God and Nature Spring 2019
Solar-Powered Life: Providing Food, Oxygen, and Protection
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By Ray Lewis
He makes grass grow for the cattle,
and plants for people to cultivate--
bringing forth food from the earth.
Psalm 104:14 (NIV)
God provides for all of life in multiple ways, and many of these ways are highlighted in Psalm 104. The provision of food is through plants, as testified by the Psalmist. Since plants and algae photosynthesize, we could say that life on earth is solar-powered. In photosynthesis, plants capture the sun’s energy as chemical energy (food), using the carbon dioxide (CO2) in the atmosphere, and producing oxygen (O2). All organisms, including plants and animals, can extract the chemical energy for life’s processes through respiration, using the food and oxygen, and releasing carbon dioxide. While the basic balance of materials between photosynthesis and respiration has been known for about 200 years, the details of the chemical processes of photosynthesis have been gradually worked out since then, and progress on understanding these complex processes is continuing to the present day. Whether focusing on the big picture of the balance of photosynthesis and respiration, or looking at the details of how photosynthesis works, our understanding of these processes have brought us to realize that photosynthesis is as amazing as it is essential for life to continue.
He makes grass grow for the cattle,
and plants for people to cultivate--
bringing forth food from the earth.
Psalm 104:14 (NIV)
God provides for all of life in multiple ways, and many of these ways are highlighted in Psalm 104. The provision of food is through plants, as testified by the Psalmist. Since plants and algae photosynthesize, we could say that life on earth is solar-powered. In photosynthesis, plants capture the sun’s energy as chemical energy (food), using the carbon dioxide (CO2) in the atmosphere, and producing oxygen (O2). All organisms, including plants and animals, can extract the chemical energy for life’s processes through respiration, using the food and oxygen, and releasing carbon dioxide. While the basic balance of materials between photosynthesis and respiration has been known for about 200 years, the details of the chemical processes of photosynthesis have been gradually worked out since then, and progress on understanding these complex processes is continuing to the present day. Whether focusing on the big picture of the balance of photosynthesis and respiration, or looking at the details of how photosynthesis works, our understanding of these processes have brought us to realize that photosynthesis is as amazing as it is essential for life to continue.
"...not only has God provided food for all of life through photosynthesis, He has provided oxygen so that the chemical energy in food could be efficiently utilized..." |
There are several ways that living organisms capture light into chemical energy, but only one way has dominated: oxygenic photosynthesis. This simply means photosynthesis that converts energy-poor carbon dioxide in the atmosphere into energy-rich sugars, while producing oxygen (by splitting water). There are other types of photosynthesis, as well as other ways of capturing light energy into chemical energy, that don’t result in producing oxygen (1). These additional types are found in a variety of bacteria, but they are rare in comparison to oxygenic photosynthesis. In order to achieve oxygenic photosynthesis, two photosystems (complexes composed of proteins and other molecules) capture and utilize the light energy. The complex called Photosystem II is responsible for splitting water (H2O) in order to make oxygen, with the hydrogen atoms and their accompanying electrons being used to convert carbon dioxide to sugar. It is not easy to split water in this way, and the details of how Photosystem II does this continue to be worked out (2).
This mechanism has been incredibly effective - there are many kinds of photosynthetic organisms that use this type of photosynthesis. While the first to come to mind may be the great diversity of green land plants, nearly half of the world’s photosynthesis is performed by simpler organisms known as algae that live in the oceans that cover most of our planet, as well as in freshwater lakes and streams. Some of these are green in color, while others are red, brown, golden, or blue-green. These various colors are due to different photosynthetic pigments in the algae that have the common function of absorbing light in order to capture the light energy so it could be stored in chemical form.
A more complete story of how oxygenic photosynthesis became the mechanism used by all these diverse kinds of photosynthesizers has emerged from studies of algae and plants over the past several decades. It appears that oxygenic photosynthesis arose in a group of bacteria known as cyanobacteria (also called blue-green algae) over three billion years ago. The several thousand species of cyanobacteria that presently occur on earth live primarily in fresh and marine waters all over the planet.
Some of these are very small single-celled organisms, and the smallest yet most abundant photosynthetic organism in the world is the cyanobacterium Prochlorococcus. The tiny cells of Prochlorococcus were first discovered by MIT oceanographer Penny Chisholm in the 1980s, using specialized flow cytometers that were able to detect these cells (they were too small for the fine mesh nets used to capture other ocean algae) (3). There are an estimated 3 X 10^27 (that is, three octillion, or three billion billion billion)
Prochlorococcus cells on earth (4), occurring in densities of about 100,000 cells/ml from the surface to 200 m deep over much of the warm oceans between latitudes of 40° N and 40° S. These cells might be small, but this recently found organism accounts for about 5% of the photosynthesis in the world, and the discovery of the large role played by Prochlorococcus and other tiny photosynthetic species has changed our understanding regarding productivity and food webs in the oceans.
