How does rising atmospheric CO2 affect marine organisms?

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Within-Species Genetic Differences in Quaking Aspen and Their Responses to Elevated CO2
Cseke, L.J., Tsai, C.-J., Rogers, A., Nelsen, M.P., White, H.L., Karnosky, D.F. and Podila, G.K. 2009. Transcriptomic comparison in the leaves of two aspen genotypes having similar carbon assimilation rates but different partitioning patterns under elevated [CO2]. New Phytologist 182: 891-911.

The authors note that certain perceived "genetic and environmental bottlenecks" may limit a plant's capacity to allocate assimilated carbon to greater biomass production; but it is logical to expect that numerous species may possess the genetic diversity needed to overcome these potential roadblocks and thereby benefit even more than is commonly anticipated from the enhanced growth that is known to be possible in a CO2-enriched atmosphere.

What was done
Working at the Aspen FACE site near Rhinelander, Wisconsin (USA), Cseke et al. grew two quaking aspen (Populus tremuloides Michx.) clones (216 and 271) from the seedling stage in replicate plots maintained at either 372 or 560 ppm CO2 throughout each year's growing season (May-September), assessing their stem volume (a surrogate for biomass) annually for a period of eight years, during and after which time they measured: (1) the trees' maximum light-saturated rates of leaf net photosynthesis, (2) the transcriptional activity of leaf elevated-CO2-responsive genes, and (3) numerous leaf primary and secondary carbon-based compounds.

What was learned
Although the CO2-induced increase in the maximum light-saturated rate of leaf net photosynthesis in clone 216 was over twice as great as that of clone 271 (37% vs. 17%, as best we can determine from Cseke et al.'s bar graphs), just the opposite relationship was manifest in the CO2-induced increases in the trees' stem volumes (only 0-10% for clone 216 vs. 40-50% for clone 271). As for why this was so, the researchers' transcript abundance and carbon/nitrogen biochemistry data suggest that "the CO2-responsive clone (271) partitions carbon into pathways associated with active defense/response to stress, carbohydrate/starch biosynthesis and subsequent growth," while "the CO2-unresponsive clone (216) partitions carbon into pathways associated with passive defense and cell wall thickening."

What it means
The seven scientists say their study indicates "there is significant variation in expression patterns between different tree genotypes in response to long-term exposure to elevated CO2," and, therefore, that "future efforts to improve productivity or other advantageous traits for carbon sequestration should include an examination of genetic variability in CO2 responsiveness." We also note that as the atmosphere's CO2 concentration continues to rise, manifestations of these fitness-promoting traits will appear on their own, as they are brought forth naturally by the changing environment. Indeed, earth's plants appear to be genetically programmed to respond positively to atmospheric CO2 enrichment, which suggests that the ongoing rise in the air's CO2 content is something they are innately well prepared to use to their advantage.

Reviewed 2 September 2009