Giant Sulfur-Oxidizing Thioploca Bacteria in Great Lakes Sediments: Where Are They and What Controls Their Abundance?

The giant, filamentous bacteria Thioploca spp. form extensive mats in marine sediments, and as sulfur oxidizers play an important role in marine nitrogen and sulfur cycling. However, high abundances of Thioploca spp. are also found in freshwater sediments, at ambient sulfate concentrations of only tens of micromolar. Little is known about the distribution patterns of freshwater Thioploca or their effects on sediment geochemistry.

In this work, we studied the ecology of Thioploca at 33 sites around the Apostle Islands (Lake Superior) and in Green Bay (Lake Michigan) across a range of depths (9 to 85 m) and water column productivity. Sediment samples were analyzed for Thioploca biomass and vertical distributions, and sediment organic C, N, and S composition, as well as vertical distributions of oxygen, pH and pore water chemistry. We found Thioploca in both the Apostle Islands and Green Bay, with the highest abundances at intermediate water depths (~30 m). Thioploca was most abundant in the top 10 cm of muddy sediments and was scarce in sandy sediments. In this contribution we discuss how the distribution of Thioploca is affected by the sediment porewater chemistry and organic C, N, and S content, and how the cm-scale transport of N and S species facilitated by these giant bacteria may be affecting sediment geochemistry. The inferred ecology of freshwater Thioploca underscores their previously unknown role in the geochemical cycling of the ecologically important elements (C,N,S) in the Great Lakes.