Recycling nutrients from greenhouse effluent with a stress-resistant cyanobacterial strain

Hydroponic agriculture is being increasingly utilized for its high productivity. Current wastewater treatment methods lead to loss of phosphate and nitrate, so a future circular economy calls for better methods to treat soilless systems' effluents, allowing reuse of nutrients. Cyanobacteria can use wastewaters as cultivation media and nutrient sources, atmospheric or anthropogenic CO₂ as a carbon source and artificial or sunlight for energy, while producing biomass and valuable products. This proof-of-concept study tests, at a laboratory scale, if the cyanobacterium Synechocystis sp. PCC 6803 can be used to remove nutrients, including nitrate and phosphate, from hydroponic greenhouse effluent (bioremediation), and if the collected Synechocystis biomass can later be used for biofertilization of lettuce (L. sativa). Cultivation in the greenhouse effluent triggered a broad stress response in Synechocystis: photosynthesis and cellular glycogen content decreased, cells flocculated, chaperone and photoprotection genes were upregulated and expression of micro- and macronutrient transporter genes altered. The GT-T control strain grew only slowly in the greenhouse effluent in high CO₂, and removal of nutrients was poor. When the more robust SigB-oe strain was used for bioremediation of the greenhouse effluent, formation of an extra RNA polymerase holoenzyme containing the stress-responsive SigB sigma factor altered gene expression, allowing fast growth and nutrient removal. Heat-treated cyanobacterial biomass could sustain the growth of lettuce seedlings for 21 days in a dose-dependent manner. Success in the use of Synechocystis biomass as a fertilizer, points to potential of cyanobacteria in a bioremediation-biofertilization cycle, warranting further studies.