Temperature dependence of inorganic nitrogen uptake: Reduced affinity for nitrate at suboptimal temperatures in both algae and bacteria

Nitrate utilization and ammonium utilization were studied by using three al gal isolates, six bacterial isolates, and a range of temperatures in chemos tat and batch cultures. We quantified affinities for both substrates by det ermining specific affinities (specific affinity = maximum growth rate/half- saturation constant) based on estimates of kinetic parameters obtained from chemostat experiments. At suboptimal temperatures, the residual concentrat ions of nitrate in batch cultures and the steady-state concentrations of ni trate in chemostat cultures both increased. The specific affinity for nitra te was strongly dependent on temperature (Q(10) approximate to 3, where Q(1 0) is the proportional change with a 10 degrees C temperature increase) and consistently decreased at temperatures below the optimum temperature. In c ontrast, the steady-state concentrations of ammonium remained relatively co nstant over the same temperature range, and the specific affinity for ammon ium exhibited no clear temperature dependence. This is the first time that a consistent effect of low temperature on affinity for nitrate has been ide ntified for psychrophilic, mesophilic, and thermophilic bacteria and algae. The different responses of nitrate uptake and ammonium uptake to temperatu re imply that there is increasing dependence on ammonium as an inorganic ni trogen source at low temperatures.