| Summary: | Mathematic models have suggested that the accumulation of anthropogenic CO2 in the atmosphere has caused a decreased in oceanic pH, and that the decrease should continue for years to come. A lower pH should affect the photosynthesis of primary producers in the sea, including seaweeds. Carbon stable isotope fractionation in photosynthesis should provide information regarding this possible influence of pH on photosynthesis. Here we show that lower pH may lead to enhanced isotopic fractionation in carbon assimilation in photosynthesis by a brown alga Undaria pinnatifida. This may reflect a shift to CO2 absorption instead of HCO3- absorption, which is consistent with a higher proportion of CO2 relative to HCO3- at lower pH. Since in photosynthesis CO2 should be absorbed with less energy costs than HCO3-, ocean acidification may be favorable to U. pinnatifida and species with similar photosynthetic mechanisms. However, other aspects of the phenomenon must be considered before predictions are made concerning the ecology of these organisms. Proceedings of 4th JSPS-VAST Joint Seminar on 'Coastal Marine Science', Hai Phong, Viet Nam, 26-28 October 2009 Oceanography departmental bulletin paper
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