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No stimulation of nitrogen fixation by non‐filamentous diazotrophs under elevated CO 2 in the South Pacific
Author(s) -
Law Cliff S.,
Breitbarth Eike,
Hoffmann Linn J.,
M Christina M.,
Langlois Rebecca J.,
L Julie,
Marriner Andrew,
Safi Karl A.
Publication year - 2012
Publication title -
global change biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 4.146
H-Index - 255
eISSN - 1365-2486
pISSN - 1354-1013
DOI - 10.1111/j.1365-2486.2012.02777.x
Subject(s) - diazotroph , nitrogen fixation , nitrogen , stimulation , environmental science , biology , chemistry , neuroscience , organic chemistry
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.