N-170. Marine Synechococcus ecotypes Are Specifically Associated with Distinct Upwelling and Oligotrophic Regimes

N. Ahlgren, G. Rocap;
Univ. of Washington, Seattle, WA.

Marine cyanobacteria provide a model system for exploring how multiple, closely-related microbial populations coexist and partition environmental niche-space. The super-genus marine Synechococcus is comprised of multiple ecotypes--genetically distinct populations that coexist throughout the world’s oceans. Previous work demonstrates that some of these ecotypes posses distinguishing physiologies and occupy distinct niches, however, for many of them, it is unclear what specific habitats they occupy. We developed and applied high-throughput real-time PCR assays for nine Synechococcus ecotypes to map their distribution in the field and thus better elucidate what environmental factors define each ecotype’s niche. Ecotypes were mapped along a transect in the Pacific cutting across oligotrophic waters and two upwelling regions: the Costa Rica upwelling Dome (CRD) and the Equatorial upwelling zone. Synechococcus dominated picophytoplankton populations in the CRD at concentrations of up to 1*10^6 cells/ml while neighboring oligotrophic waters are instead dominated by the sister genus, Prochlorococcus. It is hypothesized that the distinctive combination of trace metals in the CRD produces this unusual community structure. We find the composition of ecotypes varies greatly across the transect and that ecotypes are specifically associated with oligotrophic waters or one of the upwelling regimes. We also find evidence of depth-partitioning for several ecotypes, similar to trends seen for ecotypes of Prochlorococcus. These results demonstrate that closely-related microbial populations occupy distinct habitats that are defined by environmental gradients in the oceans. Furthermore, the ubiquitous distribution of Synechococcus throughout the world’s oceans is due to the diversity of ecotypes within the genus that occupy disparate habitats, namely oligotrophic and upwelling regimes.