Use of decomposition products of Pontederia crassipes Mart., Solms (water yacinth) for inhibition of cyanobacteria growth: A full-scale study

Eutrophication exacerbates the growth of primary producers that may reduce water quality, such as macrophytes and cyanobacteria. The floating macrophyte Pontederia crassipes and the cyanobacterium Raphidiopsis raciborskii occur worldwide in eutrophic environments, but data on the inhibition of R. raciborskii by floating macrophytes are scarce. We aimed to test the inhibitory activity of P. crassipes on cyanobacterial growth, especially R. raciborskii. First, we tested the effect of live P. crassipes exudates and decomposing macrophyte extracts on a R. raciborskii strain under culture conditions. Cyanobacterial growth was not affected by the exudate from the live macrophyte. In contrast, exposure to decaying macrophyte extracts containing 4.0 and 6.0 mg L−1 total phenols completely inhibited cyanobacterial growth after five days and decreased photosynthetic activity. Mass spectrometry FT-IRC analysis led to the identification of some molecules in P. crassipes extracts, but no association with the inhibitory effect of the was identified. We then tested the inhibitory effect of P. crassipes in mesocosms containing a natural cyanobacterial bloom. The slow release of chemicals from the dry biomass over 12 days decreased chlorophyll concentrations. Finally, in a field experiment, we applied tons of P. crassipes dry biomass to large areas (900 m3) of a reservoir to test the release of chemicals into the water during one month. At the final time, the phenol concentration in the backwater region was close to the inhibitory value found for cyanobacterial growth and an inverse correlation was observed between chlorophyll and phenol concentrations. Thus, under natural conditions, the release of phenolic compounds and other allelochemicals from the decomposing material of macrophyte mats inhibits cyanobacterial growth. Macrophyte biomass, which is usually considered waste, might be used to mitigate cyanobacterial blooms and may be a promising new and sustainable biotechnological tool to improve water quality.