Increasing atmospheric mixing ratios Of CO2 have already lowered surface ocean pH by 0.1 units compared to preindustrial values and pH is expected to decrease an additional 0.3 units by the end of this century. Pronounced physiological changes in some phytoplankton have been observed during previous CO2 perturbation experiments. Marine microorganisms are known to consume and produce climate-relevant organic gases. Concentrations of (CH3)2S (DMS) and CH2ClI were quantified during the Third Pelagic Ecosystem CO2 Enrichment Study. Positive feedbacks were observed between control mesocosms and those simulating future CO2. Dimethyl sulfide was 26% (±10%) greater than the controls in the 2× ambient CO2 treatments, and 18% (±10%) higher in the 3xCO2 mesocosms. For CHClI the 2×CO2 treatments were 46% (±4%) greater than the controls and the 3×CO2 mesocosms were 131% (±11%) higher. These processes may help contribute to the homeostasis of the planet. Copyright 2007 by the American Geophysical Union.