Unrecognized role of photosynthetic bacteria in aquaculture water purification: Producing singlet oxygen to degrade residual pharmaceuticals.

Photosynthetic bacteria (PSB) are widely used in the purification of aquaculture waters due to their ability to utilize ammonia, nitrite, hydrogen sulfide, etc. However, PSB are usually considered to be ineffective in removing biologically inert pharmaceutical residues in aquaculture waters. Herein, we found that PSB were capable of degrading pharmaceuticals in aquaculture waters, such as cimetidine and sulfamethazine, by generating extracellular singlet oxygen (1O2) under light irradiation. PSB were highly efficient to produce 1O2, and the quantum yield of 1O2 was four orders of magnitude higher than that of hydroxyl radicals. The efficient production of 1O2 by PSB arose from the photosensitization of extracellular metabolites, which produced 1O2 with an order of magnitude higher quantum yield (0.41) compared to the commonly reported dissolved organic matter (< 0.04) and could efficiently produce 1O2 even under visible light irradiation. The photosensitized extracellular metabolites were mainly hydrophobic metabolites with the molecular weight < 1 kDa, and a porphyrin (i.e., COPRO III) was identified as the dominant photosensitizer for 1O2 production. This work provides new insights into the role of PSB inoculants in aquaculture water purification, and offers new ideas for the removal of pharmaceutical residues from aquaculture waters.