An evaluation of autotrophic microbes for the removal of carbon dioxide from combustion gas streams

FUEL PROCESSING TECHNOLOGY ELSEVIER

Fuel Processing Technology 40 (1994) 139-149

An evaluation of autotrophic microbes for the removal of carbon dioxide from combustion gas streams W i l l i a m A. A p e l * , M i c h e l l e R. W a l t o n , P a t r i c k R. D u g a n Center/or Biological Processing Technology, The hlaho National Engineering Labortory. EG&G Idaho. Inc.. P.O. Box 1625. hlaho Falls, ID 83415-2203. USA Received 19 January 1994: accepted in revised form 9 May 1994

Abstract Carbon dioxide is a greenhouse gas that is believed to be a major contributor to global warming. Studies have shown that significant amounts of CO2 are released into the atmosphere as a result of fossil fuels combustion. Therefore, considerable interest exists in effective and economical technologies for the removal of CO2 from fossil fuel combustion gas streams. This work evaluated the use of autotrophic microbes for the removal of CO2 from coal fired power plant combustion gas streams. The CO2 removal rates of the following autotrophic microbes were determined: Chlorella pyrenoidosa, Euylena gracilis, Thiobacillus ferrooxidans, Aphanocapsa delicatissima, lsochrysis 9albana, Phaodactylum tricornutum, Navicula tripunctata schizonemoids, Gomphonema parvulum, Surirella ovata ot,ata, and four algal consortia. Of those tested, Chlorella pyrenoidosa exhibited the highest removal rate with 2.6 g C O / p e r day per g dry weight of biomass being removed under optimized conditions. Extrapolation of these data indicated that to remove C02 from the combustion gases of a coal fired power plant burning 2.4 × 104 metric tons of coal per day would require a bioreactor 386 km 2 × 1 m deep and would result in the production of 2.13 × 105 metric tons (wet weight) of biomass per day. Based on these calculations, it was concluded that autotrophic CO2 removal would not be feasible at most locations, and as a result, alternate technologies for CO2 removal should be explored. Keywords: Carbon dioxide bioremoval; Greenhouse effect: Greenhouse gases; Carbon dioxide fixation pathways

1. Introduction S o l a r h e a t i n g of the E a r t h ' s a t m o s p h e r e is related to the a m o u n t a n d type of i n c o m i n g solar r a d i a t i o n that enters the a t m o s p h e r e a n d reaches the E a r t h ' s surface. It is e s t i m a t e d that of the 100% of i n c o m i n g solar r a d i a t i o n a b o u t 25% is reflected by a t m o s p h e r i c gases a n d a b o u t 2 5 % is a b s o r b e d by those gases. O f the r e m a i n i n g 50% that reaches the E a r t h ' s surface, a b o u t 4 5 % is a b s o r b e d a n d 5 % is reflected. O f the 25% absorbed by atmospheric gases approximately 66% is reemitted to outer space [-1, 2]. * Corresponding author. SSD10378-3820{94100059-3

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W.A. Apel et al./Fuel Processing Technology 40 (1994) 139 149

