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I am not a biologist, but I know there exist three variants of photosynthesis, namely C3, C4 and CAM.

I would like to know what type is used by the ocean's phytoplankton? It might also be that different photosynthetic mechanisms are used by the bacteria vs. algae. I have searched the web, but the information I find is too broad.

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I'm not sure that all phytoplankton perform the same type of photosynthesis.

Originally, people thought that they all performed C4, on the basis of genome sequencing, which revealed the presence of genes important for C4 photosynthesis. However, experiments on individual species seemed to indicate that the phytoplankton were performing multiple types of photosynthesis (source).

EDIT: CAM is unlikely, as that is an adaptation for dry environments.

EDIT 2: and of course, some phytoplankton appear to have both C3 and C4 pathways. In short, you can't generalize.

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Thank you for the papers, I appreciate. –  CHM Apr 18 '13 at 4:50

Some wires crossed here. Most algae are C3, in other words they use the Calvin cycle to fix CO₂. Another way of saying it is that RUBISCO (Ribulose bisphosphate carboxylase) is dorectly responsible for CO₂ fixation. There is some evidence (disputed) that some diatoms use a form of the C4 pathway in which CO₂ is initially fixed as a 4-carbon compound (CO₂+C3 → C4). The C4 compound formed is often malic acid. Trying to establish if the C4 pathway is functional for carbon fixation is not easy. Genomics can be misleading. Practically all photosynthetic organisms have the genes required for the C4 pathway but whether they are expressed is another matter (look at KEGG). C4 photosynthesis usually requires complex anatomy because it is essentially a CO₂ supercharger mechanism because the C4 carbon form is then decarboxylated in a special location to provide RUBISCO with a very high CO₂ environment to finally fix CO₂ a 2nd time and pass the two C3 carbon compounds formed from CO₂ plus Ribulose Bisphosphate into the Calvin cycle.

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