The relationship between two Synechococcus strains and heterotrophic bacterial communities and its associated carbon flow

Abstract

Synechococcus accounts for a considerable proportion of oceanic carbon fixation and has close relation with heterotrophic bacteria at sub-micrometre scales. However, the relationship between Synechococcus and heterotrophic bacteria and their impacts on oceanic carbon flow is not well understood. Here, we separately co-cultured the heterotrophic bacterial community with two different axenic Synechococcus strains, i.e. Synechococcus sp. PCC7002 and Synechococcus sp. CCMP1334. The dissolved organic carbon (DOC) released by Synechococcus promoted the rapid growth of heterotrophic bacteria. Due in part to the slight differences in the Synechococcus-derived DOC composition, the starting heterotrophic bacterial community was reshaped by the two Synechococcus strains into significantly divergent community structures, suggesting that different heterotrophic bacterial communities may gather around different Synechococcus strains and interact with each other in the oceans. Interestingly, the two Synechococcus strains also released some very stable humic-like DOC components that resisted use by heterotrophic bacteria and were gradually accumulated in seawater, implying that in addition to photosynthetic carbon fixation, Synechococcus may also play an important role in long-term ocean carbon sequestration by producing relatively recalcitrant DOC. Moreover, the interactions of Synechococcus and the heterotrophic bacterial communities could increase aggregate formation and particle sinking, which can likely enhance the contribution of Synechococcus to the magnitude of the sinking biological carbon pump in the oceans.