Any electromagnetic radiation absorbed by molecules (whether they are gases in the atmosphere or solids at the earth's surface) will energize those molecules and be reemitted as less energetic electromagnetic energy. Some of the electromagnetic energy is converted to kinetic energy resulting in heating effects. However, it is also a thermodynamic characteristic that molecular absorption of the less energetic, longer wavelengths (i.e. infrared) translates to kinetic or heating effects in the absorbing molecules. Of the longer wavelengths reemitted from the Earth's surface, about 70% escape through the atmospheric gases to outer space whereas 30% of the outgoing radiation is either reabsorbed by the atmospheric gases (total 4%) on the way out or is rereflected back to Earth, thereby exerting a kinetic or heating effect. About 88% of the absorbed outgoing radiation is rereflected to earth and this constitutes a major heating phenomenon known as the Greenhouse Effect. The amount of outgoing radiation trapped appears to be increasing over time and is thought to be related to increasing concentrations of gases in the atmosphere that have the capacity to absorb the radiation [33. These gases have been collectively referred to as Greenhouse Gases. Essentially any atmospheric gas molecule composed of two or more atoms can act to some degree as a greenhouse gas. Important greenhouse gases include: carbon dioxide (CO2), nitrous oxide ( N 2 0 ) , and methane (CH4), all of which are natural components of the earth's atmosphere. Of these gases, CO2 is believed to be the primary contributor to the greenhouse effect and is present in the atmosphere at a concentration of 345 ppm with an average life time of two years [43. This represents a 25% increase in comparison to a pre-industrial CO2 level of 280 ppm with a total of 379 Gt of C as C O / h a v i n g been released into the atmosphere since the beginning of industrial times [5 7]. Furthermore, the atmospheric concentration of CO2 is increasing at a rate of 0.5% per year. Worldwide releases o f C O 2 a r e estimated at 7.5 billion tons per year with 5.5 billion tons of this total being derived from the combustion of fossil fuels. As a result of this increase in atmospheric CO2 concentration and its link to fossil fuel combustion, methods to effectively remove CO2 from fossil fuel combustion gas streams are being actively explored. Due to their physiological diversity, microbes represent a source of metabolic processes that can be exploited to remove or chemically convert CO2, including that from combustion gases to alternate compounds. Specifically, CO2 can be removed from gas mixtures by a variety of microorganisms including both autotrophs and heterotrophs. Autotrophic organisms are those that obtain their cellular carbon from atmospheric CO2, and as such, have the ability to fix gaseous CO2 into cellular components. A variety of organisms including a number of different bacteria, cyanobacteria, green algae, and all higher plants are autotrophic. Many autotrophic microbes fix CO2 via the Calvin cycle, a metabolic pathway that employs the enzyme ribulose bisphosphate carboxylase (RuBP carboxylase) to catalyze the incorporation of CO2 with D-ribulose 1,5-diphosphate [83. This results in the formation of 2 moles of glyceric acid-3-phosphate which is ultimately converted into nearly all cellular constituents [9]. Some autotrophs, however, fix C02 via pathways other than the Calvin cycle. Reductive tricarboxylic acid cycles have been elucidated in green photosynthetic bacteria and archaebacteria [10-12]. In these pathways CO2 is typically condensed

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with phosphoenolpyruvate (PEP) via the enzyme PEP carboxylase, to form oxaloacetic acid. It is of interest to note that many heterotrophic microorganisms also have the ability to fix CO2 by adding it to either pyruvate or PEP via the enzymes pyruvate carboxylase and/or PEP carboxylase. Also, methanogenic bacteria have the capability to convert CO2 by reducing it to methane [-13] as do acetogenic bacteria which reduce CO2 to acetate [14]. This paper reports the results and conclusions from studies performed to evaluate the potential of using microbes to remove large amounts of CO2 from fossil fuel combustion gas streams.

2. Experimental 2.1. Cultures Table 1 lists the microbes screened for CO2 removal. The cultures from University of Texas were obtained courtesy of Richard C. Starr (Culture Collection of Algae, Department of Botany, University of Texas, Austin, TX). The cultures from Loras College were obtained courtesy David B. Czarnecki (Freshwater Diatom Culture Collection, Department of Biology, Loras College, Dubuque, IA). The culture from The Ohio State University was obtained from Dr. Robert Pfister (Ohio State University, Columbus, OH). Each was cultured in the recommended medium (listed below) in sealed serum vials with CO2 added to the headspace as follows. C. pyrenoidosa, Aphanocapsa delicatissirna, and the algal consortia were grown on ASM-1 medium which consisted of the following components (g/l): NaNO3, 0.17; MgSO4"7H20, 0.024; MgCI2, 0.019; CaC12, 0.022; KEHPOa, 0.017; Na2HPO4, 0.014; and the following in mg/l: FeCI3, 0.64; HaBO3, 2.47; MnC12, 0.88; ZnCI2, 0.44; Table 1 List of the m i c r o o r g a n i s m s screened for C O 2 removal, their source, and relative ability to r e m o v e C O 2. Rates of CO2 removal: very fast ( + + + ) fast ( + + ), m o d e r a t e ( + ), very slow ( + / - ), none ( - ) Microorganism

