Effects of Atmospheric CO₂ Enrichment on Oceanic Carbon and Nitrogen Fixation

Garcia et al. (2011) 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).

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.

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."

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.

Additional References
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.