Background
The authors begin by noting that "marine cyanobacteria, both unicellular Prochlorococcus and Synechococcus and colonial Trichodesmium spp., play important roles in the ocean carbon cycle and the biological carbon pump, particularly in the subtropical and tropical gyres (e.g. Partensky et al., 1999; Capone et al., 2005)," while adding that Trichodesmium "is thought to account for about half of the total N2-fixation in the oceans," citing Barcelos e Ramos et al. (2007), Hutchins et al. (2007), Levitan et al. (2007) and Kranz et al. (2009, 2010).

What was done
Working on board the RV Atlantic Explorer between July 2009 and April 2010 at the Bermuda Atlantic Time-series Study (BATS) site in the subtropical North Atlantic Ocean about 86 km southeast of Bermuda, Lomas et al. examined the C-fixation responses of natural assemblages of cyanobacteria dominated by Synechococcus and Prochlorococcus and the N2- and C-fixation responses of isolated Trichodesmium colonies to changes in pH/pCO2 conditions between the time of the last glacial minimum (8.4/150 ppm) and projected year 2100 values (7.8/800 ppm).

What was learned
The seven scientists report that "whole community assemblages dominated by Prochlorococcus and Synechococcus, whether nutrient-replete or P-limited, did not show a clear response of C-fixation rates to changes in pH/pCO2." However, they found that "Fe- and P-replete colonies of Trichodesmium increased N2-fixation rates at pH 7.8 by 54% over ambient pH/pCO2 conditions, while N2-fixation at pH 8.4 was 21% lower than at ambient pH/pCO2." Likewise, they say that C-fixation rates of Trichodesmium "were on average 13% greater at low pH than at ambient pH and 37% greater than at high pH." And they make a point of noting that "these results for natural populations of all three 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."

What it means
Quoting the team of researchers that conducted the study, their results for Trichodesmium, along with the similar results of several other marine scientists, suggest that "ocean acidification would likely result in a positive feedback on the growth and physiology of natural populations, resulting in a positive change in their role in ocean carbon and nitrogen cycles," which is, of course, great news for the biosphere!

References
Barcelos e Ramos, J., Biswas, H., Schulz, K.G., La Roche, J. and Riebesell, U. 2007. Effect of rising atmospheric carbon dioxide on the marine nitrogen fixer Trichodesmium. Global Biogeochemical Cycles 21: 10.l029/2006GB002898.

Capone, D.G., Burns, J.A., Montoya, J.P., Subramaniam, A., Mahaffey, C., Gunderson, T., Michaels, A.F. and Carpenter, E.J. 2005. Nitrogen fixation by Trichodesmium spp.: An important source of new nitrogen to the tropical and subtropical North Atlantic Ocean. Global Biogeochemical Cycles 19: 10.1029/2004GB002331.

Hutchins, D.A., Fu, F.-X., Zhang, Y., Warner, M.E., Feng, Y., Portune, K., Bernhardt, P.W. and Mulholland, M.R. 2007. CO2 control of Trichodesmium N2 fixation, photosynthesis, growth rates and elemental ratios: Implications for past, present and future ocean biogeochemistry. Limnology and Oceanography 52: 1293-1304.

Kranz, S.A., Sultemeyer, D., Richter, K.-U. and Rost, B. 2009. Carbon acquisition by Trichodesmium: The effect of pCO2 and diurnal changes. Limnology and Oceanography 54: 548-559.

Kranz, S.A., Levitan, O., Richter, K.U., Prasil, O., Berman-Frank, I. and Rost, B. 2010. Combined effects of CO2 and light on the N2-fixing cyanobacterium Trichodesmium IMS101: physiological responses. Plant Physiology 154: 334-345.

Levitan, O., Rosenberg, G., Setlik, I., Setlikova, E., Grigel, J., Klepetar, J., Prasil, O. and Berman-Frank, I. 2007. Elevated CO2 enhances nitrogen fixation and growth in the marine cyanobacterium Trichodesmium. Global Change Biology 13: 531-538.

Partensky, F., Hess, W.R. and Vaulot, D. 1999. Prochlorococcus, a marine photosynthetic prokaryote of global significance. Microbiology and Molecular Biology Reviews 63: 106-127.