Volume 15, Number 8: 22 February 2012
In an enlightening review paper published in Photosynthesis Research, Kranz et al. (2011) note that "marine phytoplankton are responsible for almost half of all photosynthetic carbon fixation on earth and play a vital role in altering the CO2 exchange between ocean and atmosphere," citing Maier-Reimer et al. (1996) and Gruber (2004). However, they note that lack of nitrates often limits phytoplanktonic growth, and that diazotrophic cyanobacteria that fix nitrogen, such as Trichodesmium species, thus play "a crucial role in many marine ecosystems by providing a new source of biologically available nitrogen." And they go on to describe how atmospheric CO2 enrichment helps to enhance both halves of this important two-pronged phenomenon.
First of all, the three researchers - all from the Alfred Wegener Institute for Polar and Marine Research located in Bremerhaven, Germany - report that "four recent studies tested the effect of different CO2 concentrations on the growth, biomass production and elemental composition of Trichodesmium (Barcelos e Ramos et al., 2007; Hutchins et al., 2007; Kranz et al., 2009; Levitan et al., 2007)," and they say that these studies "concordantly demonstrated higher growth and/or production rates under elevated pCO2, with a magnitude exceeding those CO2 effects previously seen in other marine phytoplankton."
Focusing next on particulate organic nitrogen (PON) production, they note that Trichodesmium species are particularly effective in this regard; and they write that "the stimulation in N2 fixation and/or PON production between present-day pCO2 values (370-400 ppm) and those predicted for the year 2100 (750-1000 ppm) ranged between 35 and 240%." And last of all, they state that "data on CO2 dependency of N2 fixation rates from recent publications suggest that N2 fixation by Trichodesmium species might increase by more than 20 Tg N per year to about 100 Tg N per year until the end of this century," citing Hutchins et al. (2009).
In light of these several findings, it would appear that earth's oceans are primed to do their part in terms of preserving and protecting the biosphere, as they (1) ramp up their productivity to sustain a greater population of aquatic organisms that may be tapped to supply additional food for the planet's burgeoning human population, and as they (2) remove from the atmosphere and sequester in their sediments ever more carbon, as anthropogenic CO2 emissions continue to rise.
Sherwood, Keith and Craig Idso
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.
Gruber, N. 2004. The dynamics of the marine nitrogen cycle and its influence on atmospheric CO2. In: Follows, M. and Oguz, T. (Eds). The Ocean Carbon Cycle and Climate. Kluwer, Dordrecht, Germany, pp. 97-148.
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 552: 1293-1304.
Hutchins, D.A., Mulholland, M.R. and Fu, F. 2009. Nutrient cycles and marine microbes in a CO2-enriched ocean. Oceanography 22: 128-145.
Kranz, S.A., Eichner, M. and Rost, B. 2011. Interactions between CCM and N2 fixation in Trichodesmium. Photosynthesis Research 109: 73-84.
Kranz, S.A., Sultemeyer, D., Richter, K.-U. and Rost, B. 2009. Carbon acquisition in Trichodesmium: the effect of pCO2 and diurnal changes. Limnology and Oceanography 54: 548-559.
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.
Maier-Reimer, E., Mikolajewicz, U. and Winguth, A. 1996. Future ocean uptake of CO2: interaction between ocean circulation and biology. Climate Dynamics 12: 711-722.