No stimulation of nitrogen fixation by non-filamentous diazotrophs under elevated CO2 in the South Pacific.

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 pCO2 . To test this in natural plankton communities, four manipulation experiments were carried out during two voyages in the South Pacific (30-35o S). High CO2 treatments, produced using 750 ppmv CO2 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 CO2 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 CO2 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 CO2 . However, multiple environmental controls may also be a factor, with the low dissolved iron concentrations in oligotrophic surface waters limiting the response to elevated CO2 . If nitrogen fixation by UCYN-A is not stimulated by elevated pCO2 , then future increases in CO2 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.

In situ response of Antarctic under-ice primary producers to experimentally altered pH.

Elevated atmospheric CO2 concentrations are contributing to ocean acidification (reduced seawater pH and carbonate concentrations), with potentially major ramifications for marine ecosystems and their functioning. Using a novel in situ experiment we examined impacts of reduced seawater pH on Antarctic sea ice-associated microalgal communities, key primary producers and contributors to food webs. pH levels projected for the following decades-to-end of century (7.86, 7.75, 7.61), and ambient levels (7.99), were maintained for 15 d in under-ice incubation chambers. Light, temperature and dissolved oxygen within the chambers were logged to track diurnal variation, with pH, O2, salinity and nutrients assessed daily. Uptake of CO2 occurred in all treatments, with pH levels significantly elevated in the two extreme treatments. At the lowest pH, despite the utilisation of CO2 by the productive microalgae, pH did not return to ambient levels and carbonate saturation states remained low; a potential concern for organisms utilising this under-ice habitat. However, microalgal community biomass and composition were not significantly affected and only modest productivity increases were noted, suggesting subtle or slightly positive effects on under-ice algae. This in situ information enables assessment of the influence of future ocean acidification on under-ice community characteristics in a key coastal Antarctic habitat.