Photoheterotroph

Photoheterotrophs (Gk: photo = light, hetero = (an)other, troph = nourishment) are heterotrophic organisms that use light for energy, but cannot use carbon dioxide as their sole carbon source. Consequently, they use organic compounds from the environment to satisfy their carbon requirements. They use compounds such as carbohydrates, fatty acids and alcohols as their organic "food". Examples are purple non-sulfur bacteria, green non-sulfur bacteria and heliobacteria.^[1]

Metabolism

Photoheterotrophs generate ATP using light in one of two ways^[2][3]: they use a bacteriochlorophyll based reaction center or they use a bacteriorhodopsin. The chlorophyll based mechanism is similar to that used in photosynthesis, where light excites the molecules in a reaction center and causes a flow of electrons through an electron transport chain (ETS). This flow of electrons through the proteins causes hydrogen ions to be pumped across a membrane. The energy stored in this proton gradient is used to drive ATP synthesis. Unlike in photoautotrophs, the electrons flow only in a cyclic pathway: electrons released from the reaction center flow through the ETS and return to the reaction center. They are not utilized to reduce any organic compounds. Purple non-sulfur bacteria, green non-sulfur bacteria and heliobacteria are examples of bacteria that carry out this scheme of photoheterotrophy.

Other organisms like the halobacteria (archaea) and several eubacteria like certain flavobacteria^[4] and vibrios^[5] have purple rhodopsin based proton pumps that supplement their energy supply. The archaeal version is called bacteriorhodopsin while the eubacterial version is called proteorhodopsin. The pump consists of a single protein that has a Vitamin A derivative, retinal bound to it. It may have accessory pigments like carotenoids associated with the protein. When light is absorbed by the retinal molecule it isomerises. This drives the protein to change shape and pump a proton across the membrane. The hydrogen ion gradient can then be used to generate ATP, transport solutes across the membrane or drive the flagellar motor. In the case of one flavobacterium, though cannot reduce carbon dioxide using light, it nevertheless uses the energy from its rhodopsin system to to fix carbon dioxide through anaplerotic fixation.^[4] It is still a heterotroph as it needs reduced carbon compounds to live and cannot make do with just light and CO₂. It cannot carry out reactions in the form of

2n CO₂ + 2n DH₂ + photons → 2(CH₂O)_n + 2n DO where DH may be water, H₂S or another oxidizable compound.

However it can fix carbon in reactions like:

CO₂ + Pyruvate + ATP(from photons) → malate + ADP +P_i

where malate or other useful molecules are otherwise got by breaking down other compounds by

carbohydrate + O₂ → malate + CO₂+energy

This method of Carbon fixation is useful when energy is plentiful (from light) but reduced carbon compounds are scarce and can't be wasted as CO₂ during interconversions.

Flowchart

Flowchart to determine if a species is autotroph, heterotroph, or a subtype

• Autotroph
  • Chemoautotroph
  • Photoautotroph
• Heterotroph
  • Chemoheterotroph
  • Photoheterotroph

See also

• Primary nutritional groups

References

Sources

University of Wisconsin, Madison Microbiology Online Textbook

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