AME 66:211-222 (2012) - doi:10.3354/ame01576
FEATURE ARTICLEEffect of ocean acidification on cyanobacteria in the subtropical North Atlantic
M. W. Lomas1,*, B. M. Hopkinson2,3,**, J. L. Losh2,** D. E. Ryan2,**, D. L. Shi2,4,**, Y. Xu2,**, F. M. M. Morel2
**These authors contributed equally to the design and execution of the experiments and are listed in alphabetical order
ABSTRACT: Cyanobacteria make significant contributions to global carbon and nitrogen cycling, particularly in the oligotrophic subtropical and tropical gyres. The present study examined short-term (days) physiological and acclimation responses of natural cyanobacterial populations to changes in pH/pCO2 spanning the last glacial minimum, ~8.4/~150 ppm, to projected year 2100 values of ~7.8/~800 ppm. Fe- and P-replete colonies of Trichodesmium increased N2-fixation rates (nmol N colony−1 h−1) at pH 7.8 by 54% (range 6 to 156%) over ambient pH/pCO2 conditions, while N2-fixation at pH/pCO2 8.4 was 21% (range 6 to 65%) lower than at ambient pH/pCO2; a similar pattern was observed when the rates were normalized to colony C. C-fixation rates were on average 13% (range −72 to 112%) greater at low pH than at ambient pH and 37% (−53 to 23%) greater than at high pH. Whole community assemblages dominated by Prochlorococcus and Synechococcus (47 to 95% of autotrophic biomass), whether nutrient-replete or P-limited, did not show a clear response of C-fixation rates to changes in pH/pCO2. Comparison of initial and final C-fixation responses across pH/pCO2 treatments suggests rapid acclimation of cellular physiology to new pH/pCO2 conditions. Changes in cell size and pigment content for Prochlorococcus and Synechococcus were minor and did not vary in a consistent manner with changes in pH/pCO2. These results for natural populations of all 3 cyanobacteria concur with previous research and suggest that one important response to changes in ocean pH and pCO2 might be an increase in N2 and C fixation by Trichodesmium under nutrient-replete conditions. The response of single-cell cyanobacteria to changes in pH/pCO2 will likely be indirect and controlled by the response to other variables, such as nutrients.
KEY WORDS: Cyanobacteria · North Atlantic · Sargasso Sea · Acidification · Nitrogen fixation · Photosynthesis
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