• growth rate;
  • iron limitation;
  • nitrate reductase activity;
  • Phaeodactylum tricornutum;
  • photosynthesis rate;
  • respiratory electron transport activity;
  • temperature effect

Phaeodactylum tricornutum Bohlin (Bacillariophyceae) was maintained in exponential growth under Fe-replete and stressed conditions over a range of temperatures from 5 to 30° C. The maximum growth rate (GR) was observed at 20° C (optimal temperature) for Fe-replete and -stressed cells. There was a gradual decrease in the GR decreasing temperatures below the optimum temperature; however, the growth rate dropped sharply as temperature increased above the optimum temperature. Fe-stressed cells grew at half the growth rate of Fe-replete cells at 20° C, whereas this difference became larger at lower temperatures. The change in metabolic activities showed a similar pattern to the change in growth rate temperature aside from their optimum temperature. Nitrate reductase activity (NRA) and respiratory electron transport system activity (ETS) per cell were maximal between 15 and 20° C, whereas cell-specific photosynthetic rate (Pcell) was maximal at 20° C for Fe-replete cells. These metabolic activities were influenced by Fe deficiency, which is consistent with the theoretical prediction that these activities should have an Fe dependency. The degree of influence of Fe deficiency, however, was different for the four metabolic activities studied: NRA > Pcell > ETS = GR. NRA in Fe-stressed cells was only 10% of that in Fe-replete cells at the same temperature. These results suggest that cells would have different Fe requirements for each metabolic pathway or that the priority of Fe supply to each metabolic reaction is related to Fe nutrition. In contrast, the order of influence of decreasing the temperature from the optimum temperature was ETS > Pcell > NRA > GR. For NRA, the observed temperature dependency could not be accounted for by the temperature dependency of the enzyme reaction rate itself that was almost constant with temperature, suggesting that production of the enzyme would be temperature dependent. For ETS, both the enzyme reactivity and the amount of enzyme accounted for the dependency. This is the first report to demonstrate the combined effects of Fe and temperature on three important metabolic activities (NRA, Pcell, and ETS) and to determine which activity is affected the most by a shortage of Fe. Cellular composition was also influenced by Fe deficiency, showing lower chl a content in the Fe-stressed cells. Chl a per cell volume decreased by 30% as temperature decreased from 20 to 10° C under Fe-replete conditions, but chl a decreased by 50% from Fe-replete to Fe-stressed conditions.