Increased sulfate availability in saline water promotes hydrogen sulfide production in fish organic waste

The risk of hydrogen sulfide (H₂S) production can be a challenge in marine land-based recirculating aquaculture systems (RAS). Hydrogen sulfide is a toxic gas that can cause massive fish mortality even at low concentrations, and in addition, serious odour problems in the surroundings. It is a bacterial by-product originating from the degradation of organic matter in sulfur-rich waters such as marine waters. In order to hinder H2S production in marine land-based RAS, more information on the H₂S production conditions and the associated microbiology is needed. In this study, the production of H₂S from rainbow trout (Oncorhynchus mykiss) organic waste was examined using a novel H₂S measurement method under a range of salinities (0, 5, 10, 15, 25 and 35 g/L) in anaerobic mixed reactors, and the microbial communities as well as abundance of sulfate reducing bacteria (SRB) were characterized. The maximum H₂S concentration increased from 23.1 ± 8.2 mg H₂S/L at 0 g/L salinity to 153.9 ± 34.1 mg H₂S/L at 35 g/L salinity. Similarly, the H₂S production rate increased from 5.6 ± 0.2 at 0 g/L salinity to 26.4 ± 12.7 mg of H₂S produced per day at 35 g/L salinity. The highest H₂S production was recorded after increased availability of volatile fatty acids, which were produced by fermentative bacteria from phyla Firmicutes and Bacteroidetes that dominated the microbial communities after day 5. The traditional sulfate reducing bacteria (SRB) were found only at 0 and 5 g/L salinity, while at higher salinities, H₂S production was carried out by novel unquantifiable SRB. The results demonstrate that H2S can be a pronounced problem in marine RAS, although it can be controlled through preventing anaerobic conditions within the system.