Fulfilling iron requirements of a coastal diatom under different temperatures and irradiances

The strategies used by a coastal diatom, Thalassiosira pseudonana, for potentially different iron (Fe) requirements under different temperatures and irradiances were examined on the basis of three parameters: Fe uptake rate, cellspecific growth rate, and Fe efflux rate constant. These three variables determined the cellular Fe concentration, and they were all quantified under different temperatures and irradiances during long-term (days) and short-term (hours) 59Fe exposures. Results obtained from both exposures were consistent. Although more Fe was required under the lower irradiance, Fe uptake rate decreased 1.783 and 2.203 as the irradiance decreased from 340 to 40 mmol photons m22 s21 when measured by shortand long-term exposures, respectively. Under this condition, the cellspecific growth rate decreased from 1.30–1.50 to 0.51–0.63 d21 to keep a relatively high intracellular Fe concentration under lower irradiance. The opposite trend was observed for temperature. The higher Fe requirement at higher temperature was fulfilled mainly through an increase of Fe uptake rate with increasing temperature. For example, the Fe uptake rate increased by 1.213 and 2.553 as the temperature increased from 158C to 248C in the shortand long-term exposures, respectively. In contrast, the cell-specific growth rate was relatively constant (0.92– 1.06 d21) under these three temperatures. Fe efflux from the diatoms was significant, with an efflux rate constant ranging from 0.008 to 0.017 h21. Such a high Fe efflux suggested that it should not be neglected in the calculation of intracellular Fe concentration. Furthermore, Fe efflux might be an important process for Fe regeneration in surface seawater. However, Fe efflux had a negligible effect on the different Fe requirements under different temperatures