INFLUENCE OF TEMPERATURE ON THE TRANSIENT-RESPONSE IN NITRATE UPTAKE AND REDUCTION BY 4 MARINE DIATOMS

The transient responses to NO3- in a single addition of the limiting nutrient were investigated at different temperatures for 4 diatoms. The results from typical perturbation experiments indicated that 2 or 3 modes of uptake can be distinguished in the species and at the temperatures used. An initial rapid uptake (Vs) was observed at all temperatures in Phaeodactylum tricornutum (Bohlin) and Skeletonema costatum (Grev). Uncoupling between uptake and reduction was extensive for these species, with a great NO3- accumulation at the beginning of uptake (17% of particulate N). Temperature interfered with the time period of Vs(Ts), which was short (a few minutes) at high temperatures, where internal control acted rapidly, and long at low temperatures (1 h), even for a cold-adapted species, such as Nitzschia turgiduloides (Hasle). For Chaetoceros affinis (Lauder), no real uptake was observed at any temperature, and this species accumulated much less NO3-, indicating good coupling between uptake and reduction. The 2 last parts of uptake (Vi and Ve) were common in the 4 spp. and had a typical kinetic form during the perturbation experiment, and the kinetic parameters (half-saturation constant Ks and maximum uptake rate, Vmax) calculated from the Michaelis-Menten equation increased exponentially with temperature. These observations were considered with regard to choice of the best method of nitrate uptake kinetic determination.