Incorporation of Leucine and Thymidine by Estuarine Phytoplankton: Implications for Bacterial Productivity Estimates

Leucine and thymidine incorporation were examined in size-fractionated estuarine communities and in cultures of phytoplankton known to use dissolved organic nitrogen (DON). Cultured phytoplankton species were used to establish that phytoplankton took up leucine and thymidine into protein and DNA, respectively. Subsequently, incorporation of leucine and thymidine was measured in size-fractionated populations collected from the Lafayette River, VA, a eutrophic estuary where resident populations contain bloom-forming phytoplankton known to take up DON, and the Gulf of Mexico during a bloom of the mixotrophic red tide dinoflagellate, Karenia brevis. We examined the efficacy of size fractionation for determining phytoplankton versus bacterial incorporation of leucine and thymidine under conditions employed during bacterial productivity bioassays, and antibiotics were used to distinguish between bacterial and phytoplankton incorporation in cultured and natural populations. Results suggest that cultures and natural assemblages of phytoplankton can take up both leucine and thymidine when supplied at low concentrations (10 and 12 nmol L⁻¹, respectively) and during short incubations (15 min to 1 h). In natural populations, up to 95% of the leucine and thymidine incorporation during short bioassays was recovered in the >5.0-μm size fraction that contained ≤4.2% of the bacterial biomass.