The Effects of Seasonal Variations In Chemistry and Hydrology on the Microbial Community and Its Sulfide Oxidation Potential In a Naturally Acidic Maine Stream

Sulfide minerals oxidize through interaction with water and oxygen, releasing hydrogen ions. The process often occurs naturally near metal sulfide deposits, and can be accelerated through mining. Microorganisms accelerate the rate of sulfide oxidation. Acidified streams typically contain high metal concentrations (e.g. aluminum) and microbes in these systems may develop resistances to metal toxicity. Stream flow can affect sulfide oxidation and microbial community structure. Baseflow can influence stream chemistry from interactions with the surrounding bedrock, while stormflow affects stream chemistry and the local microbial community through dilution and addition of microbes transported by runoff. Microbial community composition is affected by seasonal shifts in water chemistry and climate conditions like temperature and precipitation. Little work has addressed the effects of acidification upon the biogeochemistry in streams located near sulfide deposits in northern New England and similar ecosystems. Maine has numerous metallic sulfide deposits. It is important to understand what microbes are present in aqueous systems near these deposits, the effects of chemistry and climate on their structure, and their sulfide oxidation potential within acidifying conditions in case one of these deposits were ever to be mined. We conducted a field study to assess community structure and its relationships with changing seasonal chemistry and hydrology within a naturally acidic stream Blood Brook. In the field, Blood Brook chemistry and microbial community was sampled and studied across an 8-month period of seasonal transition. We found that the community was resembled a typical community, with a small population of circumneutral iron-oxidizing bacteria. Changes in the microbial community structure were primarily driven by changes in stream flow throughout the study period, with stormflow overall increasing diversity. A series of three, five-week, batch reactor experiments were also conducted to assess changes within the experimental community exposed to increased amounts of pyrite, and to assess how its sulfide oxidation potential changed from differing sampling conditions. Experiments were conducted using an abiotic and biotic treatment. During the experiments, we found no significant differences between abiotic and biotic sulfate concentration changes, but there were significant differences in pH changes between treatments. Microbial community analyses of the experimental solutions revealed that there were limited classified sulfide or iron oxidizing bacteria, despite precipitate evidence of circumneutral iron oxidizing bacteria. These data suggest that there is limited bacterial sulfide oxidation occurring, and that something else was driving biological pH changes. Dominant in the final communities were genera Acidocella and Acidisoma, indicating the acidic conditions drove the microbial community to become an acidophilic one. Precipitate observations revealed structures resembling those produced by previously identified circumneutral iron oxidized bacteria. The final experimental communities resembled those that have been observed in circumneutral iron-rich groundwater and surface water communities. We concluded that the Blood Brook microbial community is primarily a stream community, whose structure is primarily influenced in changes in hydrologic flow conditions. We also conclude that under experimental acidifying conditions, changes in the community are primarily driven by decreases in pH and increases in specific conductance