Bacterial chemoautotrophy in coastal sediments

A key process in the biogeochemistry of coastal sediments is the reoxidation of reduced intermediates formed during anaerobic mineralization which in part is performed by chemoautotrophic bacteria. These bacteria fix inorganic carbon using the energy derived from reoxidation reactions. However the importance and distribution of chemoautotrophy has not been systematically investigated in coastal environments. To address these issues, diverse coastal sediments were surveyed by means of bacterial biomarker analysis (phospholipid derived fatty acids) combined with stable isotope probing (¹³C-bicarbonate). Sediment chemoautotrophy rates from this study and from literature (0.07 to 36 mmol C m⁻² d⁻¹) showed a power-law relation with benthic oxygen uptake. Benthic oxygen uptake serves as a proxy for carbon mineralization as such the ratio of the CO₂ fixed by chemoautotrophy over the total CO₂ released through mineralization can be estimated for sediments from the continental shelf (3%), nearshore (9%) and salt marsh (21%). Therefore chemoautotrophy plays an important role in C-cycling in reactive intertidal sediments (e.g. salt marsh) rather than in the organic-poor, permeable shelf sediments. Moreover, five coastal sediment regimes were linked to the depth-distribution of chemoautotrophy: 1) permeable sediments dominated by advective porewater transport, 2) bioturbated sediments, and cohesive sediments dominated by diffusive porewater transport characterized by either 3) canonical sulfur oxidation, 4) nitrate-storing Beggiatoa, or 5) electrogenic sulfur oxidation. The depth-distribution of chemoautotrophy hence varies due to biogeochemical characteristics such as grain size, organic carbon content, presence of filamentous sulfur oxidizing bacteria, and macrofaunal activity in coastal environments.