Protistan community profiles in picocyanobacteria-enriched shipboard cultures from the oligotrophic North Pacific

Abstract

Protistan consumers are thought to mediate the transfer of picocyanobacterial production in oligotrophic oceans, but the protistan lineages associated with different picocyanobacterial prey in mixed natural assemblages remain poorly resolved. We established shipboard enrichment cultures from oligotrophic North Pacific seawater collected at 50, 80, and 105 m, using the picocyanobacteria Prochlorococcus and Synechococcus, and the heterotrophic bacterium Roseobacter, as prey sources. After 10 days of incubation, we examined protistan community profiles by 18S rRNA amplicon sequencing and compared them with those of the original seawater. Stramenopiles represented only a minor fraction of eukaryotic reads in the original seawater but dominated the amplicon profiles of picocyanobacteria-enriched cultures, whereas Roseobacter-enriched cultures showed weaker and less consistent profiles. At 50 and 80 m, Prochlorococcus-enriched cultures were dominated by Synurales, whereas Synechococcus-enriched cultures were dominated by Ochromonadales at 50 m and Cafeteria spp. at 80 m. At 105 m, both picocyanobacteria-enriched cultures converged on Cafeteria-dominated profiles. Together, these patterns indicate that picocyanobacteria-enriched shipboard cultures can reveal distinct protistan assemblages associated with prey source and source-water community. They also highlight Synurales, Ochromonadales, and Cafeteria as candidate responder lineages for future direct tests of protist–picocyanobacteria interactions.