1.4. Nutrient starvation 2.1.4.1. Nitrogen stress 2.1.4.2. Phosphorus limiting condition 2.1.4.3. Carbon source 2.1.4.4. Silica 2.1.5. The effect of pH 2.1.6. UV irradiation 2.2. Pigments 2.2.1. Phycobiliproteins 2.2.1.1. Complementary chromatic adaptation (CCA) 2.2.1.2. Salinity 2.2.1.3. Temperature 2.2.1.4. Reactive oxygen species and anti-oxidants 2.2.2. Carotenoids 2.2.2.1. Light 2.2.2.2. Nutrient 2.2.2.3. Metabolic engineering 2.3. Polymers

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2.3.1. Polyhydroxyalkanoates (PHA) 2.3.2. Exopolysaccharides (EPS) 3. Future prospective 4. Conclusion Acknowledgement References

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Abstract Microalgae, due to various environmental stresses, constantly tune their cellular mechanisms to cope with them. The accumulation of the stress metabolites is closely related to the changes occurring in their metabolic pathways. The biosynthesis of metabolites can be triggered by a number of abiotic stresses like temperature, salinity, UV- radiation and nutrient deprivation. Although, microalgae have been considered as an alternative sustainable source for nutraceutical products like pigments and omega-3 polyunsaturated fatty acids (PUFAs) to cater the requirement of emerging human population but inadequate biomass generation makes the process economically impractical. The stress metabolism for carotenoid regulation in green algae is a 2-step metabolism. There are a few major stresses which can influence the formation of phycobiliprotein in cyanobacteria. This review would primarily focus on the cellular level changes under stress conditions and their corresponding effects on lipids (including omega-3 PUFAs), pigments and polymers. Keywords Abiotic stress; microalgae; metabolites; fatty acids; biofuels

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1. Introduction The growth conditions for any organism isolated from its habitat are species/strain specific, but under extreme circumstances they produce certain metabolites to overcome and acclimatize the stress conditions, which could be biotic or abiotic. Microalgae shows great adaptability towards the abiotic stress factors and produce valuable metabolites. Such unique characteristic of microalgae can be exploited for the production of desired metabolites through the abiotic stress as a tool integrated with microalgal biorefinery for its sustainable development. Microalgae have evolved from being a simple food source to the modern age high value products and energy feedstock. Microalgae left a footprint 3.5 billion years ago, when the oldest known prokaryote, belonging to cyanobacteria, fossilized in the Archean rocks of western Australia [Woese et al.1990]. Eukaryotic microalgae, with their defined organelles for specific cellular roles, have gradually evolved from the prokaryotes [Archibald, 2015]. Cyanobacteria were the first life capable of oxygenic photosynthesis on earth creating oxygen in the atmosphere for us, regulating our biosphere and with the urge to survive under any diverse extreme conditions they might have gradually developed several unique means of acclimatization with either accumulating or releasing i.e. intracellular or extracellular compounds. These intracellular and extracellular compounds, due to its potential high value applications are sought globally for which strategy for efficient exploitation of the microalgae is being developed where abiotic stresses play a role of a very sharp tool in the biorefinery machinery. One can develop the microalgal machinery of biorefinery based on the need, availability, natural environment and economics of the country where it is to be exploited. The stress tool according to the geographical location of its set up can be designed strategically, for which either a stress factor or combination of multiple stress factors can be statistically optimized to achieve sustainability. Cultivation of microalgae under continuous stress conditions might enhance the lipid or carbohydrate accumulation but, one has to

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compromise with the growth rates, thus reducing their overall productivities, which ultimately increase the biofuel production cost [Pancha et al. 2015]. Most widely used strategy to overcome this problem is the two-stage cultivation of microalgae in which cells are first grown in nutrient-sufficient conditions to achieve maximum biomass productivity and the culture conditions are then altered to induce the stress conditions, thereby stimulating the accumulations of lipid and carbohydrate in the second stage [Chen et al. 2011]. Today microalgae represent a viable alternative source for high-value products along with the biofuel. The species Chlorella protothecoides (Cp), has been widely studied providing a high amount of lutein and fatty acids (FA) profile ideal for biodiesel production [Campenni et al. 2012]. Nutrient deficiency or nutrient stress has been well documented to increase TAG accumulation in microalgae [Sheehan et al. 1998] specifically, nitrogen or phosphate limitation [Yeesang and Cheirsilp, 2011; Rodolfi et al. 2009; Hu et al. 2008; Li et al. 2008]. Lipid productivity is strain-specific function of physiological responses to many factors, including incident light intensity and cultivation temperature [Griffiths and Harrison, 2009] and salinity [Gouveia et al. 2009]. It was also proven that light dilution results in a significant enhancement in productivity of biomass [Fernández et al. 1998]. This is due to the radiation on culture surface, which increases the frequency of the dark cycles of the cells, providing a higher rate of light impulses per reaction centre [Fernández et al. 1998]. Polyunsaturated fatty acids (PUFAs) are the essential components for the growth of higher eukaryotes. Among them, eicosapentaenoic acid (EPA, C20:5) and docosahexaenoic acid (DHA, C22:6) are the most important fatty acids owing to enormous nutraceutical and pharmaceutical applications. Microalgae are the primary producer of PUFAs in the marine food chain and moreover it can grow under autotrophic, mixotrophic and heterotrophic culture conditions [Li et al., 2009], fix atmospheric carbon dioxide (CO2) [Rubio-Rodriguez

