Widespread occurrence of filamentous Thioploca bacteria in low-sulfate Great Lakes sediments with implications for sulfur and nitrogen cycling

Thioploca spp. are large sulfur oxidizing bacteria that form centimeter-scale filaments in aquatic sediments. Marine Thioploca can form dense mats that strongly affect biogeochemical cycling in sediments. While freshwater and brackish Thioploca can also achieve high abundances, their ecology is poorly understood and their metabolisms in low-sulfate environments are enigmatic. We quantified freshwater Thioploca biomass and its links to environmental characteristics at 33 sites in the Great Lakes in the Apostle Islands region (Lake Superior) and Green Bay (Lake Michigan). Vertical distributions of Thioploca and the associated sediment chemistries were analyzed in sediment cores from eight sites. Despite low sulfur availability, Thioploca attained biomasses comparable to those in marine sediments, up to 250 g m⁻² wet weight. Abundances were highest in the top 5 cm of sediment and were greater in fine-grained than coarse-grained sediments. Thioploca was common at both eutrophic and oligotrophic locations, suggesting that sediment carbon availability was not a limiting factor. Porewater profiles indicated possible ammonia oxidation in the anoxic sediment layers occupied by Thioploca, which would argue against the dissimilatory nitrate reduction to ammonium (DNRA) route of nitrate reduction common in marine Thioploca. With known metabolic capabilities for coupling oxidation of sulfide or thiosulfate to denitrification, freshwater Thioploca may play a quantitatively important role in the benthic cycling of nitrogen and sulfur, though in ways that differ from those of marine Thioploca species. Our results indicate that Thioploca may be widespread in depositional zones of the Great Lakes, with yet underappreciated effects on local elemental cycling.