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Photosynthesis: Shaping the planet
MATTHEW OLDFIELD/spl
Present: Fifty shades of green
Not so sluggish: playing host to chloroplasts gives Elysia an energy boost
Despite more than 2 billion years of evolution, the core reactions of photosynthesis have remained remarkably similar across species. Yet a variety of subtle physical and biochemical modifications have also evolved, each optimised to suit conditions in specific ecological niches. Plants, for example, have evolved slightly different forms of photosynthesis. Some 85 per cent of plant species are known as C3 plants. These use the enzyme rubisco to fix carbon from CO2 to form 3-carbon sugar molecules that provide the building blocks for sucrose. Another group of plants has evolved a way to get round the inefficiency of rubisco. C4 plants, including tropical grasses such as sugar cane, supercharge carbon fixation by using an additional enzyme, PEP carboxylase, to fix CO2 into malic acid, a 4-carbon molecule. The malic acid is then pumped into specialised cells where it is broken down to release CO2. Inside these cells, rubisco is exposed to high concentrations of CO2 which helps it work more efficiently. Although this process requires energy, it allows photosynthesis in C4 plants to be up to
50 per cent more efficient than C3 plants, giving them a competitive advantage in hot sunny conditions. A different adaption is found in dinoflagellates such as Amphidinium carterae. These live in the sea at depths where the only available light is in the blue-green part of the spectrum. They have evolved a unique light harvesting complex that uses pigments called carotenoids, which absorb blue-green light. Chlorophyll pigments in plants absorb weakly at these wavelengths. A number of creatures including jellyfish, flatworms, bivalve molluscs and salamanders also make use of photosynthesis, thanks to a symbiotic relationship with photosynthetic algae. The sea slug Elysia, for example, eats green algae and keeps their chloroplasts alive in its body, supplementing its diet using the carbohydrates they create.
right: Frans Lanting/Corbis far right: Mint Images/Rex below: DR JEREMY BURGESS/spl
Future: Plants in a changing world Since oxygenic photosynthesis evolved some 2.8 billion years ago, the proportion of the atmosphere that is made up of CO2 has dropped from about 20 per cent to just 0.04 per cent. The levels started to rise again at the start of the industrial revolution, and continue to go up. This is happening because the capacity of photosynthesis to soak up the huge volumes of gas we release by burning fossil fuels has been exceeded. What will this change mean for plants and for other life forms like us that ultimately depend on photosynthesis? Studies show that some trees are already growing bigger and faster, but perhaps the only certainty is that the effects on the composition of plant populations will be highly unpredictable. To find out more about how elevated CO2 concentrations will influence crop growth, researchers are simulating future atmospheric conditions in field experiments that expose plants to different compositions of air.
These experiments show that C4 and C3 plants respond differently. C4 plants such as maize (corn) slightly increase their rate of photosynthesis but there is little effect on growth, even when CO2 levels reach 0.06 per cent. However at this level of CO2, the rate of photosynthesis in C3 plants increases by about 40 per cent. This is reflected in the crop, with wheat, rice and soybean showing increases in yield of up to 14 per cent. There may also be an impact on water use. Plants regulate their CO2 uptake using tiny pores in their leaves called stomata. As CO2 levels rise, stomata will stay closed for longer. These pores also allow water vapour to escape, so higher CO2 levels may reduce plants’ water losses, meaning farmers would not have to water their crops as often. These benefits might come at a price. Increasing rates of photosynthesis mean plant growth may then be limited by the availability of key nutrients such as phosphorus and nitrogen. This could prove most serious for crops such as pulses that have seeds with high protein content and these may need extra fertiliser. Farmers can probably mitigate gradual changes to the atmosphere by altering farming practices – applying extra fertiliser, say, or changing crop varieties. What will be harder to deal with are sudden extremes of weather. Extensive drought in the US and heavy rain in parts of Europe drastically reduced grain yields in 2012, and global warming is expected to bring even wilder fluctuations in weather patterns.
Open and shut case: plants control the uptake of CO2 via tiny pores in their leaves iv | NewScientist | 2 February 2013
”The release of oxygen permitted the evolution of new life forms that obtained energy from respiration”
Shaping the planet When photosynthetic organisms first began to generate oxygen, it marked the start of a transformation of our world. Oxygen provided access to a more efficient source of energy through respiration, a process that would ultimately allow multicellular animals to evolve. Now, more than 2 billion years after these changes began, the world is transforming again. Our emissions are causing damaging climate change. So how will photosynthetic organisms adapt to a warmer planet, and what are the implications for our biosphere?