Source

Chlorella pyrenoidosa Euglena 9racilis Thiobacillus ferroxidans Aphanocapsa delicatissima Isochrysis oalhana Phaodactylum tricornutum Navicula tripunctata schizonemoids Gomphonema parvulum Surirella ovata ovata

Univ. OSU INEL INEL Univ. Univ. Loras Loras Loras 1NEL 1NEL INEL 1NEL

Consortium Consortium Consortium Consortium

# 1 #2 #3 #4

CO2 u p t a k e

of Texas 1230

of Texas 987 of Texas 640 College L204 College L130 College A55

+ + + + + + + + + + + +

+ + + / / / / / /

-+ + + +

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W.A. Apel et al./Fuel Processing Technology 40 (1994) 139-149

and Na2EDTA, 7.44. Eu#lena ,qracilis was grown on euglena medium which was composed of the following components (g/l): (NH4)2SO4~ 0.135; NH4C1, 0.90; KHePO4, 1.0; sodium citrate, 0.51; M g C l z ' 6 H 2 0 , 0.35; and the following in rag/l: FeClz "4H20, 4.72; (NH4)zMoO4' 2H20, 0.20; NaVO4' 16H20, 0.10; H3BO3, 0.50: Z n S O 4 . 6 H 2 0 , 0.38: CaC12, 0.20; MnCI2"4H20, 1.8; COC12, 1.35; CuSO4"5H20, 0.02: thiamine, 1.0; and vitamin B12, 0.005. The pH of the medium was 4.4. The diatoms were grown on diatom medium which consisted the following components (g/l): Ca(NO3)2"4H20, 0.1: K 2 H P O 4 " 3 H 2 0 , 0.0135; MgSO4"7H20, 0.026; Na2SiO3 • 9H20, 0.1; NazCO3, 0.02: and the following in mg/l: H3BO3, 0.568; ZnClz, 0.624: CuC12" H20, 0.268: N a 2 M o O 4 " 2 H 2 0 , 0.252; COC12"6H20, 0.42: MnCI2 • 4H20, 0.36: FeSO4" 7H20, 2.5: and sodium tartrate. 2H20, 1.76. To a 120 ml serum vial 30 ml of medium was added. The headspace was filled with 10% carbon dioxide (CO2) in air and sealed. The cultures were incubated under a 150W grow lamp with 400 footcandles of illumination on a 12h light and dark cycle. The light intensity was measured with a GE Light Meter type 214 (General Electric, Cleveland, OH). The temperature was approximately 28'~C. Cultures were shaken at 120 rpm and transferred weekly to fresh medium. Screening for CO2 removal rates also was performed under these conditions with CO2 levels in culture headspaces being followed by gas chromatography.

2.2. Analyses Analysis for C O 2 concentrations in the headspaces of the cultures was conducted on a Gow/Mac series 550P gas chromatograph equipped with a thermal conductivity detector (TCD) and a Hayesep Q column. The injector temperature was 30°C and the detector temperature was 35 r C. Helium was the carrier gas at a flow rate of 30 ml/min. The injection volume was 100 ~tl of gas. Dry weights were determined by filtering a 10 ml sample of the liquid culture through a pre-weighed 0.2 ~tm pore-size membrane filter. The filter was dried for 24 h in a 102°C oven and then weighed again. A blank filter was dried concomitantly to account for any thermal breakdown of the filter. Optical density was also used to quantify growth. Optical densities were determined on a Bausch and Lomb Spectronic 2000 spectrophotometer at a wavelength of 513 nm. A standard curve comparing each microbial dry weight and optical density was used to convert the absorbance reading to dry weight. As noted in Table 1, C. pyrenoidosa had the highest CO2 removal rates in the screening so it was the microorganism chosen for further experimentation. The objective of further experiments was to determine the optimum conditions for maximal COz removal by C. pyrenoidosa. This was done by determining rates of CO2 removal over a range of the variable being tested. Variables tested included CO2 concentration, temperature, light intensity, pH, and selected nutrient additions. Experiments were run in triplicate. An inoculum of 3 ml of stationary phase C. pyrenoidosa was added to 30 ml of ASM-1 medium in 120 ml serum vials which were sealed and placed on a gyratory shaker at 120 rpm. To buffer the medium against pH changes due to the addition of

