How does rising atmospheric CO2 affect marine organisms?

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Effects of Increases in Atmospheric CO2 and Soil Water Content on Growth in C4 Grasses
Derner, J.D., Polley, H.W., Johnson, H.B. and Tischler, C.R.  2001.  Root system response of C4 grass seedlings to CO2 and soil water.  Plant and Soil 231: 97-104.

What was done
The authors grew two C4 grasses (Schizachyrium scoparium Nash and Andropogon gerardii Vitman.) common to tallgrass prairies in pots placed within a 38-meter-long controlled environment chamber located in a ventilated glass house.  This chamber was composed of five 7.6-m lengths of a 0.76-m-deep and 0.45-m-wide soil container topped with a transparent and tunnel-shaped polyethylene cover that was attached to its upper edges.  Various other plants placed within this tunnel served as photosynthetic "sinks" for CO2 as a commercial blower moved air through the chamber sections, so that a CO2 gradient was created through the "long and winding tunnel," from near 350 ppm at its entrance to approximately 200 ppm at its end.  In addition, these two C4 grasses were subjected to relative soil water contents that were 90 or 50% of their total soil water holding capacities.  Thus, the authors were able to study how the historical rise in the air's CO2 content has already influenced growth in these grasses under different soil moisture conditions.

What was learned
Data from the two C4 grasses were pooled, as there were no significant differences between species.  With respect to aboveground growth, shoot biomass was 57% greater at the ambient (350 ppm), rather than the subambient (200 ppm), atmospheric CO2 concentration, while the increase in relative water content enhanced shoot biomass by 82%.  In addition, root growth (root length, mass, surface area, and volume) was 15-27% greater at the ambient, rather than the subambient, atmospheric CO2 concentration, and 40-51% greater at high, rather than low, soil water content.

What it means
Since the 150-ppm increase in atmospheric CO2 concentration employed in this study is actually less than the rise in this parameter that has occurred since the end of the last great ice age, the authors state that "C4 grasses may have already experienced an augmentation in root growth which is comparable to that experienced with a doubling of current CO2 concentrations."  Hence, there is every reason to believe that such grasses will continue to respond positively to future increases in the air's CO2 content.  In addition, as the atmospheric CO2 concentration continues to rise, it will likely decrease water use in these prairie grasses, due to CO2-induced reductions in stomatal conductance, causing the indirect effects of greater soil moisture to further enhance their growth.