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et al. 2009; Schenk et al. 2008], can convert light energy and CO2 into biomass rich in carbohydrates, proteins and lipids, have shorter harvesting time than plants, and have less chemical contamination than fish oil. Under abiotic stress condition, microalgae produce a large array of compounds (including PUFAs) to survive in the extreme environmental conditions through adaptation. Marine microalgal cells typically contain 10-50% (w/w) oil and can produce high percentage of total lipid (up to 30-70% of the dry weight) [Ward and Singh, 2005]. There exists a close relation between the growth phases of microalgae and the accumulation of fatty acids, as, during cell division and unfavourable growth conditions (where salinity, temperature, light, pH, and nutrients etc. may be suboptimal), fatty acids play an important role as energy stockpile. Therefore, the high-energy content of omega-3 PUFAs and its ability to maintain membrane fluidity leads to the accumulation of PUFAs (mainly omega-3 fatty acids) during stress condition [Tiez and Zeiger, 2010; Cohen et al. 2000]. To date, microalgae belonging to the genera Phaedactylum, Nannochloropsis, Thraustochytrium and Schizochytrium are known to contain EPA and/or DHA. Among them, Phaeodactylum tricornutum [Yongmanitchai and Ward, 1991] and Nannochloropsis sp. [Sukenik, 1991] reported to contain EPA up to 39% of the total fatty acids, while DHA percentage of 30-40% of the total fatty acids was observed in strains Thraustochytrium [Burja et al. 2006] and Schizochytrium limacinum [Zhu et al. 2007]. Under optimized stress conditions (i.e. optimal carbon, nitrogen and phosphorus concentrations, different salinity regime and light intensity [Mitra et al. 2015; Pal et al. 2011; Jiang et al. 1999], UV radiation [Sharma and Schenk, 2015; Liang et al. 2006] and controlled pH and temperature [Mitra et al. 2015; Jiang and Gao, 2004; Tatsuzawa and Takizawa, 1995] conditions, high biomass productivity and commercially acceptable EPA and DHA productivities are achieved with microalgae.

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Apart from external stress conditions, researchers have focused on the development of high lipid and PUFA containing microalgal strains using metabolic engineering. Metabolic engineering with genetic engineering is a promising approach, in targeted biosynthetic pathway for enzymes involved as the key players towards achieving the enhanced productivities. Only few genes encoding the enzymes involved in the fatty acid biosynthesis have been identified in some microalgal species till date [Chi et al. 2008; Tonon et al. 2005; Domergue et al. 2003]. Although, lack of information about the mechanisms involved in the fatty acid biosynthesis pathway and the role of enzymes, urge for extensive research studies on algal metabolism. Plants are rather complex in their organization and their life cycle is much dependent on the environment, the studies with their genomes are much complicated in comparison to bacteria. Therefore, studies on the molecular mechanisms of stress responses are not easy in higher plants. Cyanobacteria exhibit a close phylogenetic relationship with plant chloroplast and therefore, are regarded as most appropriate model system for studying plant responses to various stresses. It is much easier to carry out such kind of research on cyanobacteria. It should be noted that at present cyanobacteria, similarly to Escherichia coli, became one of the most actively studied groups of living organisms after the complete nucleotide sequence of the genome of Synechocystis sp. PCC 6803 was determined in 1996 [Kaneko and Tabata, 1997]. Any changes (temperature, salinity, pH, etc.) around the cell are first sensed by the specialized sensory proteins, or sensors changing their properties under the stress. Sensors transfer the signal about the changes to other polypeptides – the transducers, which in their turn regulates expression of the stress responsive genes. Transducer proteins may recognize the special regions of DNA directly, interact with them, and regulate transcription. Finally, protection proteins and/or metabolites are synthesized that help the cells and organisms to adapt or acclimate to new environment. Stress treatments may also directly affect the

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structure of DNA (chromosome packing) and cause alterations in transcription of many genes. Mostly, cells recognize the abiotic stress not always but when it exceeds a certain critical level. However, cellular sensory systems are always necessary for cells to recognize even minor changes in the environment to produce an adequate response to these changes. Cyanobacteria can regulate their tetrapyrrole content and composition in response to signals, such as nutrient availability, light intensity, light wavelength and temperature [Prassana et al. 2004]. To tolerate salinity stress, microalgae respond by accumulation of compatible solutes, also salinity stress has been reported to upregulate carotenoid synthesis genes such as canthaxanthin and astaxanthin. This may be because salinity stress generates reactive oxygen species, hence algae respond by increasing synthesis of carotenoids [Li et al. 2009]. Another mechanism by which algae survive under salinity stress is by changing membrane composition by upregulating ion transporters, and aquaporins. High salinity tolerating algae such as Dunaliella salina produce sucrose, glycerol, proline, glycine, betaine, etc as a compatible solute (osmoprotectant) under salinity stress. There is a need to understand the underlying molecular mechanism of stress responses. These would help us identify the regulatory elements involved and also identify bottlenecks in the production of biofuel precursors. This knowledge would help us engineer algae to be more resilient to stress and produce more biofuel precursors under non-stress conditions. This data can also be incorporated into the metabolic models of algae which would help us develop insilico models of algae with desirable properties. Currently our understanding of microalgal and cyanobacterial stress responses is limited to model organisms and prediction of gene function by homolog identification. Availability of genetic tools is limited and not well developed. However, by the use of transgenic microalgae, adapted to stressful conditions, we 9

can grow them under conditions which inhibit growth of contaminating/ competing microalgae.

2. Effect of different abiotic factors on metabolites 2.1. Lipids Microalgal lipids generally categorised into structural lipids (polar lipids mainly polyunsaturated fatty acids or, PUFAs) and storage lipids (non-polar lipids including saturated fatty acids (SFAs) and monounsaturated fatty acids (MUFAs)). Storage lipids mainly stored in the form of triacylglycerols (TAGs) are transesterified to produce biodiesel and supply energy to the metabolic processes. PUFAs are known to maintain membrane functions and contribute as the matrix for numerous metabolic processes. Generally, TAGs are accumulated in the cytosolic lipid bodies in presence of light while during dark hours participated in the polar lipid synthesis [Thompson, 1996]. Some microalgal species, for example Nannochloropsis sp., Pavlova lutheri, P. tricornutum etc. are known to accumulate PUFA in TAG. A sudden change in the growth conditions ameliorates microalgal lipid content. To combat with the growth limiting stress conditions (including light intensity, temperature, salinity, nutrients, pH and UV- radiation), microalgae started accumulating lipids and/or starch as a crucial part of their survival mechanism. Under varying abiotic stress conditions, total lipid along with fatty acids profile undergo extreme variation [Table 1a and 1b]. 2.1.1. Light irradiation When stressed with various light intensities, remarkable changes in the biomass and nutrient profile of microalgae have been reported. High light intensity increased the neutral lipid content (mainly TAG) with a simultaneous decrease of polar lipids owing to the oxidative damage of PUFAs [He et al. 2015; Breuer et al. 2013; Carvalho and Malcata, 2005].

