Improving phytoplankton models by using red fluorescence as a proxy for nutrient quota
In oligotrophic environments, the intracellular concentration of the most limiting nutrient is key to understand the rate at which phytoplankton populations grow. Hence, cell nutrient status (or cell “quota”) is critical to modelling phytoplankton nutrient-limited growth and analysing aquatic primary productivity, phytoplankton ecology, eutrophication and algal blooms. However, current methods to directly monitor per-capita nutrient status rely on destructive sampling techniques and are inaccurate, expensive, and time consuming. This study tested the hypothesis that nitrogen limitation triggers systematic optical changes in single cells, which can be rapidly and accurately monitored with a standard flow cytometer. The freshwater green microalgae Franceia sp., Mesotaenium sp., Scenedesmus obliquus, and Tetraedronsp. were reared in nitrogen-limited batch culture conditions across two treatments of initial population densities and monitored for total cell nitrogen, medium nitrogen, and optical flow cytometric properties of red fluorescence and forward and side light scatters. Firstly, we assessed the strength of the relationship between cell optical properties and observed total cell nitrogen. Secondly, we assessed the improvement in performance of a process-based nitrogen-quota-phytoplankton model when red fluorescence was used as a proxy for internal nitrogen.
Changes in total cell nitrogen could be described very well (R2 = 0.9) from observations of flow cytometric variables and medium nitrogen, and the relationship did not change across different species or initial population sizes. Flow cytometric red fluorescence was the most important variable, explaining 77% of the total variability in total cell nitrogen and up to 87% when combined with side light scatter, the second most important variable. Moreover, including cell red fluorescence as a proxy for internal nitrogen quota in a nitrogen-quota-phytoplankton model led to better predictions for dynamics of internal nitrogen over the course of the experiment, relative to a model in which internal quota was inferred from cell population and medium nitrogen dynamics alone. Overall, our results indicate that optical flow cytometric variables are a convenient and reliable method to improve estimates of nitrogen status in phytoplankton cells. Specifically, explicitly accounting for per-capita red fluorescence can significantly improve the performance of nutrient-limited phytoplankton models.