Influence of irradiance and temperature on the iron content of the marine diatom Thalassiosira weissflogii (Bacillariophyceae)

The marine diatom Thalassiosira weissflogii (Grun.) Fryxell et Hasle was grown over a wide range of irradiances and temperatures in both NO₃^- and NH₄⁺-amended seawater containing high Fe. Maximum growth rates were not affected by N source, but the initial slope of the growth irradiance curve was significantly greater for the NH₄⁺-grown cells. Light-limited phytoplankton contained 4 times more Fe (µmol Fe mol^-1 C) than light-sufficient cells, and at intermediate irradiances had the highest chlorophyll a concentrations (g C g^-1 chl a). Iron quotas increased as irradiance was reduced because growth rates declined by 20-fold, whereas steady-state Fe transport rates declined by only a factor of 6. Accumulation of Fe was apparently regulated by a light-induced physiological requirement. Similar reductions in growth rate with declining temperature elicited only minor changes in Fe quota, and the increase in Fe quota, was observed only at the 2 lowest temperatures. At irradiance levels between 37 and 150 µmol photons m^-2 s^-1, the relative increase in Fe quota exactly matched the relative increase predicted from the change in abundance of PSI and PSII reported to occur during photoacclimation of T. weissflogii. Subsequent increases in Fe quota at low light may represent additional changes to the photosynthetic apparatus or accumulation of Fe in a non-metabolic pool. The results demonstrate that the Fe content of phytoplankton is influenced by ambient light and imply that the extent of photoacclimation of natural populations may be constrained by the availability of Fe.