Effect of ocean acidification on cyanobacteria in the subtropical North Atlantic

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 N ₂-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.