New insights into intracellular nitrogen allocation in phytoplankton using δ15N distributions of free amino acids and chlorophyll

Chlorophyll is an essential part of the photosynthetic apparatus and therefore a molecular biomarker indicating the activity of photosynthetic organisms. The nitrogen (N) isotope content of chlorophyll is not affected by diagenesis, making it an ideal biomarker for studying the dynamics between large-scale changes in the N cycle and the responses of the photosynthetic community. Despite the assumption that all oxygenic photosynthetic organisms use the same biosynthetic pathway to produce chlorophyll, both culture studies and environmental surveys demonstrate differences in N isotope fractionation between major algal lineages such as cyanobacteria and eukaryotic algae. However, the physiological and/or biochemical basis for the observed differences in fractionation is still unknown.

Chlorophyll and its biosynthetic precursor, glutamic acid, are closely linked to N assimilation- via the GS/GOGAT pathway- and to the overall N status of the phytoplankton cell. Therefore, one hypothesis is that these fractionation patterns can be explained by fundamental differences between cyanobacteria and eukaryotic algae in intracellular N allocation. To test this hypothesis we measured the N isotope ratios of the intracellular free and protein-bound amino acid pools and chlorophyll in several model phytoplankton. The results bring us closer to understanding the mechanisms that determine δ¹⁵N values of chlorophyll, but also provide new information on how intracellular N partitioning varies among major algal groups. This is of increasing importance given that anthropogenic changes are already altering N inputs and losses in aquatic ecosystems worldwide, and elucidating N dynamics at the cellular level will likely be critical to understand changes in phytoplankton community structure that occur as a result of changing N availability.