Characterizing iron-nitrogen colimitation interactions in a chemostat model of freshwater environments

Abstract Nitrogen limitation in freshwater lakes is more common than previously understood, suggesting that nitrogen fixation rates by cyanobacteria are often insufficient to alleviate nitrogen deficiency. Nitrogen fixation rates may be restricted by iron bioavailability in pelagic freshwater environments. Continuous culturing techniques allow investigations of phytoplankton physiology at steady state, but simultaneous limitation by two or more nutrients has rarely been explored in continuous culture experiments. Here we report continuous chemostat cultures of Anabaena sp. with various low concentrations of nitrate and iron and demonstrate that iron concentrations below ca. 100 nM are limiting to growth. Addition of nitrate resulted in decreased biomass-specific nitrogen fixation rates and stimulated additional biomass accumulation, confirming that the cultures were iron-nitrogen colimited. Chemostat cultures of cyanobacteria provide an attractive new laboratory model to explore colimitation with great value for measuring the factors restricting nitrogen fixation rates and causing apparent nitrogen limitation in freshwater pelagic environments.