Anaerobic sulfur oxidation in the absence of nitrate dominates microbial chemoautotrophy beneath the pelagic chemocline of the eastern Gotland Basin, Baltic Sea

Oxic-anoxic interfaces harbor significant numbers and activity of chemolithoautotrophic microorganisms, known to oxidize reduced sulfur or nitrogen species. However, measurements of in situ distribution of bulk carbon dioxide (CO₂) assimilation rates and active autotrophic microorganisms have challenged the common concept that aerobic and denitrifying sulfur oxidizers are the predominant autotrophs in pelagic oxic-anoxic interfaces. Here, we provide a comparative investigation of nutrient, sulfur, and manganese chemistry, microbial biomass distribution, as well as CO₂ fixation at the pelagic redoxcline of the eastern Gotland Basin, Baltic Sea. Opposing gradients of oxygen, nitrate, and sulfide approached the detection limits at the chemocline at 204 m water depth. No overlap of oxygen or nitrate with sulfide was observed, whereas particulate manganese was detected down to 220 m. More than 70% of the bulk dark CO₂ assimilation, totaling 9.3 mmol C m⁻² day⁻¹, was found in the absence of oxygen, nitrite, and nitrate and could not be stimulated by their addition. Maximum fixation rates of up to 1.1 μmol C L⁻¹ day⁻¹ were surprisingly susceptible to altered redox potential or sulfide concentration. These results suggest that novel redox-sensitive pathways of microbial sulfide oxidation could account for a significant fraction of chemolithoautotrophic growth beneath pelagic chemoclines. A mechanism of coupled activity of sulfur-oxidizing and sulfur-reducing microorganisms is proposed.