In geological strata, there is evidence that the concentration of oxygen in the environment increased starting over three billion years ago, and this is likely due to the growing abundance of cyanobacteria that could perform oxygenic photosynthesis. It appears that little oxygen was present in the atmosphere for a long time, since most of it reacted with elements in the earth’s crust. Then oxygen began to accumulate in the atmosphere starting about 2.3 billion years ago in what has been called the Great Oxidation Event, with oxygen levels coming up to about 10-20% of the current concentration (5). This event is sometimes called the Oxygen Catastrophe because oxygen was toxic to many organisms, causing them to go extinct. But this setback appears to have been temporary, as evidenced by the diversity of life on the planet at present that not just tolerates oxygen but requires it to live. In other words, oxygenation by cyanobacteria was one step in preparing the way for living organisms that would utilize oxygen for life, including humans.
So it appears that oxygenic photosynthesis had its origin in cyanobacteria, and scientists conclude that photosynthesis in algae and plants originated by combining a cyanobacterium with a nonphotosynthetic cell via endosymbiosis (endo- meaning inside, symbiosis meaning life together). This would mean that a single-celled organism that ingests other cells for food captured a cyanobacterium inside but did not break it down to get the chemical energy out of it. Instead, the cyanobacterium continued to live inside the larger cell, producing food energy by photosynthesis for both itself and its larger host cell. There are many known examples of species exhibiting endosymbiosis in which a photosynthetic cell lives inside a host cell (6).
The light reactions of oxygenic photosynthesis are depicted here,
including the key role of Photosystem II in splitting water to
produce oxygen, as well as the rest of the energy capture process
to produce ATP and NADPH which are subsequently used to fix
carbon dioxide to make carbohydrates
In the origin of chloroplasts, it appears that over time, some of the DNA from the photosynthetic endosymbiont was transferred to the host cell in which it was living, while other DNA was degraded. Eventually, the endosymbiont became the photosynthetic plastids (such as the green chloroplasts in green plants) that perform photosynthesis inside the cells of algae and plants. Some of the proteins that are active inside the plastid are encoded by genes in the nucleus of the cell. The rest of the proteins that work in the plastid are encoded by genes that are on a relatively short circle of DNA inside the plastid. When scientists were able to determine the sequences of DNA from plastids and compare them to all other known forms of life, they were most similar to those of cyanobacteria. This similarity provided the strongest evidence that plastids originated from cyanobacteria via endosymbiosis. In addition, finding DNA sequences in the nucleus that were similar to those from the endosymbiont shed light on the kind of endosymbiotic gene transfer that would accompany the endosymbiotic origin of plastids.
This initial endosymbiotic event gave rise to both green and red algae, with tens of thousands of species currently known to scientists. Some green algae gave rise to the great diversity of land plants, of which we have identified several hundred thousand species. But the borrowing of photosynthetic machinery through the fusion of two kinds of cells did not stop here. There are two groups of algae (euglenids and chlorarachniophytes) in which the plastids were obtained via endosymbiosis of green algae, and there are several others (such as diatoms, dinoflagellates, brown algae, and others) that obtained their plastids through endosymbiosis involving single-celled red algae.
Thus, the green and red algae would have obtained their plastids from cyanobacteria in primary endosymbiosis, while other algae would have obtained their plastids from green or red algae in secondary endosymbiosis. In secondary endosymbiosis, the nucleus of the endosymbiotic alga eventually was lost (except in a few forms where a remnant nucleus, or nucleomorph, remains), but the plastids with their capability to perform photosynthesis were kept (7). There are even a few examples where it appears that plastids arose by tertiary endosymbiosis. Thus, oxygenic photosynthesis in all of these organisms had its start in the cyanobacteria, and oxygenic photosynthesis was multiplied many times over until we have the world we have today, with the atmosphere today containing a whopping 21% oxygen.
While oxygen is essential for life as we know it, it was toxic to the organisms that first produced it. In fact, recent research indicates that overcoming oxygen toxicity was a key step that had to accompany the endosymbiosis of the oxygen-sensitive host cell and oxygen-producing photosynthetic cell. But there is an additional global benefit to having oxygen in our atmosphere. Oxygen absorbs damaging ultraviolet (UV) light, which is part of the light coming to the earth from the sun. In the process of absorbing UV radiation, oxygen (O2) will react to produce ozone (O3). Ozone will then absorb both UV-C and most of UV-B radiation, both of which would cause damage to living organisms, especially to the DNA. The remaining UV-B radiation that comes through is able to cause sunburns and some DNA damage. But with most of the UV radiation absorbed by oxygen and ozone, life on earth is protected from significant damage.
