Ammonium uptake and growth limitation in marine phytoplankton

We used continuous culture techniques and a sensitive fluorescence method to quantify relationships among ammonium (NH4+) concentration, cellular NH4+ uptake rates, and growth rate limitation for five marine algal species ranging in diameter from 3 to 11 mu m. The growth rate of two high-nutrient-requiring coastal diatoms, Thalassiosira weissflogii and Thalassiosira pseudonana, were limited at NH4+ concentrations < 90-100 nmol L-1, while those of low-nutrient-adapted species, the oceanic coccolithophore Emiliania huxleyi, the coastal pelagophyte Aureoumbra lagunensis, and the coastal trebouxiophyte Nannochloris atomus, were limited at concentrations < 30-40 nmol L-1. In the nitrogen-limited cyclostats, the residence times of dissolved ammonium were extremely short (4-12 min at chlorophyll a levels of 20-30 mu g L-1), and these short times had to be taken into account to obtain accurate NH4+ measurements. Ammonium and nitrate concentrations in stratified surface ocean waters (3-15 nmol L-1) fall within the range that substantially limited algal growth rates in our experiments, providing evidence for nitrogen limitation in these waters. Ammonium uptake rates and associated specific growth rates were much lower for T. weissflogii (10-11 mu m in diameter) than for the other smaller (3.1-4.5 mu m in diameter) species under ammonium limitation owing to the limits imposed by NH4+ diffusion to the cell surface. Diffusion per unit of cell volume varies with the inverse square of the cell diameter and, thus, greatly restricts growth of large-celled species under NH4+ limitation. The resultant selection of small-celled algal species in ammonium-limited ocean waters should promote rapid grazing and nutrient cycling and minimum nutrient loss via settling of intact cells and zooplankton fecal material.