Acid sulfate soils are described as the nastiest soils on Earth and are broadly composed of metal sulfides. These minerals are mostly harmless in a reducing environment. However, when these soils are drained oxygen infiltration occurs. Oxidation initiates a geochemical process, thereby starting the generation of acidity. As the pH drops, a consortium of acidophilic microbiota begin to grow and flourish. These microbes catalyze the oxidation reactions, which further generates acidity, thus driving the pH even lower. The decreasing pH leads to the solubilization of any co-occurring metals within the system. During flushing events the built-up acidity and solubilized metals mobilize and flow out of the soils into surrounding waterways to the potentially lethal detriment of resident flora and fauna.

This dissertation firstly explores the microbial communities that inhabit acid sulfate soils throughout Sweden and around Vaasa, Finland, and secondly the analogous communities of a mine waste rock repository in northern Sweden. Results from Finland showed an increase in relative abundances of extremely acidophilic microbes correlated to the decreasing pH values that followed the oxidation front. Acidity generation was not mitigated by additions of lime. Further laboratory incubations found that higher volumes and finer material sizes of lime delayed acid generation but did not prevent the development of neutrophilic iron and sulfur oxidizing microbes. The survey of Sweden extended the distribution range of acid sulfate soils and found community differences between the northerly and southerly acid sulfate soils, which were hypothesized to be a result of regional temperature variation. Furthermore, regional differences of the field oxidized samples disappeared following laboratory incubations, further supporting temperature as a driver of regional differences. Lastly, the Swedish waste rock repository study suggested that there were tipping points associated with ongoing oxidation. Subsurface associated communities rapidly decreased following excavation and were slowly replaced by a simple acidophilic community; over time a radiation of acidophiles occurred leading to an increase in acidophile diversity.

These studies together show that metal sulfide rich environments are host to resident neutrophilic to extreme acidophilic microbial communities that play integral roles to the generation of acidity and metals leaching. The composition of those communities differ based on temperature, pH, substrate type, and oxidation age. With regard to remediation strategy development, the application of fine grained lime in combination with peat may hold potential to for short termed acidity mitigation. However cautionis required when transitioning from laboratory based approaches to field trials as the communities are dynamic and complex.