Nitrite excretion during nitrate uptake by three nitratelimited diatoms was measured at different temperatures.
Phaeodactylum tricornutum Bohlin and Chaetoceros affinis
Lauder Hustedt excreted nitrite over the whole range of physiological temperatures, whereas Thalassiosira pseudonana Hasle and Heimdal (Clone 13-1) excreted nitrite only at 25 ~ Parallel to growth and nitrate-uptake rates, excretion rates increased exponentially with temperature, attaining a maximum between 20 ~ and 25 ~ (optimum temperature range). At the end of nitrate uptake, when the nitrate concentration in the culture medium had decreased below 1 ktM, nitrite was reabsorbed at all temperatures, except when cells were in the dark or at very low light intensity. Nitrite uptake was also inhibited by the presence of nitrate in the medium. These results are discussed in relation to the formation and position of the maximum layer of primary nitrite in the thermocline, below the maximum layer of chlorophyll in stratified oceanic areas. this nitrite layer is always associated with a strong nitrate gradient in the thermocline (Herbland and Voituriez, 1977).
Herbland and Voituriez consider this primary nitrite layer to be of biological significance: they believe it to be an indicator of new production (as defined by Dugdale, 1967); its position is important, delimiting the depth at which fight-limitation occurs. At this depth, temperature together with low light intensity are the most important factors affecting the growth of phytoplankton, and studies of nitrite formation must consider temperature effects.
In the present study, I investigated the influence of temperature on the time-dependent response of nitrite excretion and uptake by three marine diatoms at low light intensity during nitrate uptake. One experiment simulated the sudden encounter of phytoplankton with nutrient-rich, cold, deeper water, with temperature and light variations, in order to obtain additional information on the importance of these physical parameters for nitrite formation in the field.