Iron and iron‐binding ligands as cofactors that limit cyanobacterial biomass across a lake trophic gradient

Abstract

Summary The frequency and intensity of cyanobacterial blooms (cyanoblooms) is increasing globally. While cyanoblooms in eutrophic (nutrient‐rich) freshwater lakes are expected to persist and worsen with climate change projections, many of the ‘new’ cyanobloom reports pertain to oligotrophic (nutrient‐poor) freshwater lakes with no prior history of cyanobloom occurrence. Iron (Fe) is required in nearly all pathways of cyanobacterial macronutrient use, although its precise role in regulating cyanobacterial biomass across a lake trophic gradient is not fully understood. In all lakes sampled representing a gradient in trophic status from oligotrophic to hypereutrophic (2.2–561.2 μg L−1 total phosphorus), the relative cyanobacterial biomass was highest at low predicted Fe bioavailability in eutrophic Alberta lakes (<1.0 × 10−22 mol L−1) and low Fe concentration in oligotrophic Ontario Lakes (<3.2 μg L−1). Fe‐binding organic ligands were measured within this range of low bioavailable Fe. Concentrations of ligands with reactive hydroxamate moieties were positively correlated to cyanobacterial biomass in lakes with low Fe bioavailability and supply, suggesting a possible cellular origin (i.e. siderophores) mediated by low Fe. These findings suggest that Fe serves as a possible cofactor that maintains cyanobacterial biomass across a lake trophic gradient and that cyanobacteria invoke a similar Fe‐scavenging system to overcome Fe limitation in lakes of all trophic states.