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

Law, Cliff S., Breitbarth, Eike, Hoffmann, Linn J., McGraw, Christina M., Langlois, Rebecca, LaRoche, Julie, Marriner, Andrew and Safi, Karl A. (2012) No stimulation of nitrogen fixation by non-filamentous diazotrophs under elevated CO2 in the South Pacific. Global Change Biology, 18 (10). pp. 3004-3014. DOI 10.1111/j.1365-2486.2012.02777.x.

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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 pCO2. To test this in natural plankton communities, four manipulation experiments were carried out during two voyages in the South Pacific (30–35oS). 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.

Document Type: Article
Keywords: carbon dioxide; dissolved iron; nitrogen fixation; ocean acidification; pH ; South Pacific; temperature; Trichodesmium; UCYN-A; CYANOBACTERIUM TRICHODESMIUM IMS101; MARINE-PHYTOPLANKTON GROWTH; N-2 FIXATION; OCEAN ACIDIFICATION; UNICELLULAR DIAZOTROPHS; ELEMENTAL RATIOS; CELL-SIZE; RATES; LIMITATION; PHOTOSYNTHESIS
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-BI Biological Oceanography
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-CH Chemical Oceanography
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1111/j.1365-2486.2012.02777.x
ISSN: 1354-1013
Date Deposited: 08 Oct 2012 12:21
Last Modified: 23 Sep 2019 20:37
URI: http://oceanrep.geomar.de/id/eprint/16407

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