Effect of ocean acidification on cyanobacteria in the subtropical North Atlantic

Abstract

Cyanobacteria make significant contribu- tions to global carbon and nitrogen cycling, particu- larly in the oligotrophic subtropical and tropical gyres. The present study examined short-term (days) physio- logical and acclimation responses of natural cyanobac- terial 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 commu- nity 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 con- ditions. The response of single-cell cyanobacteria to changes in pH/pCO2 will likely be indirect and con- trolled by the response to other variables, such as nutrients.