Insights into algal nutrient uptake and carbon dynamics

Abstract

Marine phytoplankton are a pivotal component of the global carbon cycle and are routinely modelled in Earth System Models as it is believed that their physiology is well understood. The key studies for nutrient uptake occurred in the period 1967 to 1985, and the subject has received little attention since. Re-examining nutrient uptake at sub-hourly scales using Lab-On-Chip (LoC) sensors was the original plan for this study, but technical issues made this impossible, and so experimental cultures were grown with daily samples taken. A compilation of all published data on algal nutrient uptake was undertaken and examined in the context of variability in cell size, taxonomy, nutrient resource and whether active or passive nutrient uptake prevails. Despite published linear relationships between cell size and nutrient uptake, this could not be found in the data compilation leading to an impression that algal nutrient uptake is not an active process supported by transmembrane transporters. The new database also revealed significant commonality in nutrient uptake kinetics between different taxa and for different nutrient sources, so that competition between microalgae is not governed by nutrients and other processes must be important for species and taxonomic succession. Dissolved Organic Carbon (DOC) is an important component of the marine carbon cycle, with Dissolved Organic Matter (DOM) including a mixture of DIC, Dissolved Organic Nitrogen (DON) and Dissolved Organic Phosphorus. Experiments to couple nutrient uptake and measurements of DOC and DON release were undertaken in parallel. Rapid (<1 day) and short-term (2-4 days) changes in the DOM composition were detected in the cultures, when growth rates were low and increases in cell density were not established. The mechanisms behind these short-term changes, whether induced by exposure to light or microbial activity in the cultures, is unclear but warrants future focus. A notable difficulty in the work was that the aged seawater used as a media base contained a significant DOC and DOM signal that overrode the potential magnitude of release by phytoplankton.