Phototrophic Microorganisms: The Basis of the Marine Food Web

Although numerous marine microorganisms can exploit solar energy for photosynthesis or photoheterotrophy, cyanobacteria and microalgae are the only ones able to perform oxygenic photosynthesis and to produce organic carbon, an essential brick of life that sustains the whole marine trophic web. Here we review recent advances in the investigation of marine oxygenic microorganisms, with a special focus on cyanobacteria. We discuss novel insights into the ecology, evolution and diversity of Synechococcus and Prochlorococcus, the two most abundant and certainly the best known oxyphototrophs at all scales of organization from the gene to the global ocean. A particular emphasis is also made on diazotrophic cyanobacteria, which constitute an important source of bioavailable nitrogen to oceanic surface waters, possibly the most important external nitrogen source, before atmospheric and riverine inputs. Diazotrophic cyanobacteria are polyphyletic and display a remarkably large range of physiologies and morphologies. These include both multicellular cyanobacteria, such as the colonial Trichodesmium or the heterocyst-forming Calothrix, Richelia and Nodularia, and unicellular cyanobacteria belonging to three major groups: the symbiotic Candidatus Atelocyanobacterium thalassa (UCYN-A), the free-living Crocosphaera sp. (UCYN-B) and the UCYN-C cluster that notably encompasses Cyanothece. Whereas some of these species can form immense blooms (Nodularia, Trichodesmium), others can also have a major ecological impact even though they represent only a minor fraction of the bacterioplankton (UCYN-C). After about one billion years of evolution, which led them to colonize any single marine niche reached by solar light, cyanobacteria appear as truly fascinating organisms that constitute a major component of the marine microbial communities and are the matter of an ebullient research area. The considerable amount of omics information recently becoming available on both isolates and natural populations of marine oxyphototrophs provide a solid basis for investigating their molecular ecology, their contribution to biogeochemical cycles, as well as their possible utilization in biotechnology, data mining, or biomimetics.