Ocean Acidification and Marine Diatoms

Wu et al. (2010) introduce their study by recounting some of the worries various people have periodically expressed about the ongoing rise in the air's CO₂ concentration, the continuing absorption of over a third of mankind's CO₂ emissions by the world's oceans, and the oceans' consequent slow trend towards a state of acidity, which many individuals consider to be detrimental to the wellbeing of the planet's marine life. However, they indicate that increased CO₂ availability may actually be beneficial, especially for marine phytoplankton, in light of the fact that "the low affinity of their carboxylating enzyme (Rubisco) for CO₂ (Badger et al., 1998) ... could lead to enhanced phytoplankton growth and photosynthetic carbon fixation" in a high CO₂ world of the future, as had earlier been suggested by the work of Riebesell et al. (1993) and Hein and Sand-Jensen (1997).

In further exploration of this phenomenon, Wu et al. cultured the diatom Phaeodactylum tricornutum -- which had been isolated from the South China Sea -- for at least 20 generations in artificial seawater equilibrated with air of either 388 or 1000 ppm CO₂, which resulted in water pH values of either 8.15 or 7.80, respectively, while they measured the diatom's photosynthetic carbon fixation, dark respiration and growth rates.

The three researchers report that photosynthetic carbon fixation was enhanced by 12% under the high CO₂ and low pH conditions. However, since dark respiration was also enhanced, the daily net photosynthetic production was stimulated by a lesser 5.8%; and this value, as they describe it, "closely agreed with the observed increase in growth," which they had independently determined to have been enhanced by 5.2% under the high CO₂ and low pH conditions.

With respect to the implications of their findings, Wu et al. write that if "the roughly 5% increase in the growth of diatoms were taken into account based on the values obtained in this study, this would allow diatoms to rapidly accumulate more biomass (by about 34% in 6 days) and drawdown available nitrogen and other nutrients, leading to a greater biological carbon flux to the deep sea." And because diatoms contribute "about half of the marine primary production," in their words, this phenomenon would provide a significant brake on the rate-of-rise of the atmosphere's CO₂ concentration and its ability to cause global warming.

Additional References
Badger, M.R., Andrews, T.J., Whitney, S.M., Ludwig, M., Yellowlees, D.C., Leggat, W. and Price, G.D. 1998. The diversity and coevolution of Rubisco, plastids, pyrenoids, and chloroplast-based CO₂-concentrating mechanisms in algae. Canadian Journal of Botany 76: 1052-1071.

Hein, M. and Sand-Jensen, K. 1997. CO₂ increases oceanic primary production. Nature 388: 526-527.

Riebesell, U., Wolf-Gladrow, D.A. and Smetacek, V.S. 1993. Carbon dioxide limitation of marine phytoplankton growth rates. Nature 361: 249-251.