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high C O 2 concentrations, 16 m M K 2 H P O 4 was added. The CO2 removal rates were determined after approximately 20h incubation, the time when the cells entered a logarithmic growth phase. The culture conditions were varied as listed below. To determine optimum C02 concentration, various amounts of CO2 ranging from 0 to 100% were added to air in the headspace of sealed serum vials. The disappearance of CO2 was monitored over time by gas chromatography as mentioned previously. The temperature and light intensity were maintained at 28°C and 400 footcandles, respectively. The optimum temperature for CO2 removal was determined by observing rates of C02 disappearance over temperatures ranging from 23-47°C. The CO2 concentration was 50% (v/v) in air and the light intensity was 400 footcandles. A range of light intensities (100 to 1000 footcandles) was tested for maximal CO2 removal. The temperature was 28 _+ 2°C, and 50% (v/v) CO2 in air was added to the headspace of the sealed serum vials. The pH was varied from 5 to 9 and its effect on CO2 removal rates was observed. For this set of experiments the CO2 concentration was 50% (v/v) in air, at 28 _+ 2°C, and the light intensity was 400 footcandles. Some of the nutrients in the ASM-1 medium were also altered. CO2 removal rates were monitored over a range of phosphate (K2HPO4) concentrations (0 to 100 mM) and nitrate (NaNO3) concentrations (2 to 64 mM). A m m o n i u m chloride (NH4C1) was substituted for N a N O 3 at concentrations ranging from 2 m M to 20 m M to determine if it was a more favorable nitrogen source for CO2 removal. The cultures were added to 120 ml sealed serum vials containing 50% (v/v) CO2 in air in the headspace. The temperature was 28 __+2°C and the light intensity was 400 footcandles (ft-c).

3. Results and discussion Under the conditions employed, Chlorella pyrenoidosa removed C O 2 at a faster rate than the other microorganisms screened (Table 1). Euylena 9racilis and the algal consortia utilized the CO2 relatively quickly but not nearly as rapidly as C. pyrenoidosa. The decision was made to use C. pyrenoidosa to determine if it would be possible to use an algal culture for scrubbing CO2 from combustion gases. Figs. 1 to 7 show the rates of CO2 removal obtained over a range of conditions affecting growth of C. pyrenoidosa. For m a x i m u m removal rates, the optimum CO2 concentration was found to be 50% (v/v) CO2 in air (Fig. 1). The temperature for the best CO2 removal rate was between 28°C and 32°C (Fig. 2). C. pyrenoidosa required a light intensity greater than 400 ft-c to attain maximum CO2 removal rates (Fig. 3). Above 400 ft-c the CO2 removal rate remained constant. CO2 removal rates appeared to increase as p H increased (Fig. 4). This may be due to increased solubility of CO2 as alkalinity increased. However, the addition of 50% CO2 actually decreased the pH to between 6.5 and 7. The phosphate seemed to inhibit removal of CO2 (Fig. 5). The rates varied considerably at the very low phosphate concentrations but then gradually decreased as the phosphate concentration increased. A m m o n i u m chloride was tested as an alternate nitrogen source to nitrate and had an optimum effect at 10raM

W.A. Apel et al./Fuel Processing Technology 40 (1994) 139 149

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10 5

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40

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C02 (%) Fig. 1. Removal rates of CO2 by C. pyrem~ido~a at various initial CO2 concentrations. Temperature was 2 8 C and light intensity was 400 ft-c.

