How does rising atmospheric CO2 affect marine organisms?

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Plant Exudate Utilization by Soil Microbes Under Elevated CO2
Hodge, A., Paterson, E., Grayston, S.J., Campbell, C.D., Ord, B.G. and Killham, K.  1998.  Characterization and microbial utilisation of exudate material from the rhizosphere of Lolium perenne grown under CO2 enrichment.  Soil Biology and Biochemistry 30: 1033-1043.

What was done
The authors grew ryegrass in sand and soil microcosms placed within controlled environmental chambers that received 450 or 720 ppm atmospheric CO2 for 21 days to determine if elevated CO2 affects the quantity or quality of exuded compounds from plant roots.

What was learned
Elevated CO2 had a tremendous positive impact on plant growth in the sand microcosms, as indicated by a total plant dry weight that was 175% greater than that reported for plants grown in ambient CO2.  Although carbon allocation in plants was not affected by atmospheric CO2 enrichment, as indicated by similar root:shoot ratios in both CO2 treatments, plants grown in elevated CO2 displayed a 30% reduction in total root exudate relative to that measured from plants grown at ambient CO2.  However, near the end of the experiment, the rate of carbon released from plants in elevated CO2 caught up to and surpassed that of plants in ambient CO2, suggesting that plants grown in elevated CO2 were beginning a trend that may have resulted in the greatest production of total root exudate if the experiment had been continued.  This analysis seems plausible, as the excess carbohydrates produced under atmospheric CO2 enrichment can only enhance biomass so much, due to plant size and sink strength, at which point, excess carbohydrates would likely be exuded from the plant to keep photosynthetic down regulation from occurring.

In the soil microcosms, elevated CO2 increased total plant dry weight by 95% over that observed in ambient CO2.  In addition, atmospheric CO2 enrichment stimulated the proliferation of culturable rhizosphere microorganisms --including bacteria, fungi and yeast-- to maintain soil microbial biodiversity.  It was also determined that these microorganisms utilized various carbon sources at faster rates when obtained from the rhizospheres of plants grown at elevated, rather than at ambient, CO2.

What it means
As the CO2 content of the air steadily rises, ryegrass will likely exhibit increases in carbohydrate synthesis, biomass accumulation, and carbon exudation to the soil.  This increased flux of carbon into the soil should increase the numbers and activities of soil microorganisms --including bacteria, fungi, and yeast-- which can make certain soil nutrients more readily available to plants.  In doing so, this soil carbon enhancement will likely maintain, or even increase, soil microorganism diversity, as increased soil carbon can cause viable non-culturable microorganisms to proliferate and become culturable.  Consequently, the increasing concentration of atmospheric CO2 may well enhance plant biodiversity, as plant diversity is tightly linked to microorganism diversity in the soil.

Reviewed 15 January 1999