Subtle bacterioplankton community responses to elevated CO2 and warming in the oligotrophic South Pacific gyre

Abstract

Bacterioplankton play a critical role in primary production, carbon cycling, and nutrient cycling in the oligotrophic ocean. To investigate the effect of elevated CO₂ and warming on the composition and function of bacterioplankton communities in oligotrophic waters, we performed two trace-metal clean deck board incubation experiments during the New Zealand GEOTRACES transect of the South Pacific gyre (SPG). High-throughput amplicon sequencing of the 16S rRNA gene revealed that bacterioplankton community composition was distinct between the fringe and ultra-oligotrophic centre of the SPG and changed consistently in response to elevated CO₂ at the ultra-oligotrophic centre but not at the mesotrophic fringe of the SPG. The combined effects of elevated CO₂ and warming resulted in a high degree of heterogeneity between replicate communities. Community-level protein synthesis rates (³ H-Leucine incorporation) and bacterioplankton abundance were not affected by elevated CO₂ alone or in combination with warming at the fringe or ultra-oligotrophic centre of the SPG. These data suggest bacterioplankton community responses to elevated CO₂ may be modulated by nutrient regimes in open ocean ecosystems and highlight the need for further investigation in expanding oligotrophic subtropical gyres.