CARBONATES ASSOCIATED WITH CYANOBACTERIAL MATS IN SINKHOLES OF LAKE HURON

At Middle Island Sinkhole (MIS) in Lake Huron, groundwater rich in sulfate and low in oxygen vents from subsurface aquifers and promotes brilliant benthic mats of filamentous purple cyanobacteria. Mats are vertically stratified, with metabolically versatile (oxygenic/anoxygenic) cyanobacteria on top, followed by sulfur-oxidizing and sulfate-reducing bacteria in the lower mat, and methanogens inhabiting sediments below. The mats are not lithified, but carbonates are associated with the mat and underlying sediment, especially within a mineral-rich layer just beneath the cyanobacteria. The unique biological and geochemical characteristics of this system present opportunities to investigate carbonates associated with cyanobacterial mats under conditions relevant to poorly understood periods during the Precambrian (low oxygen, low temperature, sulfate intermediate between modern seawater and freshwater).

X-ray diffraction identified quartz, calcite, and dolomite as the three major mineral phases associated with the mat and underlying sediments. The mineral layer beneath the cyanobacteria is highly enriched in calcite and, to a lesser extent, dolomite. Calcite and dolomite were detected in all sediments depths examined, with abundance of dolomite being more variable than calcite. We hypothesize that metabolic activities within the mat and underlying sediments influence carbonate formation and dissolution. Two approaches are underway to investigate potential microbe-mineral interactions. First, scanning electron microscopy is being used to examine spatial associations between carbonate minerals and bacterial cells. Second, by analyzing the carbon isotopic composition of the full suite of carbon-bearing biological (δ¹³C_org), mineral (δ¹³C_carb), and liquid pore water (δ¹³C_DIC) reservoirs, we are evaluating the role of different metabolic pathways in facilitating sub-mat carbonate precipitation. Finally, these findings will be placed into the context of the spatial distribution of microorganisms, metabolism, and geochemical gradients, which we have observed to vary dramatically over diel cycles due to variable light, photosynthesis, sulfur metabolism, and active vertical migration of cyanobacteria and sulfur-oxidizing bacteria.