Combined effects of temperature and iron on the growth and physiology of the marine diatom, Phaeodactylum tricornutum (Bacillariophyceae)

Phaedactylum tricornutum Bohlin (Bacillariophyceae) was maintained in expon ential growth under Fe-replete and stressed conditions over a range of temp eratures from 5 to 30 degrees C, The maximum growth rate (CR) was observed at 20 degrees C (optimal temperature) for Fe-replete and stressed cells, Th ere was a gradual decrease in the CR decreasing temperatures below the opti mum temperature; however, the growth rate dropped sharply as temperature in creased above the optimum temperature, Fe-stressed cells grew at half the g rowth rate of Fe-replete cells at 20 degrees C, whereas this difference bec ame larger at lower temperatures, The change in metabolic activities showed a similar pattern to the change in growth rate temperature aside from thei r optimum temperature, Nitrate reductase activity (NRA) and respiratory ele ctron transport system activity (ETS) per cell were maximal between 15 and 20 degrees C, whereas cellspecific photosynthetic rate (P-cell) was maximal at 20 degrees C for Fe-replete cells, These metabolic activities were infl uenced by Fe deficiency, which is consistent with the theoretical predictio n that these activities should have an Fe dependency, The degree of influen ce of Fe deficiency, however, was different for the four metabolic activiti es studied: NRA > P-cell > 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. Tn contrast, the order of influence of decreasing the tempera ture from the optimum temperature was ETS > P-cell > NRA > GR, For NRA, the observed temperature dependency could not be accounted for by the temperat ure dependency of the enzyme reaction rate itself that was almost constant with temperature, suggesting that production of the enzyme would be tempera ture dependent, For ETS, both the enzyme reactivity and the amount of enzym e accounted for the dependency, This is the first report to demonstrate the combined effects of Fe and temperature on three important metabolic activi ties (NRA, P-cell, and ETS) and to determine which activity is affected the most by a shortage of Fe, Cellular composition was also influenced by Fe d eficiency, showing lower chi a content in the Fe-stressed cells. Chi a per cell volume decreased by 30% as temperature decreased from 20 to 10 degrees C under Fe-replete conditions, but chi a decreased by 50% from Fe-replete to Fe-stressed conditions,