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Exposure to high light increased the level of TAG in many algal species including Chlorella sp. [He et al. 2015], Monoraphidium sp. [He et al. 2015], Scenedesmus obliquus [Breuer et al. 2013], Pavlova lutheri [Carvalho and Malcata, 2005], Nannochloropsis gaditana [Mitra et al. 2015a]. Although light irradiance act as a stimulant in the production of microalgal biodiesel however the %accumulation of TAG will vary for different species. Several research studies with EPA producing microalgae including Nannochloropsis sp. [Sukenik et al. 1989], N. salina [Van wagenen, 2012], N. gaditana [Mitra et al. 2015a], Nitzchia closterium [Yongmanitchai and Ward, 1989] confirmed that PUFA (especially EPA and/or DHA) content of microalgae is inversely related to light irradiation. This phenomenon may be attributed to the Reactive Oxygen Species (ROS) (which formed under unfavourable growth conditions) quenching activity of PUFA [Sukenik et al. 1989; 2009]. Moreover, PUFAs may involve in the functioning of thylakoid membrane and are essential for the photosynthesis [Cohen et al. 1988; Kates and Volcani, 1966]. Under the effect of high irradiance, the photosynthetic potential of algae becomes less, suggesting the requirement of relatively less thylakoid membranes. Owing to this phenomenon, high light acclimated algae resulted in the accumulation of less PUFA (mainly EPA and/or DHA) [Patil et al. 2005]. 2.1.2. Temperature stress Research studies till date have projected a general trend between the lipid profile of microalgae and temperature. It has been observed in most of the microalgae that polar lipid content increased with decreasing temperature whereas increased temperature leads to the higher accumulation of nonpolar lipids (TAG) [Renaud et al. 2002]. Temperature stress at 35˚C led to an elevation in the lipid content (22.7%) and increased accumulation of neutral lipid (59% of total lipids) in Acutodesmus dimorphus [Chokshi et al. 2015]. Long chain PUFAs (LC-PUFAs, mainly EPA and DHA) play a crucial role in maintaining cell membrane fluidity [Nishida and Murata, 1996]. Thus, the fatty acid profile of microalgae

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grown in relatively low temperature conditions must be composed of a good amount of PUFA (mainly EPA and DHA), suggesting the requirement of those LC-PUFAs in the metabolic profile to survive in the unfavourable conditions. In a recent study with five polar and cold-temperate microalgae, enhanced EPA and DHA production was achieved under low temperature and irradiance conditions [Boelen et al. 2013]. With a decrease in temperature from 25˚C to 10˚C, a significant increase in the ALA and DHA content was observed in Isochrysis galbana [Zhu et al. 1997]. EPA content of Pavlova lutheri augmented from 20.3 to 30.3 % when temperature was reduced to 15˚C [Tatsuzawa and Takizawa, 1995]. Similarly, when the culture temperature was shifted from 25˚C to 10˚C for 12 h, significant increase in the EPA content was observed in Phaeodactylum tricornutum [Jiang and Gao, 2004]. Mitra et al. [2015b] observed a 3.4-fold augmentation in the EPA content (%) of Nannochloropsis sp., when cultivated in a two-stage cultivation process under the combined effect of low temperature and irradiance. Algae adapt to low temperatures by increasing the production of unsaturated fatty acids such as PUFAs which maintain membrane fluidity at low temperatures. 2.1.3. Salinity stress Salinity may also tune the lipid biosynthesis pathway in microalgae, although not in an unwavering mode. Microalgae that can tolerate high salt concentration are most suitable candidate for salt tolerance study. Total lipid content along with a higher percentage of TAG was observed in Dunaliella tertiolecta while stressed with increasing NaCl concentration (0.5 M to 2.0 M) [Takagi and Yoshida, 2006]. In a two-stage process, salinity stress of 400 mM was resulted in 24.77% lipid (containing 74.87% neutral lipid) in Scenedesmus sp. [Pancha et al. 2015]. An increase in the salinity (30 to 40 g L-1) reported to reduce the biomass productivity together with lipid and EPA productivities in Nannochloropsis gaditana [Mitra et al. 2015a]. When grown in a medium containing 9 g L-1 NaCl, the DHA content of

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Crythecodinium cohnii ATCC 30556 escalated up to 56.9% of the total fatty acids [Jiang et al. 1999]. Jiang and Chen [1999] reported highest DHA yield of 131.55 mg L-1 for C. cohnii ATCC 30556 at 9 g L-1 NaCl while supplementation of 5 g L-1 NaCl showed highest DHA yield of 128.83 mg L-1 in C. cohnii RJH. 2.1.4. Nutrient starvation Microalgal growth and their lipid and fatty acid composition are closely related to the nutrient content of their cultivation medium. Nutrient starvation being one the majorly applied lipid induction strategies, has been reported to regulate the percentage of polar and nonpolar lipid although their effect differs within different species [Rodolfi et al. 2009; Hu et al. 2006]. For example, enhanced accumulation of TAG with a dwindling PUFA content was reported for diatom Stephanodiscus minutulus, when cultivated under silicon, nitrogen and phosphorus limiting conditions [Lynn et al. 2000]. 2.1.4.1.

Nitrogen stress

Nitrogen is the most applied nutrient stress factor towards the optimisation of lipid metabolism process of microalgae. In response to nitrogen starvation, accumulation of lipids (mainly TAG) was detected in numerous microalgal strains including Chlorella vulgaris [Yeh and Chang, 2011], Chlorella sp., [Praveenkumar et al. 2012], Scenedesmus sp. [Pancha et al. 2014a]. Increased accumulation of EPA in P. tricornutum under nitrate stress condition, was reported by Yongmanitchai and Ward [1991], whereas EPA productivity was maximum in N. laevis when urea was supplemented in the medium as the nitrogen source. However, utilization of ammonia as the sole nitrogen source in diatom cultivation, was found to be detrimental for growth and PUFA production. This negative correlation between PUFA content and nitrogen starvation may be related to the fact that when algal cells were exposed to nitrogen limiting condition, cells escalated TAG synthesis (consist of SFAs and MUFAs) pathway at the expense of polar lipids (mainly PUFAs) [Cohen, 1999].

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2.1.4.2.

