How does rising atmospheric CO2 affect marine organisms?

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Effects of Atmospheric CO2 Enrichment on Oceanic Carbon and Nitrogen Fixation
Garcia, N.S., Fu, F.-X., Breene, C.L., Bernhardt, P.W., Mulholland, M.R., Sohm, J.A. and Hutchins, D.A. 2011. Interactive effects of irradiance and CO2 on CO2 fixation and N2 fixation in the diazotroph Trichodesmium erythraeum (Cyanobacteria). Journal of Phycology 47: 1292-1303.

The authors note that N2 fixation by marine diazotrophic cyanobacteria (such as various species of Trichodesmium) contributes substantial new nitrogen to marine environments, including the North Atlantic, Pacific, and Indian Oceans (Carpenter et al., 1993; Capone et al., 1997, 2005; Karl et al., 2002)," and they say that phosphorus and iron have been identified as key factors that control N2 fixation in those environments. More recently, however, they note that several other studies have suggested that the partial pressure of CO2 in the atmosphere "may be another possible limiting factor for N2 fixation and CO2 fixation by Trichodesmium," citing the work of Barcelos e Ramos et al. (2007), Hutchins et al. (2007), Levitan et al. (2007, 2010) and Kranz et al. (2009).

What was done
In a laboratory study designed to explore this latter possibility, Garcia et al. examined the effects of present-day (~380 ppm) and elevated (~750 ppm) atmospheric CO2 concentrations on CO2 and N2 fixation by T. erythraeum isolates from the Pacific and Atlantic Oceans under a range of irradiance conditions.

What was learned
The seven scientists determined that "the positive effect of elevated CO2 on gross N2 fixation was large (~50% increase) under mid and/or low irradiances compared with that at high light (~20% increase)," noting that data from Kranz et al. (2010) and Levitan et al. (2010) corroborate their findings. In fact, they report that in the Kranz et al. study, "under low light, gross N2-fixation rates were 200% higher in a high-CO2 treatment (900 ppm) compared with a low-CO2 treatment (150 ppm), whereas under high light, gross N2-fixation rates were only 112% higher under elevated CO2." In the case of CO2 fixation, on the other hand, they found that CO2-fixation rates increased significantly "in response to high CO2 under mid- and high irradiances only."

What it means
As the atmosphere's CO2 concentration continues to rise, this phenomenon should boost the growth rates of marine diazotrophic cyanobacteria and enable them to make more nitrogen available to themselves and co-occurring species, which should ultimately act to significantly increase both the quantity and quality of the worldwide phytoplanktonic food base that ultimately supports all marine animal life.

Barcelos e Ramos, J., Biswas, H., Schulz, K.G., LaRoche, J. and Riebesell, U. 2007. Effect of rising atmospheric carbon dioxide on the marine nitrogen fixer Trichodesmium. Global Biogeochemical Cycles 21: 10.1029/2006GB002898.

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.

Capone, D.G., Zehr, J.P., Paerl, W.H., Bergman, B. and Carpenter, E.J. 1997. Trichodesmium, a globally significant marine cyanobacterium. Science 276: 1221-1229.

Carpenter, E.J., O'Neil, J.M., Dawson, R., Capone, D.G., Siddiqui, P.J.A., Roenneberg, G.T. and Bergman, B. 1993. The tropical diazotrophic phytoplankter Trichodesmium: biological characteristics of two common species. Marine Ecology Progress Series 95: 295-304.

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.

Karl, D., Michaels, A., Bergman, B., Capone, D., Carpenter, E., Letelier, R., Lipschultz, F., Paerl, H., Sigman, D. and Stal, L. 2002. Dinitrogen fixation in the world's oceans. Biogeochemistry 57/58: 47-98.

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.

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.

Levitan, O., Kranz, S.A., Spungin, D., Prasil, O., Rost, B. and Berman-Frank, I. 2010. Combined effects of CO2 and light on the N2-fixing cyanobacterium Trichodesmium IMS101: a mechanistic view. Plant Physiology 154: 346-356.

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.

Reviewed 21 March 2012