Global Biogeography of Picoplankton Ecotype Activity

Over the past fifteen years significant progress has been made in characterizing patterns of microbial biodiversity across oceanographic gradients. However, we have lacked the tools necessary to link this biodiversity with biogeochemical cycles and ecosystem function. One novel technique that can address this gap derives estimates of replication rate from actively growing microbial ecotypes using metagenomic gene coverage. Here, we apply this technique in order to (1) characterize the global biogeography of ecotype-specific bacterial replication and (2) determine whether bacterial ecotypes differentially contribute to marine carbon cycling. A total of 450 metagenomic samples were analyzed across all ocean basins. The magnitude of ecotype-specific replication shows significant differences between subtropical gyres, equatorial upwelling regions, and areas with freshwater-induced stratification. Moreover, activity patterns appear to be largely driven by ecotype niche partitioning. For example, parallel replication rates were observed between similar Prochlorococcus and SAR11 ecotypes. We also show that quantifying diel cycling of replication rates was critical for placing patterns of activity within the ecological context of surface communities. Finally, there was a significant correspondence between increases in metagenomically derived replication and net diel carbon accumulation. Overall, this work represents a significant advance in our ability to associate shifts in picoplankton genomic diversity with ocean carbon cycling.