Phosphorus limiting condition

Phosphorus being an essential nutrient in the algal media, plays a major role in microalgal growth and also involved in metabolic processes such as energy transfer in cells, nucleic acid biosynthesis, phospholipid biosynthesis and membrane development. Phosphorus limitation occasioned increased accumulation of TAG in P. tricornutum, Isochrysis galbana, Chaetoceros sp., Pavlova lutheri, though decrease lipid content in Tetraselmis sp. and Nannochloris atomus [Reitan et al. 1994]. In Monodus subterraneus, ~ 6-fold increase in the TGA percentage was observed with a gradually decreasing EPA content, when subjected to phosphorus limiting stress condition [Khozin-Goldberg and Cohen, 2006]. On the contrary, an elevated level of unsaturated fatty acids was observed in phosphorus starved cells of Chlorella kessleri [EI-Sheek and Rady, 1995]. Given the abundance of PUFAs in the phospholipid fraction, the concentration of phosphorous in the growth media can be directly involved in the overall yield of PUFAs [Guschina and Harwood, 2009]. When grown in a growth medium supplemented with high phosphorus, enhanced production of EPA was observed in P. tricornutum [Yongmanitchai and Ward, 1991]. 2.1.4.3.

Carbon source

Of late, microalgae are gaining interest for their ability of carbon sequestration and also to evaluate the changes occurred in their fatty acid profile when stressed with organic and inorganic carbon source. In a study carried out by Patidar et al. [2014] on Monoraphidium minutum the effect of optimum glucose, fructose and microalgae biodiesel waste residue and sodium acetate were found to increase the saturated fatty acid content of microalga. When stressed with intermittent supply of sodium hydrogen carbonate at different pH level, drastic changes in the fatty acid profile of Chlorella variabilis ATCC 12198 was observed by Patidar

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et al. 2016. Utilization of glucose as a carbon source was found to enhance the biofuel producing potential of Scenedesmus sp. [Pancha et al. 2014b]. Increased accumulation of neutral lipid (88.69 ± 2.1% of total lipid) was observed in nitrate starved hydrogen carbonate supplemented cells of Scenedesmus sp. [Pancha et al. 2015]. When cultivated in Porphyridium medium containing 5 g L-1 glucose, highest DHA content (51.12% of the total fatty acids and 7.79% of the cell dry weight) was observed in marine dinoflagellate Crypthecodinium cohnii ATCC 30556 [Jiang et al. 1999]. 2.1.4.4.

Silica

Several reports convocated diatoms as one the potential source of PUFAs mainly EPA and/or DHA. Diatom cell walls are basically composed of silica and therefor presence or absence of silica in the growth medium may affect the biomass and lipid profile of diatoms. In silicate limited cultures of Nitzscia laevis, increased accumulation of EPA was reported by Wen and Chen [2000]. The reason behind this situation may be attributed to the change in the metabolism of diatom cells during silica replete conditions, as they may divert the energy allocated for silicate uptake, towards lipid storage. 2.1.5. The effect of pH Fluctuations of the medium pH also alter the microalgal lipid composition. The highest percentage of total PUFAs (38.75%) and EPA (23.13% of the total fatty acids) were observed in Pinguiococcus pyrenoidosus 2078 at a pH level of 7 [Sang et al. 2012]. In a study with Chlorella sp. CHLOR1, alkaline pH stress directed accumulation of TAG was observed with simultaneous decrease in the membrane lipid content [Guckert and Cooksey, 1990]. 2.1.6. UV- radiation The effect of UV- irradiance in microalgae is mainly concentrated on the influence of UV-A and UV-B radiation on microalgal growth and biomass nutrient profile (including lipids). Exposure to UV-A radiation significantly increased PUFA content of N. oculata [Srinivas

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and Ochs, 2012]. In a study carried out by [Skerratt et al., 1998], increased accumulation of PUFA was observed in Phaeocystis antarctica under low UV-B radiation. In Tetraselmis sp., UV-B radiation resulted in an overall increase in the SFAs and MUFAs content with a 50% decrease in the PUFA content [Goes et al. 1995]. Under combined effect of UV-A and UV-B, increase in the MUFA content was reported in Chaetoceros mualleri [Liang et al. 2006]. In Pavlova lutheri, exposure to UV radiation trigger the accumulation of storage lipids at the expense of EPA and DHA content [Guihéneuf et al. 2010]. Generally, it has been found that abiotic stress conditions which lead to the formation of ROS and lipid peroxidation are directly proportional to the elevated PUFA content. PUFAs actively participate in the membrane repair mechanism of cells with free radical scavenging activity. An increase in the EPA content (up to 19.84%) of Phaeodactylum tricornutum was observed when exposed to UV light [Liang et al. 2006]. 2.2. Pigments Pigment content of microalgae (including cyanobacteria) can be altered under the effects of various abiotic stress factors like low light intensity, chromatic adaptation, temperature, salinity and nutrient availability [Table 2]. 2.2.1. Phycobiliproteins Phycobilisome –the light harvesting apparatus in cyanobacteria and red algae, which is composed of water-soluble phycobiliproteins covalently bound by linker peptides or proteins in a configuration designed to transfer energy to the photosynthetic reaction center of the organism. Phycobiliproteins are highly fluorescent because of their covalently bound, linear tetrapyrrole chromophores known as bilins. Natural colorants such as phycobiliproteins are gaining importance over synthetic ones, as they are nontoxic and non-carcinogenic. They are widely used in cosmetics and as label for antibodies and receptors. Bio-sensor, Neuroprotective, Antioxidants, anti-inflammatory, Anti-

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nephrolithe, Anti-hyperglycemic and hepatoprotective properties are also manifested by Phycobiliproteins [Ghosh et al. 2016; Bhayani et al. 2016; Paliwal et al. 2015]. 2.2.1.1.

