How does rising atmospheric CO2 affect marine organisms?

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The Role of Seagrasses in Earth's Carbon Cycle
Kennedy, H., Beggins, J., Duarte, C.M., Fourqurean, J.W., Holmer, M., Marba, N. and Middelburg, J.J. 2010. Seagrass sediments as a global carbon sink: Isotopic constraints. Global Biogeochemical Cycles 24: 10.1029/2010GB003848.

The authors write that "seagrass meadows are highly productive habitats found along many of the world's coastlines, providing important services that support the overall functioning of the coastal zone," and they say that "the organic carbon that accumulates in seagrass meadows is derived not only from seagrass production but from the trapping of other particles, as the seagrass canopies facilitate sedimentation and reduce resuspension."

What was done
Working with a data set that included 219 paired analyses of the carbon isotopic composition of seagrass leaves and sediments from 207 seagrass sites at 88 locations worldwide, Kennedy et al., as they describe it, provided "a comprehensive synthesis of the available data to obtain a better understanding of the relative contribution of seagrass and other possible sources of organic matter that accumulates in the sediments of seagrass meadows."

What was learned
The international team of researchers -- hailing from Denmark, the Netherlands, Spain, the United States and the United Kingdom -- determined that "the average proportional contribution of seagrass to the surface sediment organic carbon pool is ~50%." And based on this finding, they estimated that "carbon burial in seagrass meadows is between 48 and 112 Tg/year, showing that seagrass meadows are natural hot spots for carbon sequestration."

What it means
This significant finding of the seven scientists is made even more significant by the recent work of Jiang et al. (2010), who experimentally determined -- for atmospheric CO2 concentrations representative of (1) the present global ocean, with a pH of 8.10, (2) the projected ocean for 2100, with a pH of 7.75, (3) the projected ocean for 2200, with a pH of 7.50, and (4) the ocean characteristic of what they called "an extreme beyond the current predictions" (a 100-fold increase in free CO2, with a pH of 6.2) -- that the "leaf growth rate of CO2-enriched plants was significantly higher than that in the unenriched treatment," that "nonstructural carbohydrates (NSC) of T. hemprichii, especially in belowground tissues, increased strongly with elevated CO2," and that "belowground tissues showed a similar response with NSC," the combination of which factors represents a strong negative feedback to CO2-induced global warming.

Jiang, Z.J., Huang, X.-P. and Zhang, J.-P. 2010. Effects of CO2 enrichment on photosynthesis, growth, and biochemical composition of seagrass Thalassia hemprichii (Ehrenb.) Aschers. Journal of Integrative Plant Biology 52: 904-913.

Reviewed 2 March 2011