Dear AGU,
Life in submerged karst sinkholes in the Laurentian Great Lakes, where salty groundwater with high-sulfur and low-oxygen is actively venting, is almost exclusively composed of prokaryotic microbes, Bacteria and Archaea, that build colorful benthic mats capable of both photosynthesis and chemosynthesis. Aerobic organisms such as eukaryotic invertebrates and vertebrates are usually excluded from these isolated underwater benthic habitats (ranging anywhere in size from a room to a football field) due to the prevailing low-oxygen conditions.
Exceptions abound though. Diver-collected mats and observations from the fringes of these ecosystems, where the groundwater’s influence is diluted by mixing with well-oxygenated lake water, often reveal a host of eukaryotic protists (e.g., diatoms) and low-oxygen-tolerant invertebrates (e.g., nematodes and tardigrades), and even small vertebrate fish (e.g., gobies). However, it is quite uncommon to find larger mobile fish, such as this Burbot (Lota lota, a member of the Cod family, native to inland waters of North America), in the center of the sinkhole in low-oxygen groundwater. Like human divers, fish may be transient metazoan visitors to this otherworldly microbial world. Modern-day sinkholes with actively venting high-salt, high-sulfur, low-oxygen groundwater remain microbial refugia – providing a glimpse into life’s early salty, sulfurous and anoxic microbial origins.
–Jon Slayer, Force Blue (https://forceblueteam.org); Stephanie Gandulla, NOAA-Thunder Bay National Marine Sanctuary (www.thunderbay.noaa.gov); Steve Ruberg, NOAA-Great Lakes Environmental Research Lab (https://www.glerl.noaa.gov/); and Bopi Biddanda, GVSU-Annis Water Resources Institute, (www.gvsu.edu/wri/).