Uptake and inhibition kinetics of nitrogen in Microcystis aeruginosa: Results from cultures and field assemblages collected in the San Francisco Bay Delta, CA

The nitrogen (N) uptake kinetic parameters for Microcystis field assemblages collected from the San Francisco Bay Delta (Delta) in 2012 and non-toxic and toxic laboratory culture strains of M. aeruginosa were assessed. The 15N tracer technique was used to investigate uptake of ammonium (NH4+), nitrate (NO3−), urea and glutamic acid over short-term incubations (0.5-1 h), and to study inhibition of NO3−, NH4+ and urea uptake by NH4+, NO3− and NH4+, respectively. This study demonstrates that Delta Microcystis can utilize different forms of inorganic and organic N, with the greatest capacity for NH4+ uptake and the least for glutamic acid uptake, although N uptake did not always follow the classic Michaelis-Menten hyperbolic relationship at substrate concentrations up to 67 μmol N L−1. Current ambient N concentrations in the Delta may be at sub-saturating levels for N uptake, indicating that if N loading (especially NH4+) were to increase, Delta Microcystis assemblages have the potential for increased N uptake rates. Delta Microcystis had the highest specific affinity, α, for NH4+ and the lowest for NO3−. In culture, N uptake by non-toxic and toxic M. aeruginosa strains was much higher than from the field, but followed similar N utilization trends to those in the field. Neither strain showed severe inhibition of NO3− uptake by NH4+ or inhibition of NH4+ uptake on NO3−, but both strains showed some inhibition of urea uptake by NH4+.