Simon Mitrovic

Macrophytes may release allelochemicals that suppress competing phytoplankton, although it is unclear how their effectiveness could be influenced by environmental nutrient conditions. To approach this, we conducted a series of bioassays against the bloom‐forming cyanobacterium Microcystis aeruginosa using extracts and exudates from potentially allelopathic macrophytes under varying nutrient conditions. We screened six different tropical macrophytes and identified antialgal effects in both extracts and exudates of Ludwigia adscendens , Persicaria barbata , Pontederia crassipes and Vallisneria spiralis . Growth assays indicated that extract inhibitory effectiveness decreased as nutrient concentration increased from 5% to 50% strength. Focusing on the potent antialgal extract of L. adscendens , we determined that its inhibitory effects were enhanced by specifically reducing the availability of either micronutrient or bicarbonate availability, indicating a role of these nutrients in protecting algal cells from allelochemicals. Bioactivity‐guided fractionation and ultra‐performance liquid chromatography – mass spectrometry analyses of L. adscendens extract indicated that ellagitannins were the major constituent of bioactive extract fractions, suggesting an antialgal mechanism common to other tannic compounds. Novel allelopathic alkaloids and terpenoid compounds also were described, although their mechanisms of action are unknown. Our findings provide evidence that the availability of carbon and micronutrients to cyanobacteria could influence the effects of macrophyte allelochemicals. This interaction between the nutrient environment and allelochemical sensitivity of cyanobacteria can be substantial, and studies or applications involving allelopathic interactions between macrophytes and phytoplankton should consider this.