Groundwater microbial communities reflect geothermal activity on volcanic island

Studies of the effects of volcanic activity on the Hawaiian Islands are extremely relevant due to the past and current co-eruptions at both Mauna Loa and Kīlauea. The Big Island of Hawai'i is one of the most seismically monitored volcanic systems in the world, and recent investigations of the Big Island suggest a widespread subsurface connectivity between volcanoes. Volcanic activity has the potential to add mineral contaminants into groundwater ecosystems, thus affecting water quality, and making inhabitants of volcanic islands particularly vulnerable due to dependence on groundwater aquifers. As part of an interdisciplinary study on groundwater aquifers in Kona, Hawai'i, over 40 groundwater wells were sampled quarterly from August 2017 through March 2019, before and after the destructive eruption of the Kīlauea East Rift Zone in May 2018. Sample sites occurred at great distance (~80 km) from Kīlauea, allowing us to pose questions of how volcanic groundwater aquifers might be influenced by volcanic subsurface activity. Approximately 400 water samples were analyzed and temporally split by pre-eruption and post-eruption for biogeochemical analysis. While most geochemical constituents did not differ across quarterly sampling, microbial communities varied temporally (pre- and post-eruption). When a salinity threshold amongst samples was set, the greatest microbial community differences were observed in the freshest groundwater samples. Differential analysis indicated bacterial families with sulfur (S) metabolisms (sulfate reducers, sulfide oxidation, and disproportionation of S-intermediates) were enriched post-eruption. The diversity in S-cyclers without a corresponding change in sulfate geochemistry suggests cryptic cycling may occur in groundwater aquifers as a result of distant volcanic subsurface activity. Microbial communities, including taxa that cycle S, may be superior tracers to changes in groundwater quality, especially from direct inputs of subsurface volcanic activity.