Phototroph (Prokaryote Metabolism): What Is It?

Although they may seem very different or even less sophisticated at first glance, prokaryotes have at least one thing in common with all other organisms: they require fuel to power their lives. Prokaryotes, which include organisms in the domains Bacteria and Archaea, are very diverse when it comes to metabolism, or the chemical reactions that the organisms use to produce fuel.

For instance, one category of prokaryotes, called extremophiles, thrive in conditions that would obliterate other life forms, such as the super-heated water of hydrothermal vents deep in the ocean. These sulfur bacteria handle water temperatures up to 750 degrees Fahrenheit just fine, and they get their fuel from the hydrogen sulfide found in the vents.

Some of the most important prokaryotes rely on photon capture to produce their fuel through photosynthesis. These organisms are phototrophs.

The word phototroph gives the first clue revealing what makes these organisms important. It means "light nourishment" in Greek. Put simply, phototrophs are organisms that get their energy from photons, or particles of light. You probably already know that green plants use light to make energy through photosynthesis.

However, this process isn't restricted to plants. Many prokaryotic and eukaryotic organisms carry out photosynthesis to make their own food, including photosynthetic bacteria and some algae.

While photosynthesis is similar among all organisms that do it, the process of bacterial photosynthesis is less complicated than plant photosynthesis.

Just like green plants, phototrophic bacteria use pigments to capture photons as energy sources for photosynthesis. For bacteria, these are bacteriochlorophylls found in the plasma membrane (rather than in chloroplasts like plant chlorophyll pigments).

Bacteriochlorophylls exist in seven known varieties, labeled a, b, c, d, e, c_s or g. Each variant is structurally different and therefore able to absorb a specific type of light from the spectrum, ranging from infrared radiation to red light to far red light. The type of bacteriochlorophyll a phototrophic bacterium contains depends on its species.

Just like plant photosynthesis, bacterial photosynthesis occurs in two stages: **light reactions and dark reactions**.

In the light stage, the bacteriochlorophylls capture photons. The process of absorbing this light energy excites the bacteriochlorophyll, triggering an avalanche of electron transfers and ultimately producing adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH).

In the dark stage, those ATP and NADPH molecules are used in chemical reactions that transform carbon dioxide into organic carbon through a process called carbon fixation.

Different types of bacteria make fuel by fixing carbon in different ways using a carbon source such as carbon dioxide. For example, cyanobacteria use the Calvin cycle. This mechanism uses a compound with five carbons called RuBP to catch one molecule of carbon dioxide and form a molecule with six carbons. This splits into two equal pieces, and one half exits the cycle as a sugar molecule.

The other half transforms into a molecule with five carbons, thanks to reactions involving ATP and NADPH. Then, the cycle begins again. Other bacteria rely on the reverse **Krebs cycle**, which is a series of chemical reactions that use electron donors (such as hydrogen, sulfide or thiosulfate) to produce organic carbon from the inorganic compounds carbon dioxide and water.

Phototrophs that use photosynthesis (called _photoautotrophs_) form the base of the food chain. Other organisms that can't perform photosynthesis get their fuel by using photoautotrophic organisms as a food source.

Because they can't convert light to fuel on their own, these organisms simply eat the organisms that do and use their bodies as a source of energy. Since carbon fixing uses carbon dioxide to produce fuel in the form of sugar molecules, phototrophs help reduce excess carbon dioxide in the atmosphere.

Phototrophs may even be responsible for the free oxygen in the atmosphere that enables you to breathe and thrive on Earth. This possibility – called the Great Oxygenation Event – proposes that cyanobacteria performing photosynthesis and releasing oxygen as a byproduct eventually produced too much oxygen to be absorbed by iron in the environment.

This excess became part of the atmosphere and shaped evolution on the planet from that point forward, making it possible for humans to eventually emerge.