Impact of seasonal hypoxia on activity and community structure of chemolithoautotrophic bacteria in a coastal sediment

Seasonal hypoxia in coastal systems drastically changes the availability of electron acceptors in bottom water, which alters the sedimentary reoxidation of reduced compounds. However, the effect of seasonal hypoxia on the chemolithoautotrophic community that catalyzes these reoxidation reactions is rarely studied. Here, we examine the changes in activity and structure of the sedimentary chemolithoautotrophic bacterial community of a seasonally hypoxic saline basin under oxic (spring) and hypoxic (summer) conditions. Combined 16S rRNA gene amplicon sequencing and analysis of phospholipid-derived fatty acids indicated a major temporal shift in community structure. Aerobic sulfur-oxidizing <i>Gammaproteobacteria</i> (<i>Thiotrichales</i>) and <i>Epsilonproteobacteria</i> (<i>Campylobacterales</i>) were prevalent during spring, whereas <i>Deltaproteobacteria</i> (<i>Desulfobacterales</i>) related to sulfate-reducing bacteria prevailed during summer hypoxia. Chemolithoautotrophy rates in the surface sediment were three times higher in spring than in summer. The depth distribution of chemolithoautotrophy was linked to the distinct sulfur oxidation mechanisms identified through microsensor profiling, i.e., canonical sulfur oxidation, electrogenic sulfur oxidation by cable bacteria, and sulfide oxidation coupled to nitrate reduction by <i>Beggiatoaceae</i>. The metabolic diversity of the sulfur-oxidizing bacterial community suggests a complex niche partitioning within the sediment, probably driven by the availability of reduced sulfur compounds (H₂S, S⁰, and S₂O₃²⁻) and electron acceptors (O₂ and NO₃⁻) regulated by seasonal hypoxia