Relationships among photoperiod, carbon fixation, growth, chlorophyll a, and cellular iron and zinc in a coastal diatom

Abstract

We conducted culture experiments with the diatom Thalassiosira pseudonana to determine the interactive effects of day length and biologically available concentrations of iron and zinc on cellular iron (Fe), zinc (Zn), chlorophyll a (Chl a), and fixed carbon (C) throughout the light period. Specific rates of C-fixation and growth were also measured. Specific C-fixation rates showed a single linear relation with the cellular Fe:C ratio regardless of the photoperiod. Decreasing the photoperiod from 14 to 7 h increased the mean daytime cellular Fe:C ratio by 40%, the specific C-fixation rate by 34%, and the Chl a:C ratio by 91% in mildly iron-limited cultures. These changes reflect a cellular acclimation to the shortened light period. The higher cellular iron level apparently allowed for synthesis of additional iron-rich proteins (e.g., those utilized in photosynthetic electron transport) needed to support the increased rate of C-fixation. Mean cellular Chl a concentration decreased linearly with decreasing specific growth rate under iron and zinc limitation, thereby allowing the cells to maintain a balance between light harvesting and biosynthesis. Cellular concentrations of carbon, Chl a, zinc, and iron typically varied during the light period because of the day-night differences in rates of C-fixation, Chl a synthesis, growth, and metal uptake. Cell carbon concentrations increased by 36-96% during the light period, reflecting daytime storage of fixed carbon to support nighttime respiration and growth. Cellular zinc concentrations decreased by 25% during the light period owing to higher daytime specific growth rates and resulting higher rates of biodilution. By contrast, the direction of change in cellular iron concentrations was dependent on the extent of photochemical redox cycling of iron chelates, which increased iron uptake rates during the day. The direction and magnitude of daytime changes in cellular zinc and iron were also dependent on the parameter (cell volume, cell numbers, or carbon) to which the cellular metal was normalized, as each of these parameters exhibited its own unique diurnal pattern.