Bacterioplankton metabolism of phytoplankton lysates across a cyclone-anticyclone eddy dipole impacts the cycling of semi-labile organic matter in the photic zone

Mesoscale eddies, prominent physical processes in the stratified ocean gyres, affect community composition and metabolic rates of both phytoplankton and heterotrophic bacterioplankton (free-living bacterial and archaeal) communities. We hypothesized that in situ differences in organic matter production would predispose bacterioplankton communities from cyclonic vs. anticyclonic eddies toward metabolic capabilities better suited to utilizing dissolved organic matter (DOM) from the phytoplankton groups commonly associated with each eddy polarity. To test this, we established dilution batch-culture bioassay incubations along a cyclone to anticyclone spatial transect in the North Pacific Subtropical Gyre. Unamended incubations, to assess spatial variability in ambient DOM bioavailability, and incubations amended with lysates of phytoplankton cultures were established and community growth and metabolic responses were assessed. Over timescales of days, lysate type was more influential than incubation origin: Prochlorococcus lysate was rapidly remineralized, while Emiliania huxleyi lysate was efficiently incorporated into biomass and developed a unique community of copiotrophic bacteria. Over timescales of 1 week to 1 month, eddy effects were indirectly apparent in their potential to promote metabolic processes related to DOM production and consumption. Surface lysate incubations showed priming of ambient DOM, that is, the remineralization of DOM, which was otherwise not bioavailable, in the presence of labile substrates. Some incubations originating from the deep chlorophyll maximum demonstrated signatures of chemoautotrophy fueled by nitrification, coincident with eddy-driven isopycnal uplift. We conclude that eddy polarity itself does not determine community-level bacterioplankton metabolic capabilities; however, mesoscale processes may indirectly affect slower, semi-labile organic matter processing in the oligotrophic ocean.