How does rising atmospheric CO2 affect marine organisms?

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Elevated CO2 Aids C4 Grass Growth Under Conditions of Water Stress
Seneweera, S., Ghannoum, O. and Conroy, J.P.  2001.  Root and shoot factors contribute to the effect of drought on photosynthesis and growth of the C4 grass Panicum coloratum at elevated CO2 partial pressures.  Australian Journal of Plant Physiology 28: 451-460.

What was done
The authors grew the C4 grass Panicum coloratum var. makarikiense cv. Bambatsi in environmental chambers receiving atmospheric CO2 concentrations of 360 and 1000 ppm for three weeks before withholding water from half of the plants at each CO2 concentration for 10 days to induce a water stress treatment.  Thereafter, plants were re-watered for five days to promote recovery.  Thus, the authors studied the effects of elevated CO2 and water stress on grass productivity and growth.

What was learned
At the onset of water stress, shoot dry mass in CO2-enriched plants was 33% greater than that observed in ambiently-grown plants.  Although water stress reduced shoot dry mass, the reductions were less severe for CO2-enriched than ambiently-grown plants.

During the water stress treatment, leaf water potentials and leaf relative water contents dropped at a much slower rate and to a lesser degree in CO2-enriched plants than in ambiently-grown plants.  Similarly, transpiration rates of CO2-enriched plants were much less than those displayed by plants fumigated with ambient air.  This phenomenon helped contribute to the greater soil moisture contents that were always present beneath the CO2-enriched plants.

Five days later, at final harvest, the CO2-induced growth enhancement in shoot dry mass was determined to be 44 and 70% for plants that had been subjected to well-watered and water-stressed treatments, respectively.  Thus, the percentage increase in growth due to atmospheric CO2 enrichment was greatest when the environmental stress was most severe.

What it means
As the atmospheric CO2 concentration increases, it is likely that this common C4 grass will exhibit increased accumulation of biomass, even under conditions of water stress, which frequently occur within its tropical range.  This phenomenon should also lead to enhanced carbon sequestration within tropical grasslands as the air's CO2 content rises.