Rapid H2S production in layers of freshwater and marine fish organic waste from recirculating aquaculture systems

Hydrogen sulfide (H₂S) is an extremely toxic gas that has been associated with fish mass mortality events in recirculating aquaculture systems (RAS), resulting in substantial economic losses for the aquaculture industry. Previous studies have identified favorable conditions and potential hotspots for H₂S production in RAS under mixed reactors conditions, but knowledge on temporal and spatial dynamics of H₂S production within layers of accumulated organic matter within these systems remains limited. In this study, we investigated H₂S production dynamics at fine spatial (millimeter) and temporal (hourly) scales in accumulated marine and freshwater organic waste collected from RAS, using a high-resolution microsensor setup. In addition to measuring the natural H₂S production potential, we examined the response to the addition of readily available carbon sources and sulfur-rich amino acid cysteine, as well as the microbial communities present in organic waste. In both freshwater and marine organic waste, H₂S appeared only within one centimeter of the accumulated fish organic waste after oxygen was consumed. When this organic waste was disturbed, the accumulated H₂S was rapidly released into the water column and reached concentrations above the toxicity threshold for fish. Through a combination of carbon and cysteine supplementation and 16S rRNA gene analysis, we identified cysteine degradation and sulfate reduction as key microbial pathways for H₂S production in the accumulated organic waste. Overall, this study demonstrates that hydrogen sulfide can form within very thin layers of accumulated fish organic waste in both freshwater and saltwater environments and once released, can reach toxic levels for fish. Therefore, it is crucial to focus on minimizing organic matter accumulation in RAS and implementing contingency measures when the bottom of tanks is disturbed.