So not only has God provided food for all of life through photosynthesis, he has provided oxygen so that the chemical energy in food could be efficiently utilized, and ozone to absorb harmful UV radiation. He has blessed the earth with the conditions to be fruitful in many ways, and learning more about these processes helps us understand the riches of these blessings, the depth of God’s wisdom and love, and the might of his power to bring all of this about.
Image Credit: US Dept. Energy, Office of Biological and Environmental Research
References
Raymond J. Lewis (PhD, University of California, Santa Barbara) is associate professor of biology at Wheaton College. His research and teaching interests include genetics and physiology of marine algae, environmental ethics, theories of origins, and botany. He has published a number of articles in scientific journals, is coauthor of the textbook Understanding Scientific Theories of Origins (2018), and is a member of the American Scientific Affiliation, the International Phycological Society, and the Phycological Society of America.
This mechanism has been incredibly effective - there are many kinds of photosynthetic organisms that use this type of photosynthesis. While the first to come to mind may be the great diversity of green land plants, nearly half of the world’s photosynthesis is performed by simpler organisms known as algae that live in the oceans that cover most of our planet, as well as in freshwater lakes and streams. Some of these are green in color, while others are red, brown, golden, or blue-green. These various colors are due to different photosynthetic pigments in the algae that have the common function of absorbing light in order to capture the light energy so it could be stored in chemical form.
A more complete story of how oxygenic photosynthesis became the mechanism used by all these diverse kinds of photosynthesizers has emerged from studies of algae and plants over the past several decades. It appears that oxygenic photosynthesis arose in a group of bacteria known as cyanobacteria (also called blue-green algae) over three billion years ago. The several thousand species of cyanobacteria that presently occur on earth live primarily in fresh and marine waters all over the planet.
Some of these are very small single-celled organisms, and the smallest yet most abundant photosynthetic organism in the world is the cyanobacterium Prochlorococcus. The tiny cells of Prochlorococcus were first discovered by MIT oceanographer Penny Chisholm in the 1980s, using specialized flow cytometers that were able to detect these cells (they were too small for the fine mesh nets used to capture other ocean algae) (3). There are an estimated 3 X 10^27 (that is, three octillion, or three billion billion billion)
Prochlorococcus cells on earth (4), occurring in densities of about 100,000 cells/ml from the surface to 200 m deep over much of the warm oceans between latitudes of 40° N and 40° S. These cells might be small, but this recently found organism accounts for about 5% of the photosynthesis in the world, and the discovery of the large role played by Prochlorococcus and other tiny photosynthetic species has changed our understanding regarding productivity and food webs in the oceans.
In geological strata, there is evidence that the concentration of oxygen in the environment increased starting over three billion years ago, and this is likely due to the growing abundance of cyanobacteria that could perform oxygenic photosynthesis. It appears that little oxygen was present in the atmosphere for a long time, since most of it reacted with elements in the earth’s crust. Then oxygen began to accumulate in the atmosphere starting about 2.3 billion years ago in what has been called the Great Oxidation Event, with oxygen levels coming up to about 10-20% of the current concentration (5). This event is sometimes called the Oxygen Catastrophe because oxygen was toxic to many organisms, causing them to go extinct. But this setback appears to have been temporary, as evidenced by the diversity of life on the planet at present that not just tolerates oxygen but requires it to live. In other words, oxygenation by cyanobacteria was one step in preparing the way for living organisms that would utilize oxygen for life, including humans.
So it appears that oxygenic photosynthesis had its origin in cyanobacteria, and scientists conclude that photosynthesis in algae and plants originated by combining a cyanobacterium with a nonphotosynthetic cell via endosymbiosis (endo- meaning inside, symbiosis meaning life together). This would mean that a single-celled organism that ingests other cells for food captured a cyanobacterium inside but did not break it down to get the chemical energy out of it. Instead, the cyanobacterium continued to live inside the larger cell, producing food energy by photosynthesis for both itself and its larger host cell. There are many known examples of species exhibiting endosymbiosis in which a photosynthetic cell lives inside a host cell (6).
The light reactions of oxygenic photosynthesis are depicted here,
including the key role of Photosystem II in splitting water to
produce oxygen, as well as the rest of the energy capture process
to produce ATP and NADPH which are subsequently used to fix
carbon dioxide to make carbohydrates
In the origin of chloroplasts, it appears that over time, some of the DNA from the photosynthetic endosymbiont was transferred to the host cell in which it was living, while other DNA was degraded. Eventually, the endosymbiont became the photosynthetic plastids (such as the green chloroplasts in green plants) that perform photosynthesis inside the cells of algae and plants. Some of the proteins that are active inside the plastid are encoded by genes in the nucleus of the cell. The rest of the proteins that work in the plastid are encoded by genes that are on a relatively short circle of DNA inside the plastid. When scientists were able to determine the sequences of DNA from plastids and compare them to all other known forms of life, they were most similar to those of cyanobacteria. This similarity provided the strongest evidence that plastids originated from cyanobacteria via endosymbiosis. In addition, finding DNA sequences in the nucleus that were similar to those from the endosymbiont shed light on the kind of endosymbiotic gene transfer that would accompany the endosymbiotic origin of plastids.
