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.
Bibliographical noteFunding Information:
This work was funded by a Grant-in-Aid to T.O. from the University of Minnesota Duluth. S.K. was partially supported by US NSF grant 1754061. We are grateful to Audrey Huff, John Zalusky, Ted Gephart, Julie Halbur, Sandra Brovold, Jiying Li, Isabel Bibeau, and Kirstin Eaker for help in the field and the lab. We also thank Kirill Shchapov, Mojtaba Fakhraee, and Vadym Ianaiev who helped obtain preliminary data on Thioploca distributions and geochemistry in the Apostle Islands sediments.
© 2023 International Association for Great Lakes Research
- Laurentian Great Lakes
- Sediment biogeochemistry
- Sulfur oxidizing bacteria