Microdiverse high light Prochlorococcus exhibit more variable activity and abundance than higher taxonomic ranks. Prochlorococcus sp. [Taxonomy ID: 1220]

Phytoplankton communities in oligotrophic temperate and tropical oceans are numerically dominated by Prochlorococcus sp., a genetically diverse and biogeochemically important marine cyanobacterium. Previously identified phylogenetic clades of Prochlorococcus exhibit niche partitioning, but it is unknown how these clades/ecotypes differ in their in situ activity across large spatiotemporal environmental gradients. Using 23S rRNA:rDNA ratios as a proxy for specific activity, we examined high light 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 differences in the specific activity among closely related operational taxonomic units (OTUs). We sequenced (Illumina, MiSeq) 23S rRNA and rDNA amplicons from Prochlorococcus communities using the general forward primer 23S-129F and Prochlorococcus-optimized reverse primer 23S-457R-Pro. Our results showed that at the 97% OTU level, relative Prochlorococcus population abundances can serve as a proxy for activity, providing an important tool for ecosystem model development. However, uncoupling at fine molecular scales suggested that population differentiation and associated regulation of microbial activity at high genomic resolution are important but poorly understood.