Growth and ionomic responses of a freshwater cyanobacterium to supplies of nitrogen and iron

Abstract

Cyanobacterial harmful algal blooms (HABs) are increasing in frequency and magnitude worldwide. A number of parameters are thought to underlie HABs, including the ratio at which two key elements, nitrogen (N) and phosphorus (P) are supplied, although a predictive understanding eludes us. While the physiological importance of iron (Fe) in electron transport and N-fixation is well known, relatively little is known about its impacts on the growth of freshwater cyanobacteria. Moreover, there is growing appreciation for correlated changes in the quotas of multiple elements encompassing an organism (i.e. the ionome) when the supply of one element changes, indicating that growth differences arise from complex biochemical adjustments rather than limitation of a key anabolic process by a single element. In this study, the effects of supply N:P and Fe on the growth and ionome of Dolichospermum, a nitrogen-fixing cyanobacterium found in freshwater ecosystems, were examined. Changes in both supply N:P and Fe had significant effects on yield. Consistent with prior observations, cyanobacterial growth was higher at N:P = 20, compared to N:P = 5, and quotas of all elements decreased with growth. Yield was negatively related with the degree of imbalance between dissolved supply and intracellular concentrations of not only N and Fe, but also multiple other elements. Changes in Fe supply had a significant effect on yield in N-limited conditions (N:P = 5). Again, ionome-wide imbalances decreased yield. Together, these results indicate that attention to multiple elements encompassing the ionome of a HAB-forming taxon, and the supplies of such elements may help improve the ability to forecast blooms. Such elemental interactions may be critical as limnologists begin to appreciate the staggering variation in the supplies of such elements among lakes, and anthropogenic activities continue to alter global biogeochemical cycles.