Horizontal-flow Constructed Wetlands for the Remediation of Cylindrospermopsin and Microcystin-lr: a Story of Plants, Microbes, and Biodegradation Pathways

Cyanobacterial harmful blooms will be more intense and frequent in the future contaminating surface waters with cyanotoxins, posing a threat in already vulnerable communities heavily reliant on surface water usage for crop irrigation. Constructed wetlands (CWs) are proposed to ensure safe crop irrigation, but more research is needed before implementation. The present study operated 28 mesocosms in continuous mode mimicking horizontal sub-surface flow CWs. Mesocosms were fed with synthetic eutrophic lake water and spiked with two cyanotoxins, microcystin-LR (MC-LR) and cylindrospermopsin (CYN), at environmentally relevant cyanotoxins concentrations (10 µg L-1). The influence of various design factors, including plant species, porous media type, and seasonality, was explored. The mesocosms achieved maximum MC-LR and CYN mass removal rates of 95 % and 98 %, respectively. CYN removal is reported for the first time in CWs mimicking horizontal sub-surface flow CWs. Interestingly, planted-mesocosms consistently delivered higher removal rates for both cyanotoxins. Phragmites australis-planted mesocosms exhibited superior removal efficiency, while Juncus effusus yielded the effluent with the lowest concentration of cyanotoxins due to lower evapotranspiration. Using the P-k-C* model, different scaling-up scenarios were calculated, being the best compromise for future piloting an area of 6 – 36 m2 per m3 of water being treated per day. Additionally, bacterial communities’ structure was analyzed through correlation matrices and differential taxa analyses, offering promising insights into their fundamental removal role. Nevertheless, attempts to validate microcystin-LR biotransformation via the known mlrA gene degradation pathway were unfruitful, indicating alternative enzymatic degradation pathways occurring in such complex CW systems. Further investigation into the precise molecular mechanisms of removal and the identification of transformation products is needed for the comprehensive understanding of cyanotoxin mitigation in CW. All in all, this study points towards the feasibility of horizontal sub-surface flow CWs to be employed for controlling cyanotoxins in irrigation or recreational waters.