What Is Anoxygenic Photosynthesis?

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

The photosynthetic reaction that you may have seen in plants is not the only possible way to turn light energy into stored energy (sugar). In this lesson, you'll see other ways that some bacteria, the anoxygenic phototrophs, use light energy.

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:

photosynthesis equation

This equation is also, more generically, written like this:

photosynthesis equation, simplified

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.

Anoxygenic Phototrophs

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.

Acidobacterium
Acidobacterium

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:

generic photosynthesis

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.

Sulfur

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:

photosynthesis with H2S

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.

Arsenic

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:

photosynthesis with As

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