Physiological responses of three species of marine pico-phytoplankton to ammonium, phosphate, iron and light limitation

The nitrate and phosphate limitation experiments demonstrated that the small phytoplankton species could grow at low N and P concentrations. Km values were in the micro-molar (NH4+) and sub-micro-molar (PO43−) range. N and P quota were in the femto-molar range per cell and varied from nutrient-deplete to nutrient-replete conditions. Fv/Fm values were only adversely affected at the lowest N and P concentrations in these experiments. In the Fe limitation experiments, it was shown that all three species were adversely affected only at extremely low Fe concentrations. Iron chelating agents had to be added to force the species in Fe limitation till ultimately growth stopped. Km values with respect to dissolved Fe were in the femto-molar range. Fe quota were in the low zepto-molar (10−21 M) range per cell, and varied considerably from Fe limiting to Fe replete growth conditions. Fv/Fm values diminished only at the lowest iron concentrations. In the light limitation experiments, growth rates and photochemical quantum efficiencies were adversely affected only at irradiance levels below 10 μmol photons m−2 s−1. These results indicate that the pico-phytoplankton species will hardly ever be completely stopped in their growth by NH4+, PO43−, Fe or light (separately) under natural conditions.