Past: First breath When the first bacteria began to harness light energy some 3.4 billion years ago, Earth’s atmosphere was mainly composed of nitrogen and CO2. These anaerobic photosynthetic organisms relied on hydrogen, or organic or sulphur compounds as a source of electrons. Then, about 2.4 billion years ago, the “great oxygenation event” kicked off. Photosynthetic organisms evolved – probably ancestors of presentday cyanobacteria – that were capable of splitting water to produce oxygen. Now oxygen levels in the biosphere began to rise (see timeline, below). The changes that occurred over the next 2 billion years resulted in the extinction of many anaerobic organisms – it would have been a dramatic and cataclysmic transformation. Yet the arrival of oxygen was not entirely bad news. Ultraviolet radiation from the sun hit oxygen in the upper layers of the atmosphere, and the subsequent reaction created ozone (O3). The layer of it in the stratosphere filtered out this harmful ultraviolet light,
which damages DNA, helping life spread out of the deep oceans. The earliest land dwellers were mosses and liverworts, descended from green algae that thrived in warm, shallow water. Oxygen also permitted the evolution of new life forms that obtained their energy from respiration. Aerobic respiration is very efficient. The increase in energy available to support life allowed a great expansion in the number of species on our planet and, in particular, the evolution of large multicellular creatures. By 400 million years ago, oxygen levels had begun to stabilise at close to current levels, and plants such as ferns, grasses and cacti had colonised the land. The release of oxygen by photosynthetic organisms also altered Earth’s geology. For instance, oxygen in the oceans triggered the formation of iron oxide, eventually producing the red bands of iron ore deposits in sedimentary rocks. Oxygen also generated thousands of other minerals in the crust, helping to create the rich variety of materials we exploit today.
Relics: stromatolites are fossilised deposits formed by ancient cyanobacteria
Nearly all the oxygen on our planet comes from photosynthesis. This gas first began to appear when photosynthetic cyanobacteria evolved. These bacteria could split water to give them energy, rather than using hydrogen or sulphur compounds
300 mya
Atmospheric oxygen levels (%)
40
30
High oxygen peak during the Carboniferous
3.4 bya First photosynthetic bacteria evolve
20
Great oxygenation event begins
2.8 bya
Red and brown algae evolve
1.9 bya Oxygen levels drop
500 mya
First land plants evolve
750 mya Green algae evolve
Photosynthetic cyanobacteria begin to release oxygen
10
0
1.2 bya
2.4 bya
4 Billion years ago (bya)
3
2
1
0 2 February 2013 | NewScientist | v
Present: Fifty shades of green
Not so sluggish: playing host to chloroplasts gives Elysia an energy boost
Despite more than 2 billion years of evolution, the core reactions of photosynthesis have remained remarkably similar across species. Yet a variety of subtle physical and biochemical modifications have also evolved, each optimised to suit conditions in specific ecological niches. Plants, for example, have evolved slightly different forms of photosynthesis. Some 85 per cent of plant species are known as C3 plants. These use the enzyme rubisco to fix carbon from CO2 to form 3-carbon sugar molecules that provide the building blocks for sucrose. Another group of plants has evolved a way to get round the inefficiency of rubisco. C4 plants, including tropical grasses such as sugar cane, supercharge carbon fixation by using an additional enzyme, PEP carboxylase, to fix CO2 into malic acid, a 4-carbon molecule. The malic acid is then pumped into specialised cells where it is broken down to release CO2. Inside these cells, rubisco is exposed to high concentrations of CO2 which helps it work more efficiently. Although this process requires energy, it allows photosynthesis in C4 plants to be up to
50 per cent more efficient than C3 plants, giving them a competitive advantage in hot sunny conditions. A different adaption is found in dinoflagellates such as Amphidinium carterae. These live in the sea at depths where the only available light is in the blue-green part of the spectrum. They have evolved a unique light harvesting complex that uses pigments called carotenoids, which absorb blue-green light. Chlorophyll pigments in plants absorb weakly at these wavelengths. A number of creatures including jellyfish, flatworms, bivalve molluscs and salamanders also make use of photosynthesis, thanks to a symbiotic relationship with photosynthetic algae. The sea slug Elysia, for example, eats green algae and keeps their chloroplasts alive in its body, supplementing its diet using the carbohydrates they create.
right: Frans Lanting/Corbis far right: Mint Images/Rex below: DR JEREMY BURGESS/spl
Future: Plants in a changing world Since oxygenic photosynthesis evolved some 2.8 billion years ago, the proportion of the atmosphere that is made up of CO2 has dropped from about 20 per cent to just 0.04 per cent. The levels started to rise again at the start of the industrial revolution, and continue to go up. This is happening because the capacity of photosynthesis to soak up the huge volumes of gas we release by burning fossil fuels has been exceeded. What will this change mean for plants and for other life forms like us that ultimately depend on photosynthesis? Studies show that some trees are already growing bigger and faster, but perhaps the only certainty is that the effects on the composition of plant populations will be highly unpredictable. To find out more about how elevated CO2 concentrations will influence crop growth, researchers are simulating future atmospheric conditions in field experiments that expose plants to different compositions of air.
