How does rising atmospheric CO2 affect marine organisms?

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The Impact of Atmospheric CO2 Enrichment on a Major Oceanic N2-Fixing Cyanobacterium
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.

The authors write that "marine phytoplankton contribute up to 50% of global primary production (Falkowski et al., 1998) and influence earth's climate by altering various biogeochemical cycles (Schlesinger, 2005)." They also note, with respect to the latter subject and, more specifically, the marine nitrogen (N) cycle, that among diazotrophic cyanobacteria (dinitrogen-fixers), the species Trichodesmium "contributes about half of all marine N2 fixation (Mahaffey et al., 2005)," supporting "a large fraction of biological productivity in tropical and subtropical areas" and exerting, "over long timescales, a significant influence on global carbon cycles by providing a major source of reactive N to the water column (Falkowski and Raven, 2007)."

What was done
To see how the ongoing and projected increase in the air's CO2 concentration may impact one of the global ocean's most important diazotrophic cyanobacteria (Trichodesmium erythraeum IMS101), Kranz et al. grew the ubiquitous marine N2-fixer in semicontinuous batch cultures through which they bubbled air with CO2 concentrations of either 370 or 1000 ppm. After the cultures were acclimated to their respective CO2 concentrations for at least 14 days (more than 5 generations), they then measured rates of particulate organic carbon (POC) and particulate organic nitrogen (PON) fixation.

What was learned
The four researchers report there was "a strong increase in photosynthesis and N2 fixation under elevated CO2 levels," such that POC and PON production rates rose "by almost 40%."

What it means
In discussing the generality of their results, the German scientists note that - working with the same Trichodesmium species - "Barcelos e Ramos et al. (2007) and Levitan et al. (2007) observed stimulation in N2 fixation by approximately 40% and even up to 400%, while Hutchins et al. (2007) obtained stimulation by up to 35% over the respective CO2 range." And in discussing the significance of these similar findings, they state that "the observed increase in photosynthesis and N2 fixation could have potential [global] biogeochemical implications, as it may stimulate productivity in N-limited oligotrophic regions and thus provide a negative feedback on rising atmospheric CO2 levels," slowing the rate of CO2 rise and reducing the degree of CO2-induced global warming.

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.

Falkowski, P.G., Barber, R. and Smetacek, V. 1998. Biogeochemical controls and feedbacks on ocean primary production. Science 281: 200-206.

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.

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.

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

Schlesinger, W.H. 2005. Biogeochemistry. Elsevier, Amsterdan.

Reviewed 5 August 2009