Trace metal and nitrogen concentrations differentially affect bloom forming cyanobacteria of the genus Dolichospermum

Abstract

The role of nitrogen and phosphorus in promoting cyanobacterial blooms has been researched for many decades. In contrast, much less is known about the effect of trace metals on regulating cyanobacterial bloom formation. This is particularity relevant for nitrogen-fixing species, as some trace metals act as co-factors in nitrogenase and nitrite reductase enzymes. In the present study, we assessed the response of two nitrogen-fixing species, Dolichospermum lemmermannii and Dolichospermum planctonicum, to copper (Cu), cobalt (Co), molybdenum (Mo), iron (Fe) and nitrogen (N) using a culture-based approach. Response surface modelling showed growth rates in D. planctonicum were positively influenced by increased Co and Fe (p < 0.05), whereas Fe (p < 0.001) and Mo (p < 0.05) increased growth rates in D. lemmermannii. Heterocyst frequency in D. planctonicum was associated with nitrogen (negative response, p < 0.001) and all trace metals tested (positive; p < 0.001 or < 0.05) except Mo. Nitrogen had a strong negative effect (p < 0.001), and Fe a positive (p < 0.01) impact on heterocyst frequency in D. lemmermannii. These results indicate that in addition to regulating nitrogen and phosphorus inputs, management efforts should consider minimising trace metals from entering waterbodies.