Is iron a limiting factor of Nodularia spumigena blooms?

It is well known that a deficiency of iron, a trace element essential to every living organism, limits the growth of algae and cyanobacteria. Nodularia spumigena Mertens is a blue-green algae species inhabiting the Baltic region that often forms toxic blooms. The aim of the study was to assess the growth of the toxic cyanobacteria with respect to iron bioavailability. The measured growth parameters were the numbers of cells (optical density), chlorophyll a and pheopigment a concentrations. The iron concentrations used ranged from 10−7 to 10−4 mol dm−3. Under iron stress conditions (< 5 × 10−7 mol dm−3), growth inhibition, gradual pigment decay and cell mortality were observed. However, enriching the medium with complexing factors like citric acid and EDTA significantly stimulated the growth rate and chlorophyll a production. The citric acid – EDTA – Fe (5 × 10−7 mol dm−3) complex was demonstrably effective in stimulating the rate of cell division. Starting with 10−6 mol dm−3, the higher the iron(III) concentration used in the media, the more intensive the growth of the cyanobacteria populations. This was most rapid in the presence of high iron concentrations (10−4 mol dm−3), regardless of the presence of complexing agents. It appears that the growth of toxic cyanobacteria N. spumigena, and thus also its ability to form blooms, may well depend on iron availability in the environment.