Complementary chromatic adaptation (CCA)

Photosynthetic organism like cyanobacteria regulates their relative pigment production and content in response to changes in light intensity and light wavelength. The effect of light wavelength on the pigment content of the cells, known as complementary chromatic adaptation (CCA) in cyanobacteria. The term “complementary chromatic adaptation” was used by Engelmann. During CCA, cyanobacterial cells change from brick red to bright blue green, depending on their light color. In this type of adaptation, changes in cell pigmentation in response to specific spectral illuminations result from moderation of the relative amounts of the red colored phycoerythrin (PE) and the blue-colored phycocyanin (PC), with a predominance of PE in green-light-grown cells and of PC in red-light grown cells. These phycobiliproteins (PE and PC) are the major light harvesting pigments used to drive photosynthesis. Therefore, this mode of chromatic control allows cells to trap the available light energy with maximum efficiency [Tandeau de Marsac, 1983]. Several studies have shown that the regulation of CCA is complex and involves total three pathways. One is regulate by a phytochrome-class photoreceptor that is responsive to green and red light and a complex two component signal transduction pathway, whereas another is based on sensing the redox state. Thus, CCA can only occur in cyanobacterial species that make both PC and PE, although not all such species are capable of this process [Tandeau de Marsac, 1977]. Pseudanabaena sp. grown under different quality of light. Here, maximum phycoerythrin production was observed in green light (39.2 mg L-1), while phycocyanin production was maximum in red light (10.9 mg L-1) [Mishra et al. 2012]. Cyanobacterial species that undergo CCA must have stockpile the structural and regulatory genes needed for this response. Fujita and Hattori [1960] demonstrated that in the soil cyanobacterium Tolypothrix tenuis (PCC

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7101), CCA was controlled by a photoreversible pigments. These results suggested that CCA was controlled by a photoreceptor rather than via photosynthesis or another cellular process. Within species that produce both PE and PC, three response groups were defined. Group I strains, which made up about 27% of the total, were not capable of CCA because they altered neither PE nor PC abundance in response to changing light colors. Group II strains comprised 16% of the total. These had elevated PE levels in green light but did not vary PC levels in response to green-red light shifts. The remaining 57% were group III strains, which varied both PE and PC, increasing PE in green light and PC in red light. It has become clear that many additional cellular responses are affected by shifts between green and red light, including changes in cell and filament assembly [Bogorad et al. 1982; Bennett and Bogorad, 1973], cell differentiation states [Damerval et al. 1991; Lazaroff and Schiff, 1962], and the abundance of many RNAs and proteins that do not encode PBS components [Stowe-Evans et al. 2004; Gendel et al. 1979]. There is considerable morphological changes and cellular response during CCA. Under red light, cells are smaller and more rounded than during growth in green light. Filament length is approximately 10 times less in red light due to the conversion of approximately 20% of the cells to necridia (dead cells in filament) [Bogorad et al. 1982; Bogorad 1975; Bennett and Bogorad, 1973]. The synthesis of hormogonia is CCA regulated in F. diplosiphon. Hormogonia are short, differentiated, motile filaments that develop pili and gas vesicles. They are resistant to adverse environmental conditions and appear to play a role in survival [Tandeau de Marsac, 1983; Rippka et al. 1979]. CCA also affects the growth rate of F. diplosiphon cells grown with glucose in darkness. A daily five-minute green-light exposure depresses the dark growth rate by up to 50%, and this effect can be reversed by brief exposure with red light [Diakoff and Scheibe, 1975]. The above findings suggest that there are many changes in protein abundance during CCA in addition to those involved in the

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restructuring of PBS, several studies have confirmed this. Protein-labeling experiments demonstrated that the abundance of several unidentified membrane-associated polypeptides changes during CCA [Gendel et al. 1979]. CCA responses in cyanobacteria are responsive to the redox state of photosynthetic electron transport chain and some photoreceptors that are maximally sensitive to green and red light [Haury et al. 1997; Campbell et al. 1993]. The redox state of the plastoquinone pool controls the cellular differentiation of green-lightacclimated cells. Shifting these cells to red light partially oxidizes the plastoquinone pool, leading to inhibition of heterocyst development and the production of hormogonia [Campbell et al. 1993]. 2.2.1.2.

Salinity

Salinity is one of the most important factors that limits the growth and productivity of microorganisms [Inabha et al. 2001]. Salinity has a noticeable effect on agriculture, affecting huge part of the world’s irrigated land area. Salt has been shown to have detrimental effects on growth in cyanobacterial systems. Salt adaptation in the cyanobacterium initiates two mechanisms; the initial mechanism stimulates photosynthetic activity and modification of the photosynthetic apparatus with the aggregation of sucrose as an osmoregulator. The secondary mechanism involves the adaptation of N2 fixation activity and protein biosynthesis [Blumwald and Tel-Or, 1982]. 200 mM NaCl in combination with CCA was found to maximally inhibit cell growth and chlorophyll levels, and accumulation of PE and PC under green and red light, respectively. NaCl also affected cellular morphology resulting in a larger cell size under both light conditions. Cell length decreased while width increased under green light in the presence of salt, and both cell length and width were increased under red light with salt. The addition of osmoprotectant glycine betaine(GB) to the growth medium in the presence of salt resulted in a change in the morphology of cells growing in the absence of salt, whereas GB treatment in the presence of salt did not have a major effect on PE and PC

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biosynthesis or accumulation. Salt impacts photosynthetic pigment content and morphology in Fremyella [Singh et al. 2013]. The chlorophyll and cellular protein contents increased, when 50 mM NaCl incorporated in media. Further increment in NaCl concentration, shows significant decrease in both chlorophyll and cellular protein with morphological variations of organism by having alteration in their size and volume, thus increased level of protein was recorded in NaCl adapted cells of A. cylindrica in response to NaCl stress [Bhadauriya et al. 2007]. Studies have shown that protein content varied from 37.3% to 56.1% under salt stress [Rafiqul et al. 2003]. 2.2.1.3 Temperature Temperature is the most foundational factor for organisms as it influences metabolic processes and biochemical composition of cells. The optimal growth temperature and tolerance to the extreme values usually changes from strain to strain. Sudden temperature changes exert stress on the organisms. Environmental stress influences the organisms to inhibit or enhance the functioning and production of some physiologically important proteins. One such physiologically important group of proteins is phycobiliproteins. Different components of PBSs evolved independently from each other according to need of cyanobacteria under different stress conditions. Phycocyanin from Synechococcus lividus has the same amino acid composition, molecular weight, sedimentation, properties as the Phycocyanin from non-thermal blue green algae [Edwards and Gantt, 1971]. Thermo tolerant Oscillatoria sp. produced phycoerythrin and phycocyanin (PC) at 30 ± 2 °C and 55 ± 2 °C, respectively whereas at 42 ± 2 °C temperature, it produced phycoerythrocyanin [Singh et al. 2012]. Cyanidioschyzon merolae from sulfuric hot springs and habitat near volcanic areas accumulates thermostable phycocyanin growing at 83 °C. These properties make the C. merolae phycocyanin an interesting alternative to Arthrospira platensis as a natural blue food colorant [Rahman et al. 2016].

