Mixotrophic metabolism by natural communities of unicellular cyanobacteria in the western tropical South Pacific Ocean

International audienceCyanobacteria are major contributors to ocean biogeochemical cycling. However, mixotrophic metabolism and the relative importance of inorganic and organic carbon assimilation within the most abundant cyanobacteria are still poorly understood. We explore the ability of Prochlorococcus and Synechococcus to assimilate organic molecules with variable C:N:P composition and its modulation by light availability and photosynthetic impairment. We used a combination of radiolabeled molecules incubations with flow cytometry cell sorting to separate picoplankton groups from the western tropical south Pacific Ocean. Prochlorococcus and Synechococcus assimilated glucose, leucine, and ATP at all stations, but cell-specific assimilation rates of N and P containing molecules were significantly higher than glucose. Incubations in the dark or with an inhibitor of photosystem II resulted in reduced assimilation rates. Light-enhanced cell-specific glucose uptake was generally higher for cyanobacteria (~50%) than for the low nucleic acid fraction of bacterioplankton (LNA, ~35%). Our results confirm previous findings, based mainly on cultures and genomic potentials, showing that Prochlorococcus and Synechococcus have a flexible mixotrophic metabolism, but demonstrate that natural populations remain primarily photoautotrophs. Our findings indicate that mixotrophy by marine cyanobacteria is more likely to be an adaptation to low inorganic nutrient availability rather than a facultative pathway for carbon acquisition

Marine cyanophages and light Marine cyanophages and light

In contrast to the phages of heterotrophic hosts, light can play a key role in all aspects of the life cycle of phages infecting ecologically important marine unicellular cyanobacteria of the genera Synechococcus and Prochlorococcus. Phage adsorption, replication, modulation of the host cell metabolism, and survival in the environment following lysis, all exhibit light-dependent components. The analysis of cyanophage genomes has revealed the acquisition of key photosynthetic genes during the course of evolution, such as those encoding central components of the light harvesting apparatus. These discoveries are beginning to reveal novel features of the interactions between parasite and host that shape the biology of both