How does rising atmospheric CO2 affect marine organisms?

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Atmospheric CO2 Enrichment Effects on Marine Picocyanobacteria
Lu, Z., Jiao, N. and Zhang, H. 2006. Physiological changes in marine picocyanobacterial Synechococcus strains exposed to elevated CO2 partial pressure. Marine Biology Research 2: 424-430.

"Unicellular marine picocyanobacteria," in the words of the authors, "are ubiquitous in both coastal and oligotrophic regimes," and they report that "the contribution of these organisms to primary production and nutrient cycling is substantial on a global scale." Hence, they say "they should be one of the focus points regarding biological responses to the rise in atmospheric CO2 concentration."

What was done
Lu et al. studied physiological changes in phycocyanin (PC)-rich and phycoerythrin (PE)-rich Synechococcus strains of picocyanobacteria under atmospheric CO2 concentrations of 350, 600 and 800 ppm in batch cultures maintained in one-liter glass flasks under a 12:12 hour light:dark regime for periods of 12 days, during which time they measured a number of physiological parameters related to the picocyanobacteria's growth and well-being.

What was learned
Growth of the Synechococcus PE strain was unaffected by atmospheric CO2 enrichment. However, the Synechococcus PC strain grown at 800 ppm CO2 experienced a significant 36.7% increase in growth compared to when grown at 350 ppm CO2 (P<0.05). On the other hand, the PC strain showed no significant change in carbohydrate content over the CO2 range investigated; but the PE strain exhibited a significant CO2-induced increase of 37.4% at 800 ppm CO2 (P<0.05). Nevertheless, the PC strain exhibited a 36.4% increase in RNA/DNA ratio between 350 and 800 ppm CO2, which ratio, in the words of Lu et al., "provides a good estimate of metabolic activities and has been used extensively as a biochemical indicator of growth rate in a variety of marine organisms." In addition, in both Synechococcus strains, cellular pigment contents were generally greater in the CO2-enriched treatments than in the ambient-air controls. At day 12 in the PE strain, for example, they averaged in excess of 70% greater at 800 ppm CO2 than at 350 ppm CO2.

What it means
Both strains of Synechococcus picocyanobacteria benefited from the extra CO2 supplied to them in this study, albeit in a variety of different ways; and in comparing the responses of the two strains, Lu et al. conclude that "the PC strain would probably benefit more than the PE strain from future increases in atmospheric CO2 concentration." However, they note that "differences in photosynthetic characteristics may allow the coexistence of the two picocyanobacterial strains through a subtle form of niche differentiation," citing the work of Ernst et al. (2003) and Stomp et al. (2004). Hence, we may reasonably expect to see a significant increase in primary production and nutrient cycling throughout the world's oceans as the air's CO2 content continues to climb in the years and decades to come, driven by its several and varied positive impacts on these very tiny organisms.

Ernst, A., Becker, S., Wollenzien, U.I.A. ande Postius, C. 2003. Ecosystem-dependent adaptive radiations of picocyanobacteria inferred from 16S rRNA and ITS-1 sequence analysis. Microbiology 149: 217-228.

Stomp, M., Huisman, J., de Jongh, F., et al. 2004. Adaptive divergence in pigment composition promotes phytoplankton biodiversity. Nature 432: 104-107.

Reviewed 13 June 2007