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2.2.1.4.

Reactive oxygen species and anti-oxidants

Reactive oxygen species (ROS) have been implicated both as secondary messengers as well as harmful by-products of various cellular processes. Their detrimental activities increase under adverse environmental stress conditions [Das and Roychaudhury, 2014]. Among these environmental conditions, heavy metal stresses and UV-B exposure are among the chief culprits which give rise to harmful ROS in the organisms. Exposure to UV-B component of sunlight induces the generation of ROS in many plant cells. However, a ‘low’ exposure is not sufficient for damage to the cellular components; on the contrary, a ‘low’ UV-B exposure prepares the anti-oxidant defences of the plant cells and prepares them for a ‘fight’. This phenomenon has been termed as eustress, which is an important part of the acclimatization process [Hideg et al. 2013]. In order to cause damages, a ‘high’ exposure is often required. However, the threshold level of intensity which separates ‘high’ and ‘low’ often varies considerably according to the terrain and environment of growth. To protect themselves from these ROS and their ill effects, cyanobacteria have evolved different protection mechanisms, chief among which are ROS scavenging enzymes such as superoxide dismutase, peroxidase and catalase [Zeeshan and Prasad, 2009]. They all have a collective role in scavenging superoxide and peroxide radicals, converting them to relatively less harmful substances. Another way to reduce the effects of ROS on cellular functions is by non-enzymatic scavenging using various anti-oxidants such as ascorbic acid, reduced glutathione, carotenoids and osmolytes like proline [Das and Roychoudhury, 2014]. Phycobiliproteins (PBPs) have a significant role as anti-oxidants in vitro, which has been extensively demonstrated by various study groups over many years. Their anti-oxidant activity has been demonstrated both in the form of crude extracts and in their purified forms [Paliwal et al. 2015; Thangam et al. 2013; Benedetti et al. 2004; Pinero Estrada et al. 2001].

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Our group has studied the antioxidant activities of the water extracts of different cyanobacterial species, with the result that their phycobiliprotein content has been directly implicated as the reason for their anti-oxidative properties [Ghosh et al. 2016; Paliwal et al. 2015]. However, their in vivo effects are still largely confined to cell line and animal model studies. A detailed view of the mechanisms involved in ROS scavenging and corresponding antioxidant mechanisms in vivo is a lacuna which should be addressed. 2.2.2.

Carotenoids Carotenoids are natural tetraterpenoids produced by all photosynthetic organisms to

protect themselves from photodamage and to aid in photosynthesis [Miller et al. 2002]. Keeping in view that photosynthesis is the primary conversion of sunlight to usable energy, major producers adopt all possible measures to protect and enhance this process. They are widely used as a nutraceutical and natural colorant in the cosmetic industry, but they also find application in the chemotaxonomy and therapeutics [Paliwal et al. 2016; Ghosh et al. 2015]. 2.2.2.1.

Light

Microarray and proteomic approaches have been used to identify genes and proteins induced by high light in Chlamydomonas. Exposure of dark-grown cells to high light intensity (800µEs m-2 s-1) seemed to trigger induction of both phytoene synthase (psy) and phytoene desaturase (Pds) as after 1 hour of exposure to very high light their transcript levels were improved by2- and 4-fold, respectively. LCYE and LCYB, involved in the cyclization of lycopene to yield "-carotene and/or -carotene, showed a slight decrease in their mRNA level. For ZEP, which catalyses the synthesis of violaxanthin and is directly involved in the xanthophyll cycle, where it was observed that there is a very slight decrease in its transcription levels during high illumination condition. The most interesting results

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concerned the expression of the P450 cytochrome–dependent "- and -ring carotene hydroxylase genes [Couso et al. 2012]. 2.2.2.2. Nutrient The carotenoid synthesis is highly controlled and the transcriptional regulation of Lycopene beta-cyclase (Lcy-β) gene expression in D. salina has been investigated previously with different environmental stress conditions [Ramos et al. 2008]. The study states that there is an increase in Steady-state DsLcy-β mRNA levels when D. salina cells were subjected to abiotic stress like high salinity and light condition. Also, excessive salinity and high light conditions results into highest level of steady-state DsLcy-β mRNA when combined with nutrient depletion strategy. These results show that nutrient depletion is critical for β-carotene accumulation in this microalga. Coesel et al. [2008] also suggests that D. salina Psy and Pds are also similarly regulated. There are certain transcriptional inhibitors that stop carotenoid biosynthesis in D. salina, which are crucial factors in their regulation in D. salina and other carotenogenic organisms. It is also observed that in other algae, namely H. pluvialis and C. reinhardtii, the up-regulation of other carotenogenic genes is caused due to stressful conditions. Similar type of regulatory control also exists during the development of fruit and flowers in higher plants at transcriptional level of carotenoid biosynthesis. Moreover, there is a need to understand about regulatory factors including posttranscriptional, translational and metabolic like feedback mechanism through metabolites (e.g. retinol and other β-ringcontaining compounds or end-products) and redox control. Salt-stress up-regulated the carotenoid ketolase (BKT) in Chlorella zogingiensis and enhanced the accumulation of canthaxanthin and astaxanthin. ROS generation is stimulated due to high salinity, triggering the up-regulation of characteristic carotenogenic genes and improving the carotenoid yield. Although, there are limited studies on the effect of nutrients on the carotenoid composition of

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cyanobacteria, we have found selective carotenoid accumulation in Synechocystis sp.in our study [Paliwal et al. 2015]. 2.2.2.3.Metabolic engineering There have been various research focussing on enhancement of astaxanthin in plants due to its potential health benefits in some of the life-threatening diseases like cardiovascular diseases, diabetes, etc. [Fassett and Coombes, 2011]. The study by Kim and Portis, [2004] suggest that under the influence of chromoplast-associated PDS promoter from green alga Haematococcus pluvialis in the nectary, CrtO ketolase gene overexpresses resulting in the high levels of astaxanthin and other ketocarotenoids. Similarly, overexpression of bacterial CrtZ and CrtW enzymes (encoding, respectively, β-carotene hydroxylase and ketolase) lead to high levels of ketocarotenoids in nectary, and low levels in leaf tissue in tobacco. 2.3.

