Microbial Observatory for Photoheterotroph Exploration (MOPE)

Why are photoheterotrophs important? The reasons why photoheterotrophs are important vary with the type of photoheterotroph. Some eukaryotic photoheterotrophs appear to rely mainly on photoautotrophy (light for energy, CO₂ for carbon), but obtain nitrogen and phosphorus by grazing on other small microbes, including bacteria, whereas other eukaryotic photoheterotrophs augment a mainly heterotrophic lifestyle with some carbon from photoautotrophy.

Prokaryotic photoeheterotrophs are also common and potentially important in the oceans. There are three main groups:

Cyanobacteria (Synechococcus and Prochlorococcus ): Cyanobacteria are bacteria but functionally are phytoplankton. In fact, these microbes can account for a large (>75%) fraction of phytoplankton biomass and primary production, especially in the open oceans. Cyanobacteria depend on light and CO₂ for energy and carbon, respectively, and most strains cultured in the lab cannot grow in the dark heterotrophically.

Photomicrograph of a Synechococcus strain (CCMP1334) under epifluorescence microscopy. The cells are fluorescing without any stains ("autofluorescence") because of their pigments (mainly phycoerythrin).

However, data from this project and from other labs indicate that cyanobacteria have some heterotrophic activity in the oceans. This seems especially the case for Prochlorococcus but there is some evidence of Synechococcus using DOM. We are testing the hypothesis that these cyanobacteria use only some DOM components, mainly small nitrogen-containing compounds like free amino acids.

Aerobic anoxygenic phototrophic bacteria (AAP bacteria): Like nearly all other microbes in the oceans, these microbes require oxygen (they are “aerobic”), but unlike cyanobacteria, eukaryotic phytoplankton and plants, AAP bacteria do not produce oxygen during phototrophy. For this reason, these microbes are “anoxygenic” whereas the other phototrophic are “oxygenic” because they do produce oxygen. Again, unlike the other phototrophs, AAP bacteria do not appear to “fix” CO₂ and synthesize organic material; instead of being autotrophic, they are heterotrophic and must use other organic compounds (mainly DOM) as a carbon source. However, they appear to gain some energy from phototrophy in addition to the energy from the oxidation of organic material.

AAP bacteria had been found in isolated marine habitats, but were not thought to be important in oceans until they were found by Z. Kolber and colleagues in 2000.

Proteorhodopsin-bearing bacteria: Rhodopsins are light-sensitive proteins found in all three domains of life (archaea, bacteria, and eukaryotes). Some rhodopsins, such as those in the eyes of humans and other vertebrates, are light receptors and are found in photosensing cells. Another type of rhodopsin is a proton pump that converts light energy into chemical energy (ATP).

This form of rhodopsin was first found in halophilic archaea before these microbes were discovered to form their own domain, separate from bacteria. Consequently, this type of rhodopsin is called bacteriorhodopsin.

Similar to bacteriorhodopsin, proteorhodopsin is a light dependent proton pump first found by Béjà and colleagues in an uncultured type of bacteria (SAR86) belonging to the Proteobacteria. Consequently, it was called proteorhodopsin. However, after the initial studies of Béjà and colleagues, proteorhodopsin was found in other bacterial divisions, such as the Bacteroidetes.

So, proteorhodopsin-bearing bacteria may harvest some energy from light, but they are not autotrophic and must rely on organic material for carbon. This organic material (mainly DOM) also probably supplies most of the energy for these microbes.

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