Diurnal variations of intracellular nitrate storage by marine diatoms: effects of nutritional state

Abstract

Nitrate uptake and accumulation were measured in N-sufficient, N-limited, and 24 h N-starved cells of Phaeodactylum tricornutum Bohlin and Skeletonema costatum Grev., growing under a light-dark cycle. In N-sufficient cells the uptakerate was reduced at night and showed possible variation during the light period. In N-limited and N-starved cells such diurnal changes in uptake were absent, except extremely rapid, but short-lived nitrate uptake was observed early in the morning in N-limited cells. The nitrate accumulation inside the cells reflects a transient uncoupling between uptake and reduction mostly due to the light-dark cycle and strongly influenced by the physiological state of the cells. This accumulation is high during the night and at the beginning of the day, but decreases during the light period in N-sufficient cells. On the other hand, nitrate storage in N-sufficient and N-limited cultures shows a strong diurnal pattern, with maximum accumulation, suggesting the greatest uncoupling between uptake and assimilation, in the morning. In N-starved cells, accumulation is high and constant during the entire light period. Consequently, the uncoupling between nitrate uptake and reduction decreases during the light period but increases with N deficiency. These results indicate the importance of light periodicity and nutritional state of the cells on the nitrate utilization. They reveal the need for more systematic studies on N dynamics in relation to nutrient-light regimes.