Iron is not responsible for Didymosphenia geminata bloom formation in phosphorus-poor rivers

Abstract

Blooms of the river benthic diatom Didymosphenia geminata are an enigma because they occur under phosphorus-poor conditions. A recent proposal that ferric–ferrous iron redox shifts sequester the additional phosphorus needed to stimulate and sustain D. geminata blooms does not agree with published experimental data showing that blooms only occur when cells are phosphorus-limited. The “iron hypothesis” also infers that blooms would be favoured in rivers with elevated iron, and management should target iron. Surveys of rivers around the world affected by D. geminata show that blooms most often occur in iron-poor rivers. Phosphorus uptake experiments conducted under realistic environmental conditions with living D. geminata colonies showed no effect of iron enrichment on phosphorus uptake. Iron does not solve the mystery of D. geminata growth causing nuisance blooms worldwide.