Steph has a PhD in Entomology and teaches college biology and ecology.
The Photosynthetic Reaction
What's so special about the photosynthetic reaction?
If you've studied photosynthesis for a while, you may have seen this equation, which describes the reaction by which photosynthetic organisms convert carbon dioxide, water and light energy into stored energy (sugar) and oxygen:
This equation is also, more generically, written like this:
But there's no particular reason that this reaction should be the only possible way to convert light energy into chemical energy. Some organisms convert light energy into stored energy by other means. Meet the anoxygenic phototrophs, or anoxygenic photosynthesizing bacteria. They find similar ways to store light energy as chemical energy, but they don't use water, so they don't give off molecular oxygen as a waste product.
Only bacteria are capable of anoxygenic photosynthesis, but anoxygenic photosynthesis exists in multiple groups of bacteria. Green and red filamentous anoxygenic phototrophs, green sulfur bacteria, and some heliobacteria are anoxygenic phototrophs. Some Acidobacteria, a fairly mysterious phylum that has only been known since the 1990s, are also part of this group, as well as some purple bacteria. Purple bacteria are particularly interesting because many scientists think they may be related to our mitochondria.
The way in which anoxygenic photosynthesizing bacteria use light energy is similar to the way in which plants use light energy. They both use inorganic carbon in the form of carbon dioxide to create energy (which means they are called photoautotrophs).
But there are a few minor differences. Anoxygenic photosynthesizing bacteria use slightly different pigments, called bacteriochlorophylls. In addition, anoxygenic photosynthesizing bacteria use only one system for harvesting energy from light, Photosystem I. Plants use Photosystem I and Photosystem II.
But there are some bigger differences, too. Let's look at the reactants and products used in anoxygenic photosynthesis.
Anoxygenic Photosynthesis Equation
Let's go back to the basic photosynthesis equation, but make it a bit more generic:
This equation is demonstrating that some molecule is acting as an electron donor, but it isn't necessarily the oxygen in a water molecule. We will look at two other possibilities.
Very often, that donor is sulfur. Sulfur is very reactive in various oxidative and reductive states, which makes it a great electron donor. All organisms on earth use sulfur- it is found in amino acids, in polypeptides, in antibiotics, in enzyme cofactors, and in vitamins.
So in that case, the equation for photosynthesis using hydrogen sulfide is this:
Water actually requires more energy to complete the photosynthetic reaction than hydrogen sulfide. Therefore, in a world where hydrogen sulfide was abundant, you might expect to see more anoxygenic photosynthesis than oxygenic photosynthesis.
Some microbes, such as those found in biofilms in Mono Lake, California, use compounds containing arsenic. This equation shows how microbes can use arsenite in photosynthesis:
Anoxygenic phototrophs are believed to have evolved long before oxygenic phototrophs. Anoxygenic phototrophs probably evolved about 3.4 billion years ago, compared with perhaps 2.7 billion years ago for oxygenic phototrophs. It is not known what the original electron donor was, but sulfur compounds and iron compounds are considered likely candidates.
So one reason you might be interested in anoxygenic phototrophs is that they are some of the earliest forms of life on earth, and they are partially responsible for the evolution of life as we know it! Another reason is that they are being investigated for their role in bioremediation- that is, their ability to break down pollutants in sites contaminated by waste.
The photosynthetic reaction that you see in plants is pretty important because it converts light energy into stored energy, but there are other ways to do the same thing.
Some bacteria, called anoxygenic phototrophs, perform a similar reaction. But instead of water, they use other molecules as electron donors. For example, some microbes use hydrogen sulfide, and others use arsenite. This is called anoxygenic photosynthesis. Anoxygenic phototrophs are photoautotrophs like plants, but they only use Photosystem I, and they use special pigments called bacteriochlorophylls to harvest light energy.
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