Microscale effects of oxygen and light on bacterial sulfate reduction in organic-rich lacustrine sediments

Abstract

Biogeochemical cycling involves the exchange of electrons between chemical species through redox reactions. Microorganisms can utilize energy released by redox reactions for their metabolism following a thermodynamic sequence. Among these reactions, the dissimilatory reduction of sulfate (SO_4²⁻) to produce hydrogen sulfide (H₂S) is one of the most important microbially-mediated pathways. The use of microelectrodes at the water-sediment interface provided a better understanding of sulfate reduction processes in the suboxic and anoxic zones. The goal of this study was to assess the microzonation of H₂S and O₂ under different conditions of light and oxygen in the water column. For this purpose, organic-rich lacustrine sediments from Pétrola Lake (SE Spain) were used. Sediment incubations were performed in mesocosm devices. The highest production of H₂S (up to 0.36 nmol/cm^-3s^-1) was observed under anoxic and dark conditions. Production under those conditions was several orders of magnitude higher than those measured when oxygen was present in the water column. Furthermore, the absence of O₂ in the water column significantly altered the microzonation of H₂S in depth. The absence of light seems not to affect the dynamics of O₂ and H₂S in depth. The study contributes to our understanding of microzonation in organic-rich sediments.