Pluses and minuses of ammonium and nitrate uptake and assimilation by phytoplankton and implications for productivity and community composition, with emphasis on nitrogen-enriched conditions

Anthropogenic activities are altering total nutrient loads to many estuaries and freshwaters, resulting in high loads not only of total nitrogen (N), but in some cases, of chemically reduced forms, notably . Long thought to be the preferred form of N for phytoplankton uptake, may actually suppress overall growth when concentrations are sufficiently high. has been well known to be inhibitory or repressive for uptake and assimilation, but the concentrations of that promote vs. repress uptake, assimilation, and growth in different phytoplankton groups and under different growth conditions are not well understood. Here, we review N metabolism first in a “generic” eukaryotic cell, and the contrasting metabolic…