Effect of high iron concentrations on iron uptake and growth of a coastal diatom Chaetoceros sociale

The growth and iron uptake of the coastal marine diatom Chaetoceros sociale were experimentally measured in batch experiments at 10°C to which an acidic Fe(III) stock solution was added. The direct input of Fe(III) into the culture media induced the highest iron uptake rate (~3.4 to 4.2 × 10–16 mol Fe cell–1 d–1) by C. sociale during the first day of the incubation, resulting from the supply of bioavailable inorganic Fe(III) species at levels above its expected equilibrium value (~0.1 nmol l–1) with solid amorphous Fe(III) hydroxide in seawater. The iron uptake rate during the first day of incubation in solid amorphous Fe(III) hydroxide medium aged for 1 d at 10°C was approximately 50% lower than that in the direct Fe(III) input media. We used a modified approach in which further iron uptake by C. sociale from external iron in the direct Fe(III) input media was prevented by adding hydroxamate siderophore desferrioxamine B (DFB) during cultivation. After the addition of DFB, the highest growth rate (~0.5 to 0.6 d–1) of C. sociale by intracellularly stored Fe in the direct Fe(III) input media was maintained for a few days since no iron uptake was observed after the DFB addition. The growth rate was independent of the amount of intracellularly stored Fe. However, the maximal cell yields appeared to be relatively dependent on the amount of intracellularly stored Fe, suggesting the presence of a critical concentration of intracellular Fe (minimum cellular Fe for growth) for phytoplankton growth (~1 × 10–16 mol Fe cell–1 for C. sociale). In the present study, maximal and minimal Fe quotas were 3.4 to 4.2 × 10–16 and ~1 × 10–16 mol Fe cell–1 (the maximal/minimal Fe ratio of 3.4 to 4.2), respectively. The high iron uptake and storage capacity in C. sociale allows this species to accumulate excess iron at high concentrations of bioavailable inorganic Fe species and to support up to 1.8 to 2.1 cell divisions without any additional iron uptake. In addition, we attempted to model the effect of luxury uptake on growth, as biodilution of cellular Fe eventually decreases the Fe quota to a critical threshold