VARIATIONS IN CHEMISTRY AND HMICROBIAL COMMUNITY STRUCTURE IN A NATURALLY ACIDIC MAINE STREAM

Sulfide minerals oxidize through interaction with water and oxygen, releasing hydrogen ions and lowering local pH. The process occurs naturally near sulfide deposits and can be accelerated through mining. Microorganisms also accelerate the rate of sulfide oxidation. Acidified streams often contain high metal concentrations, and microbes in these systems may develop resistance to metal toxicity. Microbial community composition is affected by seasonal shifts in water chemistry and climate conditions like temperature and precipitation. Little work has addressed the effect of acidification on the biogeochemistry of streams located near sulfide deposits in colder climates such as northern New England. We conducted a field study to assess community structure and its relationship to changing seasonal chemistry and hydrology within the naturally acidic stream Blood Brook near Brownville Junction, Maine. Blood Brook chemistry and microbial community were sampled across an 8-month period. We found that the community resembled a typical stream 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 increasing overall diversity.

A series of five-week batch reactor experiments was also conducted to assess changes in the experimental community in contact with pyrite. We found no significant differences between sulfate in abiotic and biotic experiments, but there were significant differences in pH changes between treatments. Microbial community analyses of final experimental solutions showed limited classified sulfide or iron oxidizing bacteria, despite likely precipitate evidence of circumneutral iron oxidizing bacteria observed via SEM. These data suggest that limited bacterial sulfide oxidation is occurring, and that something else drives biological pH changes. The genera Acidocella and Acidisoma were dominant in the final communities, indicating that decreasing pH drove the microbial community to become acidophilic.