The assessment of direct effects of anthropogenic CO2 increase on themarine biota has received relatively little attention compared to theintense research on CO2-related responses of the terrestrial biosphere.Yet, due to the rapid air-sea gas exchange, the observed past and predictedfuture rise in atmospheric CO2 causes a corresponding increase inseawater CO2 concentrations, [CO2], in upper ocean waters.Increasing [CO2] leads to considerable changes in the surface oceancarbonate system, resulting in decreases in pH and the carbonateconcentration, [CO32-]. These changes can be shown to have strong impactson the marine biota.Here we will distinguish between CO2-related responses of the marinebiota which a) potentially affect the ocean's biological carbon pumps and b)are relevant to the interpretation of diagnostic tools (proxies) used toassess climate change on geological times scales. With regard to the former,three direct effects of increasing [CO2] on marine plankton have beenrecognized: enhanced phytoplankton growth rate, changing elementalcomposition of primary produced organic matter, and reduced biogeniccalcification. Although quantitative estimates of their impacts on theoceanic carbon cycle are not yet feasible, all three effects increase theocean's capacity to take up and store atmospheric CO2 and hence, canserve as negative feedbacks to anthropogenic CO2 increase.With respect to proxies used in paleo-reconstructions, CO2-sensitivityis found in carbon isotope fractionation by phytoplankton and foraminifera.While CO2-dependent isotope fractionation by phytoplankton may be ofpotential use in reconstructing surface ocean PCO2 at ancient times,CO2-related effects on the isotopic composition of foraminiferal shellsconfounds the use of the difference in isotopic signals between planktonicand benthic shells as a measure for the strength of marine primaryproduction. The latter effect also offers an alternative explanation forthe large negative swings in delta13C of foraminiferal calcite between glacial andinterglacial periods. Changes in [CO32-] affect the delta18O in foraminiferalshells. Taking this into account brings sea surface temperature estimatesfor the glacial tropics closer to those obtained from other geochemicalproxies.