How does rising atmospheric CO2 affect marine organisms?

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Elevated CO2 Alters Microbial Populations
Montealegre, C.M., Van Kessel, C., Blumenthal, J.M., Hur, H.G., Hartwig, U.A. and Sadowsky, M.J.  2000.  Elevated atmospheric CO2 alters microbial population structure in a pasture ecosystem.  Global Change Biology 6: 475-482.

What was done
After three years of exposure to atmospheric CO2 concentrations of 350 and 600 ppm in FACE plots, white clover (Trifolium repens) was harvested and the genetic structure of 120 isolates of the symbiotic bacterium Rhizobium leguminosarum were determined.  In addition, isolates favored by elevated CO2 were subsequently mixed with strains favored by ambient CO2 to determine which ones would be more successful at colonizing white clover under controlled environmental conditions of 350 and 600 ppm CO2.

What was learned
Elevated CO2 altered the composition of Rhizobium leguminosarum strains associated with root nodules of white clover and favored some isolates over others.  When strains favored by ambient and elevated CO2 concentrations were mixed together and grown with white clover at an atmospheric CO2 concentration of 600 ppm, 17% more nodules were formed by isolates selected for by elevated CO2 than by ambient CO2.  Thus, atmospheric CO2 enrichment influenced the competitive ability of certain bacterial strains in forming symbiotic relationships with plant roots.

What it means
As the atmospheric CO2 concentration rises, it is likely that microbial populations will change, as some bacterial strains may be favored over others.  However, the results of such changes in microbial community structure on plant performance and competition are difficult to ascertain and require much more complex and lengthy experiments.

Reviewed 30 August 2000