No stimulation of nitrogen fixation by non‐filamentous diazotrophs under elevated CO 2 in the South Pacific

Abstract

Nitrogen fixation by diazotrophic cyanobacteria is a critical source of new nitrogen to the oligotrophic surface ocean. Research to date indicates that some diazotroph groups may increase nitrogen fixation under elevated pCO 2 . To test this in natural plankton communities, four manipulation experiments were carried out during two voyages in the South Pacific (30–35 o S). High CO 2 treatments, produced using 750 ppmv CO 2 to adjust pH to 0.2 below ambient, and ‘Greenhouse’ treatments (0.2 below ambient pH and ambient temperature +3 °C), were compared with Controls in trace metal clean deckboard incubations in triplicate. No significant change was observed in nitrogen fixation in either the High CO 2 or Greenhouse treatments over 5 day incubations. qPCR measurements and optical microscopy determined that the diazotroph community was dominated by Group A unicellular cyanobacteria (UCYN‐A), which may account for the difference in response of nitrogen fixation under elevated CO 2 to that reported previously for Trichodesmium . This may reflect physiological differences, in that the greater cell surface area:volume of UCYN‐A and its lack of metabolic pathways involved in carbon fixation may confer no benefit under elevated CO 2 . However, multiple environmental controls may also be a factor, with the low dissolved iron concentrations in oligotrophic surface waters limiting the response to elevated CO 2 . If nitrogen fixation by UCYN‐A is not stimulated by elevated pCO 2 , then future increases in CO 2 and warming may alter the regional distribution and dominance of different diazotroph groups, with implications for dissolved iron availability and new nitrogen supply in oligotrophic regions.

Bibliographic Information