How does rising atmospheric CO2 affect marine organisms?

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Rising Atmospheric CO2 Will Likely Boost Ocean Productivity
Volume 10, Number 26: 27 June 2007

In a newly published study of major significance, Levitan et al. (2007) note that "among the principal players contributing to global aquatic primary production, the nitrogen (N)-fixing organisms (diazotrophs) are important providers of new N to the oligotrophic areas of the oceans," and they cite several studies which demonstrate that "cyanobacterial (phototrophic) diazotrophs in particular fuel primary production and phytoplankton blooms which sustain oceanic food-webs and major economies and impact global carbon (C) and N cycling." These facts thus compelled them to examine how the ongoing rise in the air's CO2 content might impact these relationships; and they began by exploring the response of the cyanobacterial diazotroph Trichodesmium to changes in the atmosphere's CO2 concentration, choosing this particular diazotroph because it dominates the world's tropical and sub-tropical oceans in this regard, contributing over 50% of total marine N fixation.

The eight Israeli and Czech researchers grew Trichodesmium IMS101 stock cultures in YBCII medium (Chen et al., 1996) at 25░C and a 12-hour:12-hour light/dark cycle (with the light portion of the cycle in the range of 80-100 Ámol photons m-2 s-1) in equilibrium with air of three different CO2 concentrations (250, 400 and 900 ppm, representing low, ambient and high concentrations, respectively), which was accomplished by continuously bubbling air of the three CO2 concentrations through the appropriate culture vessels throughout various experimental runs, each of which lasted a little over three weeks, during which time they periodically monitored a number of diazotrophic physiological processes and properties.

So what did the scientists learn? Levitan et al. report that Trichodesmium in the high CO2 treatment "displayed enhanced N fixation, longer trichomes, higher growth rates and biomass yields." In fact, they write that in the high CO2 treatment there was "a three- to four-fold increase in N fixation and a doubling of growth rates and biomass," and that the cultures in the low CO2 treatment reached a stationary growth phase after only five days, "while both ambient and high CO2 cultures exhibited exponential growth until day 15 before declining."

In discussing possible explanations for what they observed, the researchers suggest that "enhanced N fixation and growth in the high CO2 cultures occurs due to reallocation of energy and resources from carbon concentrating mechanisms required under low and ambient CO2." Consequently, they conclude, in their words, that "in oceanic regions, where light and nutrients such as P and Fe are not limiting, we expect the projected concentrations of CO2 to increase N fixation and growth of Trichodesmium," and that "other diazotrophs may be similarly affected, thereby enhancing inputs of new N and increasing primary productivity in the oceans." And to emphasize these points, they write in the concluding sentence of their paper that "Trichodesmium's dramatic response to elevated CO2 may consolidate its dominance in subtropical and tropical regions and its role in C and N cycling, fueling subsequent primary production, phytoplankton blooms, and sustaining oceanic food-webs."

Yes, it's no longer just the land area of the planet that is participating in the CO2-induced greening of the earth phenomenon; the world's oceans are involved as well.

Sherwood, Keith and Craig Idso

References
Chen, Y.B., Zehr, J.P. and Mellon, M. 1996. Growth and nitrogen fixation of the diazotrophic filamentous nonheterocystous cyanobacterium Trichodesmium sp IMS101 in defined media: evidence for a circadian rhythm. Journal of Phycology 32: 916-923.

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.