How does rising atmospheric CO2 affect marine organisms?

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Effects of Nutrient and Genotype on CO2 Response of Leucadendron Species
Midgley, G.F., Wand, S.J.E. and Pammenter, N.W.  1999.  Nutrient and genotypic effects on CO2-responsiveness: photosynthetic regulation in Leucadendron species of a nutrient-poor environment.  Journal of Experimental Botany 50: 533-542.

What was done
The authors collected seed from four closely related Leucadendron species growing in different nutrient-poor environments of South Africa and grew their respective seedlings for about one year in open-top chambers receiving atmospheric CO2 concentrations of 350 and 700 ppm.  In addition, plants were irrigated with solutions of low and high nitrogen to determine the effects of elevated CO2 and nutrient availability on photosynthesis in Leucadendron of differing genotypes.

What was learned
No significant interactions were found between CO2 and nutrient levels, or between CO2 and species.  Elevated CO2 significantly increased leaf starch concentrations in all genotypes and nutrient treatments by 19 to 24%.  In addition, elevated CO2 increased photosynthesis in every species and fertility regime by an average of 40%, in spite of the fact that down regulation occurred, as indicated by an approximate 30% decrease in rubisco activity.  This acclimation of the photosynthetic apparatus invariably contributed to a near 50% enhancement of photosynthetic nitrogen-use efficiency in plants grown at 700 ppm compared to plants grown at 350 ppm atmospheric CO2.

What it means
As the concentration of CO2 in the air continues to rise, different but closely related species of Leucadendron will likely display similar increases in photosynthesis and starch production, regardless of genotype and soil fertility.  Thus, because the increasing CO2 content of the atmosphere does not act as a selective factor among Leucadendron genotypes, it should preserve the genetic diversity that exists within these species.  Photosynthetic acclimation, which may also happen as a consequence of the air's rising CO2 content, should increase the photosynthetic nitrogen-use efficiency of these species, thus allowing them to use valuable nitrogen resources for other physiological processes, which may actually delay the onset or reduce the degree of acclimation that occurs.

Reviewed 15 June 1999