Effects of short-term manipulations in iron nutrition of Nodularia spumigena from near-coast blooms (southern Baltic Sea)

Randomly collected, dense cyanobacterial bloom material of coastal origin showed a differential response to short-term manipulations of iron availability in microcosms under natural light conditions. Temporally significant iron-mediated promotion of net photosynthetic performance was detected by oxygen saturation and pH measurements in two experiments, although surplus iron was rapidly transformed to insoluble forms even during short incubation periods of 2 days. Stimulating effects of added iron were more pronounced at continuously high incident irradiance. A bi-directional approach of iron manipulation comprising either iron or siderophore addition revealed a medium level of iron starvation in near-coast Nodularia bloom material. Significant stimulation of the quantum efficiency of photosystem II (up to 100%) also indicated photosynthetic performance as a main target of iron fertilization. Missing treatment effects in two other experiments coincided with partially decaying bloom material. Sunscreen effects of iron minerals in fertilized microcosms were unlikely due to insignificant shifts of cellular chlorophyll:carotenoid ratios. The stimulation by iron was most probably linked to physiological photoprotection mechanisms. This link can support the understanding of less consistent iron fertilization effects on the toxic cyanobacterium Nodularia spumigena among the few studies on summer phytoplankton of the Baltic Sea.