Interactions Between Sulphur-Oxidizing Bacteria and Sediment Chemistry in Brackish Water Ecosystems

Brackish water ecosystems like estuaries and lagoons are subject to high organic matter loads and the sulphide produced by microbial sulphate reduction in the sediments can build up to toxic levels, diffuse to the overlying waters and exacerbate hypoxia. Biological barrier to this sulphide consists primarily of sulphur-oxidizing bacteria (SOB), but their relationship with the sediment chemistry along the salinity gradient is not well quantified. A one-dimensional reaction–transport model (1 D RTM) of an 8 cm nonbiological column of brackish sediment was used to investigate the distribution of oxygen, sulphide, and sulphate for three functional configurations (abiotic – only oxidation, canonical aerobic SOB, and SOB with filamentous cable bacteria) at 3, 8, 15 and 25 under normoxic (250 μmol L⁻¹) and hypoxic (60 μmol L⁻¹) bottom water. The salinity increases from 3 to 25 led to an increase in sulphate reduction from 3.5 to 6.6 mmol m⁻² d⁻¹, and in maximum porewater sulphide from 350 to 702 μmol L⁻¹. If not for biological oxidation, there would have been overlap in the presence of oxygen and sulphide for 13.5 mm and approximately 71% of sulphide would have been lost to the water column. Aerobic SOB oxidized 82% of the sulphide, reduced the oxic layer to 4.4 mm and reduced the sulphide efflux by approximately 800-fold. Cable bacteria accounted for up to 83% of sulphide oxidation and achieved 82% reduction of peak sulphide. Hypoxia caused a significant decrease in buffering capacity in canonical SOB while the electrogenic pathway also reduced sulphide efflux by 84%. The findings suggest that the composition of sulphur-oxidising community – not salinity per se – determines whether brackish sediments are sulphide sinks or sources.