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The Diverse Impacts of Ocean Acidification on Marine N2 Fixers

Paper Reviewed
Eichner, M., Rost, B. and Kranz, S.A. 2014. Diversity of ocean acidification effects on marine N2 fixers. Journal of Experimental Marine Biology and Ecology 457: 199-207.

Owing to their unique ability to fix atmospheric nitrogen, Eichner et al. (2014) write that ocean-dwelling "diazotrophic cyanobacteria play a crucial role in the biogeochemical cycles of nitrogen as well as carbon," in light of nitrogen (N) "being the limiting nutrient in vast regions of ocean," as discussed by Moore et al. (2013), where they say that "N2 fixers fuel primary productivity by providing a source of new N to the phytoplankton community." And they indicate, in this regard, that laboratory studies of the abundant diazotroph Trichodesmium sp. have suggested "a strong CO2 sensitivity for this species, with an increase in N2 fixation or particulate organic nitrogen (PON) production ranging between 35 and 140% for pCO2 levels expected by the end of this century (e.g. Hutchins et al., 2007; Kranz et al., 2011; Levitan et al., 2007)."

In an effort to "improve our knowledge of the CO2 responses of other diazotrophs," in the words of Eichner et al., they acclimated the single-celled Cyanothece sp. and two heterocystous species, Nodularia spumigena and the symbiotic Calothrix rhizosoleniae, to two different pCO2 levels - 380 and 980 µatm - after which they measured their growth rates, cellular composition, and carbon and N2 fixation rates, which they then compared to what had been found in prior similar studies with still other related species.

The three researchers found that "the three species of functionally different N2 fixers investigated in this study responded differently to elevated pCO2, showing enhanced, decreased as well as unaltered growth and production rates." However, it is important to note that Barcelos e Ramos et al. (2007) concluded that Trichodesmium - with its large positive response to atmospheric CO2 enrichment - is "the single most important nitrogen fixer in today's ocean."

In addition to Trichodesmium, Eichner et al. indicate that a number of single-celled and symbiotic open-ocean species have also been found "to respond positively to high pCO2," as in their study, thus confirming prior "predictions of an increase in N2 fixation with ocean acidification on a global scale."

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.

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., Eichner, M. and Rost, B. 2011. Interactions between CCM and N2 fixation in Trichodesmium. Photosynthesis Research 109: 73-84.

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.

Moore, C., Mills, M., Arrigo, K., Berman-Frank, I., Bopp, L., Boyd, P., Galbraith, E., Geider, R., Guieu, C. and Jaccard, S. 2013. Processes and patterns of oceanic nutrient limitation. Nature Geoscience 6: 701-710.

Posted 15 October 2014