Combined Effects of Ocean Acidification and Light or Nitrogen Availabilities on (13)C Fractionation in Marine Dinoflagellates

Along with increasing oceanic CO(2) concentrations, enhanced stratification constrains phytoplankton to shallower upper mixed layers with altered light regimes and nutrient concentrations. Here, we investigate the effects of elevated pCO(2) in combination with light or nitrogen-limitation on (13)C fractionation (ε(p)) in four dinoflagellate species. We cultured Gonyaulax spinifera and Protoceratium reticulatum in dilute batches under low-light (‘LL’) and high-light (‘HL’) conditions, and grew Alexandrium fundyense and Scrippsiella trochoidea in nitrogen-limited continuous cultures (‘LN’) and nitrogen-replete batches (‘HN’). The observed CO(2)-dependency of ε(p) remained unaffected by the availability of light for both G. spinifera and P. reticulatum, though at HL ε(p) was consistently lower by about 2.7‰ over the tested CO(2) range for P. reticulatum. This may reflect increased uptake of ((13)C-enriched) bicarbonate fueled by increased ATP production under HL conditions. The observed CO(2)-dependency of ε(p) disappeared under LN conditions in both A. fundyense and S. trochoidea. The generally higher ε(p) under LN may be associated with lower organic carbon production rates and/or higher ATP:NADPH ratios. CO(2)-dependent ε(p) under non-limiting conditions has been observed in several dinoflagellate species, showing potential for a new CO(2)-proxy. Our results however demonstrate that light- and nitrogen-limitation also affect ε(p), thereby illustrating the need to carefully consider prevailing environmental conditions.