Carbon isotope fractionation by marine phytoplankton

Abstract

Stable carbon isotope fractionations by seventeen species of marine phytoplankton, representing the classes of Bacillariophyceae, Chlorophyceae, Prasinophyceae, Chrysophyceae, Haptophyceae and Dinophyceae, have been determined in laboratory culture experiments using bicarbonate enriched artificial sea water. The Δ¹³C values (Δ¹³C = δ¹³C of plant HCO₃) range from -22.1 0/00 to -35.5 °/oo Nitzschia closterium shows the smallest fractionation of -22.1 °/oo and Isochrysis galbana, the greatest of -35.5 °/oo. Within the Bacillariophyceae, the Δ¹³C values vary from -22.1 °/oo (Nitzschia closterium) to -29.9 °/oo (Skeletonema costatum). With the exception of Cyclotella sp., the centric diatoms seem to fractionate more than the pennate diatoms. Haptophyceae and Chrysophyceae are always more depleted in ¹³C than other marine microalgae. For Prasinophyceae, Chlorophyceae and Dinophyceae, Δ¹³C values of -28.9 °/oo (Platymonas sp.), -31.9 °/oo (Dunaliella sp.), -29.1 °/oo (Chlorococcum sp.) and -27.2 °/oo (Glenodinium foliaceum) have been measured. Since these algae were cultured under identical laboratory conditions, the wide range of Δ¹³C values are indicative that fractionating processes other than RuDP and/or PEP carboxylases are operable in these organisms. Differential accumulation of photosynthetic products by marine algae has been suggested to be a possible explanation for the variation of Δ¹³C in these organisms..