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Fig. 2. Removal rates of C O t by C. pyrenoidosa over a range of temperatures. C O / c o n c e n t r a t i o n was 50% and light intensity was 400 ft-c.

35 30 "0

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CO 2

concentration was 50%

W.A. Apel et al./Fuel Processing Technology 40 (1994) 139-149 A

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-~

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pH Fig. 4. Removal rates of CO2 by C. pyrenoidosa over a range of pH. CO2 concentration was 50%, temperature was 28:C, and light intensity was 400 ft-c.

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Phoephate (mM) Fig. 5. Removal rates of COz by C. pyrenoidosa at various concentrations of K z H P O 4 . CO2 concentration was 50%, temperature was 28:C, and light intensity was 400 ft-c.

~

60

20 E ec 10 O

o" o

O0

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1'0

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Ammonium Chloride (mM) Fig. 6. Removal rates of CO2 by C. pyrenoidosa at various concentrations of NH4CI. CO2 concentration was 50%, temperature was 28~C, and light intensity was 400 ft-c.

146

W.A. Apel et al./ Fuel Processing Technology 40 (1994) 139-149

120[ 100

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ao

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Sodlum Nltrate (mM) Fig. 7. Removal rates of C02 by C. pyrenoidosa at various concentrations of NaNO3. CO 2 concentration was 50%, temperature was 28' C, and light intensity was 400 ft-c,

Table 2 Assumptions used in extrapolation of laboratory CO2 removal data to coal-fired power plant combustion gas streams An autotrophic bioreactor is used to remove CO, The stream is from a power plant burning 24000 metric tons of coal per day C. pyrenoidosa is used in the bioreactor The alga has a maximum CO2 fixation rate of 2.6 g/d/g drywt, biomass and grows at a density of 0.7 gdry wt./1 The cells are 45% carbon (drywt.) and 85°'0 (w/wt water There are 12h optimum light/d The cells are 100% efficientcompared to best rates observed in the laboratory The coal is 60% (w/w) carbon, all of which is oxidized to combustion gas CO2

(Fig. 6). However, the o r g a n i s m had far greater C O 2 removal rates in the presence of N O 3 t h a n it did in the presence of NH4C1. The highest rate of CO2 removal achieved was 110 lag C O 2 / h / m g dry weight biomass u n d e r the following conditions: 8 m M N a N O 3 (substituted for the N a N O 3 in the ASM-1 medium), 50% CO2, 28°C, 400 ft-c, a n d 16 m M K z H P O 4 {Fig. 7). 3.1. Extrapolation o f laboratory C O 2 remot, al data to combustion gas streams

In practical terms, the results reported above can be evaluated relative to a CO2 stream at a coal fired power plant. F o r example, assume that an a u t o t r o p h i c bioreactor would be used to treat a c o m b u s t i o n gas stream from a moderately large coal fired power plant u n d e r the c o n d i t i o n s outlined in Table 2. O p e r a t i n g u n d e r these a s s u m p t i o n s , the algal biomass p r o d u c e d when all COE is t r a p p e d by the algae, a n d 100% conversion occurs, can be determined. As shown in

W.A. Apel et al./Fuel Processing Technology 40 (1994) 139-149

147

Eq. (1), approximately 2.13 x 105 metric tons algal biomass is produced daily from the combustion of 2.4 × 104 metric tons of 60% carbon coal. 2.4 x 104 metric tons coal day

×

0.6 metric ton C metric ton coal

×

1 metric ton algae (dry wt) 0.45 metric ton C

1 metric ton algae (wet wt) 2.13 x 105 metric tons algae (wet wt) x 0.15 metric ton algae (dry wt) = day

(1)

