Background
According to the authors, "Trichodesmium, a colony-forming cyanobacerium, fixes nitrogen in an area corresponding to almost half of Earth's surface (Davis and McGillicuddy, 2006) and is estimated to account for more than half of the new production in parts of the oligotrophic tropical and subtropoical oceans (Capone et al., 2005; Mahaffey et al., 2005)," making it "the single most important nitrogen fixer in today's ocean."

What was done
Semi-continuous batch cultures of the N2-fixing cyanobacterium were maintained for approximately two months in the exponential growth phase (by means of repeated sampling and dilution) throughout a range of conditions corresponding to atmospheric CO2 concentrations stretching from 140 to 850 ppm, while a number of the organism's physical, chemical and physiological characteristics were repeatedly measured.

What was learned
In the words of the five German researchers who conducted the work, "over the experimental CO2 range (140 to 850 ppm), cell division rate of Trichodesmium increased about twofold," while "nitrogen fixation rate normalized to cellular phosphorus quota and Chlorophyll a content increased threefold," which "corresponds to a 50% increase in P-normalized N2 fixation for atmospheric CO2 increasing from its present value (380 ppm) to that projected for 2100 (750 ppm) assuming a business as usual CO2 emission scenario."

What it means
Barcelos e Ramos et al. conclude their paper by saying their work shows that "not only Trichodesmium responds to rising CO2, but as one of the oldest life forms on planet Earth, it is more sensitive than other groups previously considered (e.g., coccolithophores and diatoms)," and that "if the observed effect on Trichodesmium is a general phenomenon in diazotrophic cyanobacteria, our results would predict an increase in global ocean N2 fixation at CO2 levels expected for the future ocean," and that "this in turn, would increase the nitrogen inventory, resulting in increased future primary productivity and oceanic carbon sequestration," which "could thereby provide a strong negative feedback to atmospheric CO2 increase."

References
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.

Davis, C.S. and McGillicuddy Jr., D.J. 2006. Transatlantic abundance of the N2-fixing colonial cyanobacterium Trichodesmium. Science 312: 1517-1520.

Mahaffey, C., Michaels, A.F. and Capone, D.G. 2005. The conundrum of marine N2 fixation. American Journal of Science 305: 546-595.