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Effects of Atmospheric CO2 Enrichment on N2-Fixing Oceanic Cyanobacteria
Volume 10, Number 50: 12 December 2007

In the introduction to their important new study published in the July 2007 issue of Limnology and Oceanography, Hutchins et al. (2007) note that Trichodesmium species and other diazotrophic cyanobacteria support a large fraction of the total biological productivity of earth's tropical and subtropical seas, and that they exert a significant influence on the planet's carbon cycle by supplying much of the nitrogen that enables marine phytoplankton to maintain a level of productivity that removes vast amounts of CO2 from the atmosphere. Hence, they speculated that if either an increase in the air's CO2 content or its temperature led to an increase in oceanic N2 fixation, it could also lead to the biological extraction of more CO2 from the atmosphere and a tempering of the CO2 greenhouse effect via this negative feedback process.

To explore this intriguing possibility, the eight researchers grew cultures of both Pacific and Atlantic Ocean isolates of Trichodesmium ecotypes across a range of atmospheric CO2 concentrations characteristic of earth's past (150 ppm), its current state (380 ppm) and possible future conditions (750, 1250 and 1500 ppm) at two temperatures (25 and 29░C) and at sufficient and limiting phosphorus concentrations (20 and 0.2 Ámol L-1 of phosphate, respectively), in situations where the carbonate buffer system parameters in their artificial seawater culture media were "virtually identical to those found in natural seawater across the relevant range of CO2 values."

As Hutchins et al. describe it, their work revealed that at atmospheric CO2 concentrations projected for the year 2100 (750 ppm), "N2 fixation rates of Pacific and Atlantic isolates increased 35-100%, and CO2 fixation rates increased 15-128% relative to present day CO2 conditions (380 ppm)." And in what they call one of their "most striking results," they found that "increased CO2 enhanced N2 and CO2 fixation and growth rates even under severely phosphorus-limited steady-state growth conditions [our italics]." They also report that "neither isolate could grow at 150 ppm CO2," but that "N2 and CO2 fixation rates, growth rates, and nitrogen:phosophorus ratios all increased significantly between 380 and 1500 ppm," and that, "in contrast, these parameters were affected only minimally or not at all by a 4░C temperature change."

In discussing the implications of their findings, Hutchins et al. note that current global estimates of N2 fixation by Trichodesmium are about 60 x 109 kg N yr-1, and that if their experimental results can be extrapolated to the world's oceans, by 2100 this amount could increase to 81-120 x 109 kg N yr-1. In addition, they say that "if these estimates are coupled with modeling predictions of a 27% warming-induced expansion of suitable habitat (Boyd and Doney, 2002), calculations suggest that global N2 fixation by Trichodesmium alone could range from 103-152 x 109 kg N yr-1 by the end of this century," which is to be compared to recent estimates for total pelagic N2 fixation of 100-200 x 109 kg N yr-1 (Galloway et al., 2004). What is more, they note that free-living unicellular cyanobacteria in the ocean are believed to fix at least as much nitrogen as Trichodesmium (Montoya et al., 2004), and that endosymbiotic cyanobacteria also contribute substantially to N2 fixation. Hence, they conclude that "if N2 fixation rates in these groups show commensurate increases with rising CO2, the cumulative effect on the global nitrogen cycle could be considerably larger (e.g., a doubling)." In addition, they say their results suggest that "like N2 fixation, CO2 fixation by Trichodesmium should also increase dramatically in the future because of CO2 enrichment."

In light of these several observations, Hutchins et al. state in their concluding sentence that "many of our current concepts describing the interactions between oceanic nitrogen fixation, atmospheric CO2, nutrient biogeochemistry, and global climate may need re-evaluation to take into account these previously unrecognized feedback mechanisms between atmospheric composition and ocean biology."

How right they are!

Sherwood, Keith and Craig Idso

References
Boyd, P.W. and Doney, S.C. 2002. Modelling regional responses by marine pelagic ecosystems to global climate change. Geophysical Research Letters 29: 10.1029/2001GL014130.

Galloway, J.N., Dentener, F.J., Capone, D.G., Boyer, E.W., Howarth, R.W., Seitzinger, S.P., Asner, G.P., Cleveland, C.C., Green, P.A., Holland, E.A., Karl, D.M., Michaels, A.F., Porter, J.H., Townsend, A.R. and V÷osmarty, C.J. 2004. Nitrogen cycles: past, present, and future. Biogeochemistry 70: 153-226.

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.

Montoya, J.P., Holl, C.M., Zehr, J.P., Hansen, A., Villareal, T.A. and Capone, D.G. 2004. High rates of N2 fixation by unicellular diazotrophs in the oligotrophic Pacific Ocean. Nature 430: 1027-1031.