Polymers

Cyanobacteria have many unexplored potential for natural products with a huge variability in structure and biological activity. Their products are species specific and growth condition specific. Under stress conditions they are reported to produce biopolymers like EPS and PHA, which can be produced extracellularly and intracellularly, respectively. 2.3.1. Polyhydroxyalkanoates (PHA) Polyhydroxyalkanoates (PHAs) are linear polyesters produced by microbial fermentation of sugar or lipids storing carbon and energy. Halophilic bacteria like Halomonas hydrothermalis accumulates PHA under different stress conditions utilizing various raw substrates are (PHAs) [Bera et al. 2015] Interest in cyanobacterial PHAs gained attention due to its minimum requirement for its growth. PHAs have different applications in packaging films, disposable items, bone replacements, blood vessel replacements and scaffold material in tissue, etc. The stress condition of salinity in which organism grows almost reduces the contamination problem A. subsalsa is reported to produce 129.8 mg/g 24

CDW of PHA when cultivated in saline ASN-III media [Shrivastav et al. 2010]. A study conducted on A. platensis suggested that in phosphate limited condition poly 3hydroxybutyrate (PHB) biosynthesis occurs even after 30 days of growth [Panda et al. 2005]. 2.3.2. Exopolysaccharides (EPS) Cyanobacteria extracellularly secretes heteropolysaccharides, which are frequently associated with small amounts of non-carbohydrate substituents, such as peptides, fatty acids [De Philippis and Vincenzini, 1998]. Exopolysaccharides (EPS) from A. platensis secretes sulfated EPS to the surrounding media which plays an important role in the survival of the producer organisms under extreme conditions [Potts, 2001]. Trabelsi et al. 2009 observed the role of temperature (330C-350C) indicating optimum EPS productivity while decrease in temperature increases the growth of Arthrospira. 3.

Future Prospective

In order to achieve cost effective and sustainable production of microalgal products, a suitable biorefinery approach should be opted based on the biomass nutrient profile of microalgae. Abiotic stresses can be used to alter the microalgal metabolite profiles and thereby utilization of nutrient rich biomass for both high value and low value products. Establishment of sustainable and economically viable biorefinery urge for development of less harsh cell disruption methods that do not degrade high value products such as proteins (including phycobiliproteins) and PUFA. To improve the productivity of microalgae, genetic engineering efforts need to be directed to improve the stress tolerance. Stress tolerance genes from other organisms can be expressed in host organism to improve the stress tolerance to achieve higher biomass productivity in extreme conditions and also to improve the yield percentage of desired end products. Conditional expression of genes is another area which could help us to metabolically re-program microalgae to behave differently under stress conditions.

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4.

Conclusion

Microalgal species respond to the changing environmental condition (abiotic stress factors) by modulating their metabolites. Nitrogen limitation and salinity stress is found to be the most widely applied stress condition for the commercial production of feedstock for biodiesel production whereas, temperature can be considered as the most tuning factor for PUFA production. Various abiotic stress conditions affect genes which enhance pigment production in cyanobacteria; thus, the field of cyanobacterial genetic engineering will require attention both at basic and applied levels for higher pigment production.

Acknowledgement This manuscript was assigned by CSIR-CSMCRI registration no. 047/2017. Authors deeply acknowledge Dr. Amitava Das, Director, CSIR-CSMCRI and Dr. Arvind Kumar, DC, Salt and Marine Chemicals for their encouragement. Authors would like to acknowledge DST and CSIR for financial support through projects GAP 2006, CSC 0105, CSC 0203 and OLP 0084. MM and VBSV would like to acknowledge CSIR for SRF. All the authors acknowledge AcSIR for PhD enrolment. KB is thankful to Bhavnagar University for PhD enrolment.

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76. Singh, S. P., Montgomery, B. L., 2013. Salinity impacts photosynthetic pigmentation and

cellular

morphology

changes

by distinct

mechanisms

in

Fremyella

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Table Captions Table 1a: Effect of abiotic stress factors on neutral lipid (TAG) content of microalgae Table 1b: Effect of abiotic stress factors on PUFA content of microalgae Table 2: Effect of abiotic stress factors on microalgal pigment

39

Microalgae

Neutral lipid (TAG)

Abiotic stress factors applied

Major observations

References

Chlorella sp.

TAG

High light intensity

Increase in TAG

He et al., 2015

Monoraphidinium sp.

TAG

High light intensity

Increase in TAG

He et al., 2015

Scenedesmus obliquus

TAG

High light intensity

Increase in TAG

Breuer et al., 2013

Pavlova lutheri

TAG

High light intensity

Increase in TAG

Carvalho and Malcata, 2005

Acutodesmus dimorphus

TAG

Temperature stress at 35°C

Increased accumulation of neutral lipid

Chokshi et al., 2015

Dunaliella tertiolecta

TAG

Increasing NaCl concentration (0.5 M to 2.0 M)

Increase in total lipid content and TAG

Takagi and Yodhida, 2006

Scenedesmus sp.

TAG

Salinity stress of 400 mM

Increase accumulation of neutral lipid

Pancha et al., 2015

Chlorella sp.

TAG

Nitrogen starvation

Increased accumulation of lipids (mainly TAG)

Praveenkumar et al., 2012

Chlorella vulgaris

TAG

Nitrogen starvation

Increased accumulation of lipids (mainly TAG)

Yeh and Chang, 2011

Scenedesmus sp.

TAG

Nitrogen starvation

Increased accumulation of lipids (mainly TAG)

Pancha et al., 2014a

Phaeodactylum tricornutum

TAG

Phosphorus limitation

Increased accumulation of TAG

Reitan et al., 1994

Isochrysis galbana

TAG

Phosphorus limitation

Increased accumulation of TAG

Reitan et al., 1994

Chaetoceros sp.