Algal biomass production also can be calculated based on C. pyrenoidosa's experimentally determined m a x i m u m CO2 fixation rate of 2.6 g CO2/g dry wt biomass/day as shown in Eq. (2). 2.4 x 104 metric tons coal x 0.6 metric ton C x 1 metric ton CO2 metric ton coal 0.2727 metric ton C 1 metric ton algae (dry wt) 1 metric ton algae (wet wt) x x 2.6 metric ton CO2 fixed.day 0.15 metric ton algae (dry wt) =

1.35 x 105 metric ton algae (wet wt) day

(2)

The difference between the theoretical m a x i m u m biomass value of 2.13 × 105 metric tons algae wet weight and the calculated m a x i m u m biomass of 1.35 × 105 metric tons algae wet weight produced with C. pyrenoidosa is probably attributable, at least in significant part, to soluble metabolic byproducts produced by the algae under actual growth conditions. These soluble products would not have been quantified by the filtration technique used to determine biomass production. One then may estimate the bioreactor size that would be needed to support this conversion. 1.35 × 105 metric tons algae (dry wt) x

×

11 = 1.93 0.7 g algae (dry wt)

106 g . . metric ton

x 10111

This assumes the bioreactor has light for 24 h day-x. If the bioreactor has light for 1 2 h d a y -1 then a 3.86x10111 or 3.86x 10am 3 bioreactor would be required to remove all the CO2 generated at a 2.4 x 104 metric ton/day power plant. Assume for optimum light penetration a bioreactor 1 m deep is required, then 3.86 x 108 m 2 of surface area would be required. This equates to a pond 19.65 km x 19.65 km x 1 m deep or 386 km 2 x 1 m deep. In the above calculations, the conversion efficiencies are unrealistically high, and as such, represent an absolute best case based on 100% efficiency. In reality, conversion efficiencies would be expected to be considerably lower with bioreactor size increasing proportionally to the decrease in efficiency. These calculations show that biological removal of CO2 from combustion gas streams is probably not widely applicable as an

148

W.A. Apel et al./Fuel Processing Tectmologr 40 (1994) 139-149

independent point source treatment due to the large amount of biomass produced in combination with the very large bioreactor size required when using C. pyrenoidosa or other comparable microbes. While it might be possible to devote the necessary space for massive autotrophic fixation of COz in some locations; e.g. in the southwest US where land, sunlight, and warmer climate are more generally available than in other locations, significant amounts of atmospheric COz would not be permanently removed unless the algal biomass produced was preserved or somehow sequestered from the biosphere for long periods of time. An example of one way to potentially accomplish this type of sequestration would be injection of the biomass down abandoned deep wells. Otherwise, the relatively rapid degradation of any biomass produced would reintroduce CO2 into the atmosphere with no net atmospheric COz decrease. Barring a significant break through in biological CO2 removal methods, other C O / removal technologies should be examined. If a biological solution to help combat atmospheric CO2 increases is desired, the most practical is to promote the growth of significant forest tracts. While this approach is not a point source solution, the trees composing these forests not only convert CO2 to biomass, which in mature, old growth forests is a relatively long term CO2 sink, but also actively convert COz to 0 2 by photosynthesis. Although rates of algal COz fixation into biomass were determined, these calculations are irrelevant to conclusions related to amounts of biomass production required for conversion of CO2 into biomass. That is, if the efficiency of CO2 fixation was increased significantly, e.g. by selection of more efficient autotrophs or via genetic engineering, more rapid generation of biomass would result. However, the total amount of biomass production resulting from the reduction of the COz would not be decreased. This would lead to significant biomass handling problems that in a practical sense would be difficult to address. It should be noted, however, that partial removal of COz from a combustion gas stream may be feasible, particularly if the biomass produced can be used as a fuel or alternative feedstock for the production of chemicals. This would be particularly advantageous if the use of this biomass allowed decreased reliance on fossil fuels which would be associated with decreased addition of CO2 into the biosphere. The question of tolerance of autotrophs to other combustion gas components such as sulfur oxides and nitrogen oxides was not addressed by the research presented above. Based on the arguments presented, these considerations are not germane.

