Experimental assessment of diazotroph responses to elevated seawater <i>p</i>CO₂ in the North Pacific Subtropical Gyre

Abstract

Abstract We examined short‐term (24–72 h) responses of naturally occurring marine N 2 fixing microorganisms (termed diazotrophs) to abrupt increases in the partial pressure of carbon dioxide ( p CO 2 ) in seawater during nine incubation experiments conducted between May 2010 and September 2012 at Station ALOHA (A Long‐term Oligotrophic Habitat Assessment) (22°45′N, 158°W) in the North Pacific Subtropical Gyre (NPSG). Rates of N 2 fixation, nitrogenase ( nifH ) gene abundances and transcripts of six major groups of cyanobacterial diazotrophs (including both unicellular and filamentous phylotypes), and rates of primary productivity (as measured by 14 C‐bicarbonate assimilation into plankton biomass) were determined under contemporary (~390 ppm) and elevated p CO 2 conditions (~1100 ppm). Quantitative polymerase chain reaction (QPCR) amplification of planktonic nifH genes revealed that unicellular cyanobacteria phylotypes dominated gene abundances during these experiments. In the majority of experiments (seven out of nine), elevated p CO 2 did not significantly influence rates of dinitrogen (N 2 ) fixation or primary productivity (two‐way analysis of variance (ANOVA), P &gt; 0.05). During two experiments, rates of N 2 fixation and primary productivity were significantly lower (by 79 to 82% and 52 to 72%, respectively) in the elevated p CO 2 treatments relative to the ambient controls (two‐way ANOVA, P &lt; 0.05). QPCR amplification of nifH genes and gene transcripts revealed that diazotroph abundances and nifH gene expression were largely unchanged by the perturbation of the seawater p CO 2 . Our results suggest that naturally occurring N 2 fixing plankton assemblages in the NPSG are relatively resilient to large, short‐term increases in p CO 2 .