The effects of marine photosynthesis and calcification on the partial pressure of carbon dioxide in seawater (PCO2) are examined in the light of recent studies and using original model calculations. The ratio of organic carbon to inorganic carbon production (ROI) determines whether an ecosystem is a net sink or source for atmospheric CO2. ThePCO2 maintains its initial value when the photosynthetic rate is approximately 0.6 times the calcification rate under normal sea surface condition. In case of higherROI, thePCO2 decreases and seawater can absorb atmospheric CO2. The ratio of organic carbon to inorganic carbon production can be used as a potential indicator of sink-source behavior in aquatic photo-calcifying systems.