TAG

Phosphorus limitation

Increased accumulation of TAG

Reitan et al., 1994

Pavlova lutheri

TAG

Phosphorus limitation

Increased accumulation of TAG

Reitan et al., 1994

Monodus subterraneus

TAG and PUFA

Phosphorus limitation

~6-fold increase in TAG and decrease in EPA content

Khozin-Goldberg and Cohen, 2006

Scenedesmus sp.

TAG

Nitrate starvation

Increased accumulation of neutral lipid

Pancha et al., 2015

Monoraphidinium minutum

SFA

Optimum carbon source

Increase in SFA content

Patidar et al., 2014

Chlorella sp.

TAG

Alkaline pH stress

Increased accumulation of TAG

Guckert and Cooksey, 1990

Tetraselmis sp.

SFA and MUFA

UV-B radiation

Overall increase in the SFA and MUFA content

Goes et al., 1995

Chaetoceros mualleri

MUFA

Combined effect of UV-A and UV-B

Increase in the MUFA content

Liang et al., 2006

Pavlova lutheri

TAG

UV radiation

Increase in TAG with decrease in EPA and DHA

Guihéneuf et al. 2010

Table 1a: Effect of abiotic stress factors on neutral lipid (TAG) content of microalgae

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Microalgae

PUFA

Abiotic stress factors applied

Major observations

References

Chaetoceros brevis

EPA and DHA

Low temperature and low irradiance

Enhanced production of EPA and DHA

Boelen et al., 2013

Thalassiosira weissflogii

EPA and DHA

Low temperature and low irradiance

Enhanced production of EPA and DHA

Boelen et al., 2013

Pyramimonas sp.

EPA and DHA

Low temperature and low irradiance

Enhanced production of EPA and DHA

Boelen et al., 2013

Emiliania huxleyi

EPA and DHA

Low temperature and low irradiance

Enhanced production of EPA and DHA

Boelen et al., 2013

Fibrocapsa japonica

EPA and DHA

Low temperature and low irradiance

Enhanced production of EPA and DHA

Boelen et al., 2013

Isochrysis galbana

ALA and DHA

Decrease in temperature 25°C to 10°C

Significant increase in ALA and DHA content

Zhu et al., 1997

Pavlova lutheri

EPA

Decrease in temperature (10°C)

Increase in EPA content

Tatsuzawa and Takizawa, 1995

Phaeodactylum tricornutum

EPA

Decrease in temperature 25°C to 10°C

Increase in EPA content

Jiang and Gao, 2004

Nannochloropsis sp.

EPA

Combined effect of low temperature and low irradiance

3.4-fold increase in the EPA content

Mitra et al., 2015b

Nannochloropsis gaditana

EPA

Increase in salinity

Decrease in EPA productivity

Mitra et al., 2015a

-1

Crypthecodinium cohnii

DHA

9 g L NaCl

Increase in DHA content

Jiang et al., 1999

Phaeodactylum tricornutum

EPA

Nitrate stress

Increased accumulation of EPA

Yongmanitchai and Ward, 1991

Nitzschia laevis

EPA

Urea used as a nitrogen source

Increase in EPA productivity

Yongmanitchai and Ward, 1991

Chlorella kessleri

PUFA

Phosphorus starvation

Increase in unsaturated fatty acid content

EI-sheek and Rady, 1995

Phaeodactylum tricornutum

EPA

High phosphorus supplementation in the media

Increase in EPA content

Yongmanitchai and Ward, 1991

Crypthecodinium cohnii

DHA

Utilization of glucose as carbon source

Increase in DHA content

Jiang et al., 1999

Nitzschia laevis

EPA

Silicate limitation

Increased accumulation of EPA

Wen and Chen, 2000

Pinguiococcus pyrenoidosus

PUFA

pH 7

Increase in PUFA content including EPA

Sang et al., 2012

Nannohloropsis oculata

PUFA

UV-A radiation

Increased accumulation of PUFA

Srinivas and Ochs, 2012

Phaeocystis antarctica

PUFA

Low UV-B radiation

Increased accumulation of PUFA

Skerratt et al., 1998

Tetraselmis sp.

PUFA

UV-B radiation

50% decrease in the PUFA content

Goes et al., 1995

Phaeodactylum tricornutum

EPA

UV radiation

Increase in EPA content

Liang et al., 2006

Table 1b: Effect of abiotic stress factors on PUFA content of microalgae

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Table 2: Effect of abiotic stress factors on microalgal pigment

Microalgae

Pigments

Abiotic stress factors applied

Major observations

References

Chlamydomonus reinhardtii

Carotenoids

high light stress

induced synthesis of violaxanthin

Couso et al., 2012

Dunaliella salina

Carotenoids

salt, light and nutrient depletion

upregulates carotenoid production

Ramos et al., 2008

Synechocystis sp.

Carotenoids

salinity

higher production of β-carotene

Paliwal et al., 2015

Chamaesiphon sp.

C-PC and C-PE

chromatic adaptation

upregulates production of C-PC and C-PE

Tandeau de Marsac N., 1977

Dermocarpa sp.

C-PC and C-PE

chromatic adaptation

upregulates production of C-PC and C-PE

Tandeau de Marsac N., 1977

Xenococcus sp.

C-PC and C-PE

chromatic adaptation

upregulates production of C-PC and C-PE

Tandeau de Marsac N., 1977

Phormidium sp.

C-PC and C-PE

chromatic adaptation

upregulates production of C-PC and C-PE

Tandeau de Marsac N., 1977

Pseudanabaena sp.

C-PC and C-PE

chromatic adaptation

higher production of C-PC and C-PE

Mishra et al., 2012

Fremyella diplosiphon

C-PC and C-PE

salt stress and chromatic adaptation

increased production of C-PC and C-PE

Singh et al., 2013

Spirulina fusiformis

C-PC

salt stress

increased production of C-PC and C-PE

Rafiqul et al., 2003

Oscillatoria sp.

C-PC and C-PE

chromatic adaptation and temperature

upregulates production of C-PC and C-PE

Singh et al., 2012

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43

Highlights

•

Abiotic stresses are important tools for metabolites in microalgae.

•

Temperature, nutrient starvation, salinity and light influence PUFAs.

•

Nitrogen and light stress influence phycobiliproteins.

•

Salinity, light and nutrients influence carotenoids.

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