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 (SO42−) 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.
Highlights
Biogeochemical cycling involves the exchange of electrons between chemical species through reduction and oxidation reactions
Microorganisms can utilize chemical energy released from redox reactions by using the terminal electron acceptors that have the highest reduction potential (Eh) available following a redox reaction sequence [1]
The thermodynamic sequence starts by using oxygen (O2) as an electron acceptor, followed by nitrate (NO3−), manganese and iron oxides, sulfate (SO42−) and carbon dioxide (CO2) in the different redox zones
Summary
Biogeochemical cycling involves the exchange of electrons between chemical species through reduction and oxidation (redox) reactions. The thermodynamic sequence starts by using oxygen (O2) as an electron acceptor, followed by nitrate (NO3−), manganese and iron oxides, sulfate (SO42−) and carbon dioxide (CO2) in the different redox zones (oxic, suboxic, sulfidic, and methanic). This redox sequence establishes a vertical succession of redox reactions driven by microbial respiration beyond oxic-anoxic interfaces (e.g. water-sediment interface). Microelectrodes are used for the analysis of selective physico-chemicals parameters such as H2S These devices consist of small needle-shaped probes with a tip diameter of 1–20 mm that minimally disturb sediments or microenvironments [3]. In fine-grained sediments, mass transfer is governed by diffusion, and local activities can be calculated using diffusion-conversion models [5]
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