Comment on bg-2023-62

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> The surface of intertidal estuarine sediments is typically covered with a photosynthetic biofilm. A large fraction of the carbon that is fixed is in the form of exopolymeric substances (EPS), providing the biofilm matrix. The consumption of organic carbon within the sediment column by heterotrophs bacteria is stratified according to the availability of electron acceptors used for organic matter degradation. This sequential use of electron acceptors strongly impacts geochemical gradients and early diagenetic processes within the sediment. In most studies, the distribution and role of the predominant microbial metabolisms is deduced from porewater chemistry and restricted to the upper decimeters of the sediment column, but rarely from direct measurements of microbial activity, potentially leading to erroneous conclusions of biogeochemical processes. We measured geochemical gradients in three estuarine sediment cores to a depth of 6 meters. Geochemical analyses of porewater and sediment were combined with measurements of microbial activity. In situ microelectrode measurements were performed for pH, oxygen and sulfide. Porewater was extracted and analyzed for major elements using Ion Chromatography, Inductively-Coupled-Plasma, and colorimetric assays for iron speciation. Porewater chemistry was compared to measurements of microbial activity including isothermal calorimetry and metabolic assays (triphenyltetrazolium chloride (TTC) and fluorescein diacetate (FDA)) and concentrations of EPS (sugars, proteins) measured in a previous study on the same cores. Finally, sediment composition was characterized through X-Ray Fluorescence core scanning. Results show that: (i) aerobic respiration occurred between 0 and 1 cm, (ii) nitrate reduction between 6 and 16 cm, (iii) sulfate reduction between 10 and 50 cm, (iv) manganese oxide reduction between 2&ndash;6 and 35&ndash;50 cm and (v) iron oxide reduction between 16&ndash;18, 24&ndash;26 and 35&ndash;45 cm. This is concomitant with the area where the microbial activity is the highest. In contrast to the literature, we conclude that some reactions, for example sulfate and nitrate reduction, were locally coupled or at least occurred concomitantly. Impacts of microbial metabolism on early diagenesis have been modeled via PhreeQc and predicted potential precipitation of metastable iron and/or sulfides. This is confirmed by iron and sulfur increases in sediments characterized through XRF. All these observations have been used to propose a biogeochemical model linking microbial metabolisms and early diagenesis that can be used as a basis for the study of other geochemical profiles in the future.