This initial endosymbiotic event gave rise to both green and red algae, with tens of thousands of species currently known to scientists. Some green algae gave rise to the great diversity of land plants, of which we have identified several hundred thousand species. But the borrowing of photosynthetic machinery through the fusion of two kinds of cells did not stop here. There are two groups of algae (euglenids and chlorarachniophytes) in which the plastids were obtained via endosymbiosis of green algae, and there are several others (such as diatoms, dinoflagellates, brown algae, and others) that obtained their plastids through endosymbiosis involving single-celled red algae.
Thus, the green and red algae would have obtained their plastids from cyanobacteria in primary endosymbiosis, while other algae would have obtained their plastids from green or red algae in secondary endosymbiosis. In secondary endosymbiosis, the nucleus of the endosymbiotic alga eventually was lost (except in a few forms where a remnant nucleus, or nucleomorph, remains), but the plastids with their capability to perform photosynthesis were kept (7). There are even a few examples where it appears that plastids arose by tertiary endosymbiosis. Thus, oxygenic photosynthesis in all of these organisms had its start in the cyanobacteria, and oxygenic photosynthesis was multiplied many times over until we have the world we have today, with the atmosphere today containing a whopping 21% oxygen.
While oxygen is essential for life as we know it, it was toxic to the organisms that first produced it. In fact, recent research indicates that overcoming oxygen toxicity was a key step that had to accompany the endosymbiosis of the oxygen-sensitive host cell and oxygen-producing photosynthetic cell. But there is an additional global benefit to having oxygen in our atmosphere. Oxygen absorbs damaging ultraviolet (UV) light, which is part of the light coming to the earth from the sun. In the process of absorbing UV radiation, oxygen (O2) will react to produce ozone (O3). Ozone will then absorb both UV-C and most of UV-B radiation, both of which would cause damage to living organisms, especially to the DNA. The remaining UV-B radiation that comes through is able to cause sunburns and some DNA damage. But with most of the UV radiation absorbed by oxygen and ozone, life on earth is protected from significant damage.
So not only has God provided food for all of life through photosynthesis, he has provided oxygen so that the chemical energy in food could be efficiently utilized, and ozone to absorb harmful UV radiation. He has blessed the earth with the conditions to be fruitful in many ways, and learning more about these processes helps us understand the riches of these blessings, the depth of God’s wisdom and love, and the might of his power to bring all of this about.
Image Credit: US Dept. Energy, Office of Biological and Environmental Research
References
- Bryant, D.A. and Frigaard, N.-U. 2006. Prokaryotic photosynthesis and phototrophy illuminated. Trends in Microbiology 14 (11): 488-496.
- For example, Suga, M. et al. 2017. Light-induced structural changes and the site of O=O bond formation in PSII caught by XFEL. Nature 543, 131–135; Kawashima, K., Takaoka, T., Kimura, H., Saito, K. & Ishikita, H. 2018. O2 evolution and recovery of the water-oxidizing enzyme. Nature Communications 9, Article number: 1247.
- Chisholm, S. W., R. J. Olson, E. R. Zettler, J. Waterbury, R. Goericke, and N. Welschmeyer. 1988. A novel free-living prochlorophyte occurs at high cell concentrations in the oceanic euphotic zone. Nature (London) 334: 340–343.
- Pennisi, E. 2017. Meet the obscure microbe that influences climate, ocean ecosystems, and perhaps even evolution. Science doi:10.1126/science.aal0873.
- Holland, Heinrich D. 2006. The oxygenation of the atmosphere and oceans. Phil. Trans. R. Soc. Lond. B Biol. Sci. 361: 903–915.
- A variety of photosynthetic endosymbionts as well as other types of endosymbionts are reviewed by Nowack, E.C.M. and Melkonian, M. 2012. Endosymbiotic associations within protists. Phil. Trans. R. Soc. Lond. B Biol. Sci. 365: 699–712.
- Keeling, P.J. 2013. The number, speed, and impact of plastid endosymbioses in eukaryotic evolution. Ann. Rev. Plant Biol. 64:583-607.
Raymond J. Lewis (PhD, University of California, Santa Barbara) is associate professor of biology at Wheaton College. His research and teaching interests include genetics and physiology of marine algae, environmental ethics, theories of origins, and botany. He has published a number of articles in scientific journals, is coauthor of the textbook Understanding Scientific Theories of Origins (2018), and is a member of the American Scientific Affiliation, the International Phycological Society, and the Phycological Society of America.