These experiments show that C4 and C3 plants respond differently. C4 plants such as maize (corn) slightly increase their rate of photosynthesis but there is little effect on growth, even when CO2 levels reach 0.06 per cent. However at this level of CO2, the rate of photosynthesis in C3 plants increases by about 40 per cent. This is reflected in the crop, with wheat, rice and soybean showing increases in yield of up to 14 per cent. There may also be an impact on water use. Plants regulate their CO2 uptake using tiny pores in their leaves called stomata. As CO2 levels rise, stomata will stay closed for longer. These pores also allow water vapour to escape, so higher CO2 levels may reduce plants’ water losses, meaning farmers would not have to water their crops as often. These benefits might come at a price. Increasing rates of photosynthesis mean plant growth may then be limited by the availability of key nutrients such as phosphorus and nitrogen. This could prove most serious for crops such as pulses that have seeds with high protein content and these may need extra fertiliser. Farmers can probably mitigate gradual changes to the atmosphere by altering farming practices – applying extra fertiliser, say, or changing crop varieties. What will be harder to deal with are sudden extremes of weather. Extensive drought in the US and heavy rain in parts of Europe drastically reduced grain yields in 2012, and global warming is expected to bring even wilder fluctuations in weather patterns.
Open and shut case: plants control the uptake of CO2 via tiny pores in their leaves iv | NewScientist | 2 February 2013
”The release of oxygen permitted the evolution of new life forms that obtained energy from respiration”
Shaping the planet When photosynthetic organisms first began to generate oxygen, it marked the start of a transformation of our world. Oxygen provided access to a more efficient source of energy through respiration, a process that would ultimately allow multicellular animals to evolve. Now, more than 2 billion years after these changes began, the world is transforming again. Our emissions are causing damaging climate change. So how will photosynthetic organisms adapt to a warmer planet, and what are the implications for our biosphere?
Past: First breath When the first bacteria began to harness light energy some 3.4 billion years ago, Earth’s atmosphere was mainly composed of nitrogen and CO2. These anaerobic photosynthetic organisms relied on hydrogen, or organic or sulphur compounds as a source of electrons. Then, about 2.4 billion years ago, the “great oxygenation event” kicked off. Photosynthetic organisms evolved – probably ancestors of presentday cyanobacteria – that were capable of splitting water to produce oxygen. Now oxygen levels in the biosphere began to rise (see timeline, below). The changes that occurred over the next 2 billion years resulted in the extinction of many anaerobic organisms – it would have been a dramatic and cataclysmic transformation. Yet the arrival of oxygen was not entirely bad news. Ultraviolet radiation from the sun hit oxygen in the upper layers of the atmosphere, and the subsequent reaction created ozone (O3). The layer of it in the stratosphere filtered out this harmful ultraviolet light,
which damages DNA, helping life spread out of the deep oceans. The earliest land dwellers were mosses and liverworts, descended from green algae that thrived in warm, shallow water. Oxygen also permitted the evolution of new life forms that obtained their energy from respiration. Aerobic respiration is very efficient. The increase in energy available to support life allowed a great expansion in the number of species on our planet and, in particular, the evolution of large multicellular creatures. By 400 million years ago, oxygen levels had begun to stabilise at close to current levels, and plants such as ferns, grasses and cacti had colonised the land. The release of oxygen by photosynthetic organisms also altered Earth’s geology. For instance, oxygen in the oceans triggered the formation of iron oxide, eventually producing the red bands of iron ore deposits in sedimentary rocks. Oxygen also generated thousands of other minerals in the crust, helping to create the rich variety of materials we exploit today.
Relics: stromatolites are fossilised deposits formed by ancient cyanobacteria
Nearly all the oxygen on our planet comes from photosynthesis. This gas first began to appear when photosynthetic cyanobacteria evolved. These bacteria could split water to give them energy, rather than using hydrogen or sulphur compounds
300 mya
Atmospheric oxygen levels (%)
40
30
High oxygen peak during the Carboniferous
3.4 bya First photosynthetic bacteria evolve
20
Great oxygenation event begins
2.8 bya
Red and brown algae evolve
1.9 bya Oxygen levels drop
500 mya
First land plants evolve
750 mya Green algae evolve
Photosynthetic cyanobacteria begin to release oxygen
10
0
1.2 bya
2.4 bya
4 Billion years ago (bya)
3
2
1
0 2 February 2013 | NewScientist | v