ZIJ presented on the diversity and activity of Prochlorococcus at the OCB2017 summer meeting at WHOI in Woods Hole, MA. The presentation, “Ribosomal RNA and DNA (23S) of Prochlorococcus reveal variable activity and abundance relationships across taxonomic ranks” was based on a study with lead author and former student Alyse Larkin, now at UCI. The work highlights how the fine taxonomic scale of Prochlorococcus has unexpected patterns in activity and suggests population ecological differentiation, regulation and biogeochemistry that are still poorly understood for this keystone microbe.
Abstract: Phytoplankton communities in oligotrophic temperate and tropical oceans are numerically dominated by Prochlorococcus sp., a genetically diverse and biogeochemically important marine cyanobacterium. The phylogenetic clades and subgroups of Prochlorococcus exhibit niche partitioning based on light, temperature and other resources in the ocean, but it is unknown how these clades/ecotypes differ in their in situ activity across large spatiotemporal environmental gradients. Here using relative 23S rRNA:rDNA ratios as a proxy for specific activity, we examine high light (HL) adapted Prochlorococcus across environmental gradients in the surface North Pacific Ocean to (1) determine the coupling between activity and abundance across taxonomic ranks and (2) examine the specific activity among closely related operational taxonomic units (OTUs). We show that activity and abundance are highly correlated for all 97% similarity OTUs of Prochlorococcus across all sites in the surface ocean. However, finer molecular scale resolution (oligotyping) shows significantly more variability in rRNA:rDNA ratios and reveals differing trends among closely related OTUs including different patterns between high and low abundance oligotypes. These results suggest that HL Prochlorococcus populations respond quickly to (a)biotic changes and the mechanisms that lead to uncoupling between activity and abundance (e.g. density dependent processes) are less important for this community at course molecule scales. These findings also suggest that relative Prochlorococcus population abundances at a given location can serve as a proxy for activity, providing an important tool for ecosystem model development. However, uncoupling at fine molecular scales suggests population differentiation and mechanisms of regulation that are still poorly understood, but important for understanding their relative role to biogeochemical cycles.
Alyse Larkin & Zackary Johnson OCB217