4. Conclusions Conclusions from this research are as follows: (1) Under the laboratory test conditions employed, C. pyrenoidosa exhibited the best CO2 removal rates of the autotrophic organisms tested. (2) The maximum rate of CO2 removal by C. pyrenoidosa under optimized conditions was 2.6 g COz/day/g dry wt. biomass.

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(3) If the CO2 removal rate data generated in the l a b o r a t o r y are extrapolated to removal of C 0 2 from the c o m b u s t i o n gas stream of a coal fired power p l a n t b u r n i n g 2 . 4 x 104 m e t r i c t o n s of coal per day, a b i o r e a c t o r 386 k m E x 1 m deep w o u l d be needed u n d e r o p t i m u m conditions. (4) This research indicates that a u t o t r o p h i c removal of C O / f r o m coal fired power p l a n t c o m b u s t i o n gas streams is n o t widely applicable based on c u r r e n t technology, a n d that alternate m e t h o d s for CO2 control should be sought.

Acknowledgements This work was s u p p o r t e d u n d e r c o n t r a c t no. D E - A C 0 7 - 7 6 I D 0 1 5 7 0 from the US D e p a r t m e n t of Energy, Office of A d v a n c e d Research a n d T e c h n o l o g y D e v e l o p m e n t , Office of Fossil Energy to the I d a h o N a t i o n a l E n g i n e e r i n g L a b o r a t o r y / E G & G Idaho, Inc.

References [1] Schneider, S.H., 1987. Climate Modeling. Sci. Am., 256(5): 72 80. [2] Schneider, S.H., 1989. The Greenhouse effect: Science and Policy. Sci., 243:771 781. [3] Lashof, D.A., 1989.The dynamic greenhouse:feedback processes that may influencefuture concentrations of atmospheric trace gases and climatic change. Climatic Change, 14: 213-242. [4] Bouwman, A.F., 1989. The role of soils and land use in the greenhouse effect. Netherlands J. Agricul. Sci., 37:13 19. [5] Neftel, A., Oeschger, H., Schwander, J., Stauffer, B. and Zumbrunn, R., 1982. Ice core sample measurements give atmospheric COz content during the past 40000 yr. Nature, 295:220 223. [6] Sundquist, E.T., 1993. The global carbon dioxide budget. Sci., 259: 934-941. [7] Raynaud, D., Jouzel, J., Barnola, J.M., Chappellaz, J., Delmas, R.J. and Lorius, C., 1993. The ice record of greenhouse gases. Sci., 259: 926-934. [8] Tabita, F.R., 1988. Molecular and cellular regulation of autotrophic carbon dioxide fixation in microorganisms. Microbiol. Rev., 52: 155-189. [9] Doelle, H.W., 1975. Bacterial Metabolism. 2nd edn., Academic Press, New York, 738 pp. [10] Fuller, R.C., Smillie,R.M., Sisler, E.C. and Kornberg, H.L., 1961. Carbon metabolism in Chromatium. J. Biol. Chem., 236:2140 2148. [11] Fuchs, G., Stupperich, E. and Eden, G., 1980. Autotrophic COz fixation in Chlorobium lumincola. Evidence for the operation of a reductive tricarboxylic acid cycle in growing cells. Arch. Microbiol., 128:64 71. [12] Schafer, S., Barkowski, C. and Fuchs, G., 1986. Carbon assimilation by the autotrophic thermophilic archaebacterium Thermoproteus neutrophilus. Arch. Microbiol., 146: 301-308. [13] Jones, W.J., Nagle, D.P. and Whitman, W.B., 1987. Methanogens and the diversity of archaebacteria. Microbiol. Rev., 39: 135-177. [14] Fuchs, G., 1986. CO2 fixation in acetogenic bacteria: variations on a theme. FEMS Microbiol. Lett., 39:181 213.