Uncoupling growth from phosphorus uptake in Lemna: Implications for use of duckweed in wastewater remediation and P recovery in temperate climates.

Phosphorus (P) is an essential nutrient for crop growth and the second most limiting after N. Current supplies rely on P-rich rocks that are unevenly distributed globally and exploited unsustainably, leading to concerns about future availability and therefore food security. Duckweeds (Lemnaceae) are aquatic macrophytes used in wastewater remediation with the potential for nutrient recycling as feed or fertilizer. The use of duckweeds in this way is confined to tropical regions as it has previously been assumed that growth in the colder seasons of the temperate regions would be insufficient. In this study, the combined effects of cool temperatures and short photoperiods on growth and P uptake and accumulation in Lemna were investigated under controlled laboratory conditions. Growth and P accumulation in Lemna can be uncoupled, with significant P removal from the medium and accumulation within the plants occurring even at 8°C and 6-hr photoperiods. Direct measurement of radiolabeled phosphate uptake confirmed that while transport is strongly temperature dependent, uptake can still be measured at 5°C. Prior phosphate starvation of the duckweed and use of nitrate as the nitrogen (N) source also greatly increased the rate of P removal and in-cell accumulation. These results form the basis for further examination of the feasibility of duckweed-based systems for wastewater treatment and P recapture in temperate climates, particularly in small, rural treatment works.