Protistan diversity and activity inferred from RNA and DNA at a coastal ocean site in the eastern North Pacific

Microbial eukaryotes fulfill key ecological positions in marine food webs. Molecular approaches that connect protistan diversity and biogeography to their diverse metabolisms will greatly improve our understanding of marine ecosystem function. The majority of molecular-based studies to date use 18S rRNA gene sequencing to characterize natural microbial assemblages, but this approach does not necessarily discriminate between active and non-active cells. We incorporated RNA sequencing into standard 18S rRNA gene sequence surveys with the purpose of assessing those members of the protistan community contributing to biogeochemical cycling (active organisms), using the ratio of cDNA (reverse transcribed from total RNA) to 18S rRNA gene sequences within major protistan taxonomic groups. Trophically important phytoplankton, such as diatoms and chlorophytes exhibited seasonal trends in relative activity. Additionally, both radiolaria and ciliates displayed previously unreported high relative activities below the euphotic zone. This study sheds new light on the relative metabolic activity of specific protistan groups and how microbial communities respond to changing environmental conditions.