How does rising atmospheric CO2 affect marine organisms?

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Aquatic Algae Are An Important Part of Earth's "Bio-Thermostat"
Reference
Sciare, J., Mihalopoulos, N. and Dentener, F.J. 2000. Interannual variability of atmospheric dimethylsulfide in the southern Indian Ocean. Journal of Geophysical Research 105: 26,369-26,377.

Background
Dimethylsulfide (DMS) is a climatically-important trace gas produced by certain types of marine phytoplankton that is believed to play a major role in keeping the temperature of the earth within bounds that are conducive to the continual existence of life. In response to some impetus for warming (such as an increase in atmospheric greenhouse gases), for example, the climate-stabilizing mechanism begins with a warming-induced increase in the productivity of marine phytoplankton, which results in a greater production of oceanic DMS and its release to the atmosphere, where greater gas-to-particle conversions increase the air's population of cloud condensation nuclei and, ultimately, the albedos of marine stratus and altostratus clouds via a narrowing of the cloud droplet spectrum and a decrease in the mean radius of the cloud droplets, both of which phenomena tend to counteract the initial impetus for warming.

What was done
The authors made continuous measurements of atmospheric DMS concentrations over a 10-year period (1990-1999) at Amsterdam Island in the southern Indian Ocean. They then compared their results with a number of environmental parameters measured over the same period.

What was learned
Atmospheric DMS concentration, in the words of the authors, showed "a clear seasonal variation with a factor of 20 in amplitude between its maximum in January (austral summer) and minimum in July-August (austral winter)." In addition, DMS anomalies were found to be "closely related to sea surface temperature anomalies, clearly indicating a link between DMS and climate changes." Indeed, the authors found that a sea surface temperature (SST) increase of only 1C was sufficient to increase the atmospheric DMS concentration by as much as 50% on a monthly basis.

What it means
"To our knowledge," the authors say, "this is the first time that a direct link between SSTs and atmospheric DMS is established for a large oceanic area." And the effect is huge. If the relationship between DMS and SST is linear, for example - and the authors' data suggest that it likely is - the type of warming predicted to accompany a doubling of the air's CO2 content would increase the atmosphere's DMS concentration by a factor of three or more! And in the estimation of the authors, such an increase in DMS would have a "very important" negative feedback on the original impetus for warming. In fact, it could well overwhelm it.