How does rising atmospheric CO2 affect marine organisms?

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Effects of Fungi on Prairie Plants
Wilson, G.W.T. and Hartnett, D.C.  1998.  Interspecific variation in plant responses to mycorrhizal colonization in tallgrass prairie.  American Journal of Botany 85: 1732-1738.

What was done
The authors grew 36 grass and 59 forb species common to tallgrass prairie ecosystems with and without arbuscular mycorrhizal fungi to determine the importance of fungal symbiosis on their growth.

What was learned
Perennial warm-season C4 grasses benefited significantly from fungal inoculation, as indicated by an average plant dry mass value that was 85% greater than that reported for plants not inoculated with arbuscular mycorrhizal fungi.  The average dry mass of perennial cool-season C3 grasses, however, was not affected by mycorrhizal colonization, as was also the case for cool- and warm-season annual grasses.

Similar results were observed for the many forb species that were studied.  Over 80% of the perennial forbs, for example, exhibited significant increases in dry mass with fungal inoculation, while only 15% of the annual forbs displayed significant increases in growth.

What it means
As the CO2 content of the air steadily rises, most C3 plants will experience greater increases in carbohydrate production than most C4 plants; this observation has been used by some individuals to predict vast decreases in C4 species in ecological communities where they coexist with C3 species.  However, there are other phenomena that come to bear upon this issue.

Consider one of the primary results of this paper --the fact that perennial C4 grasses experience much greater mycorrhizal fungi-induced growth enhancements than do perennial C3 grasses.  This phenomenon could enable C4 plants to match the greater photosynthetic gains of C3 plants that typically occur with atmospheric CO2 enrichment, as much research has shown that elevated CO2 levels enhance the mycorrhizal colonization of roots, which is known to make certain soil minerals and water more available for plant growth.  And if these fungal-mediated processes, as well as others, are significantly favored in C4 as opposed to C3 plants in tallgrass prairies, it is much more prudent to suggest that species biodiversity will be maintained in these ecosystems as the CO2 content of the air rises steadily higher.

Reviewed 1 February 1999