Influence of nutrients on carbon isotope fractionation by natural populations of Prymnesiophyte algae in NE Pacific

Isotope effects by Prymnesiophyte algae were determined on samples of marine particulate organic material collected by MULVFS pumps during three C-JGOFS cruises along Line P in the northeast Pacific. Carbon isotope fractionation associated with photosynthetic carbon uptake was calculated from 13 C / 12 C ratios of dissolved carbon dioxide and the C37:2 alkenone, a biomarker originating from Prymnesiophyte algae. In the presence of nitrate (N) the magnitude of the carbon isotope fractionation (ep) is dependent on dissolved phosphate concentration (PO4) normalized to the concentration of dissolved carbon dioxide (Ce), i.e. ep∝PO4/Ce for N≠0. In nitrate-depleted waters, the magnitude of carbon isotope fractionation is lower than expected from the ep vs PO4/Ce relationship. These results suggest that growth conditions approaching nitrogen starvation can severely affect carbon isotope fractionation in these paleo-climatically important algae.