Genomic insights into the psychrotrophic microbial leaching of low sulfide waste rock in kinetic testing systems

Abstract

The microbial activity occurring within mine wastes is a known driver of the development and perpetuation of acid generation from mining waste products, including waste rock. The acid generating capacity of waste rock can pose a great environmental liability to surrounding watersheds, which often prompts substantial management of large volumes of waste rock throughout the development, operation, and closure of a mining operation. Kinetic testing systems are a standard means of predicting the acid generating potential of waste rock, but are dependent on abiotic or artificial biological amendments to measure this key parameter. Waste rock from mine sites can be classified as potentially acid generating (PAG), which suggests uncertainty around the acid generating status of the waste rock in question. Temperature is a key control on the weathering of waste rock as the acceleration of abiotic and biotic sulfide oxidation at warmer temperatures can induce faster weathering rates. Nonetheless, the relationship between PAG rock geochemistry in colder regions and its native microbial communities have not been well characterized in the field or in kinetic testing systems. This thesis aims to address the microbial contribution to humidity cell and field leach bin testing systems of low sulfur waste rock native to Boreal climate. A laboratory-based humidity cell experiment was conducted over an extended 28 week period where the microbial community was surveyed over time and compared to weekly leachate geochemistry. The same rock was subjected to a field bin experiment, where the microbial community and leachate geochemistry was profiled over 11 months. Both humidity cell and field bin experiments remained at near neutral conditions over the course of the experiment and exhibited significant neutralization potential. The elemental release rates differed between the humidity cell and field bin experiments, although all experiments predicted the time to acid generation to be under 75 years. Temperature exerted a greater influence on element release in the field bin experiments. Additionally, those elements in the humidity cell experiments that exhibited significant differences in their release rates between temperature treatments were not released at the same rates as they were in the field bin experiments. Microbial communities in both experiments were analyzed through a combination of 16S rRNA gene sequencing and shotgun metagenomics. The original, untreated, waste rock from the mine site contained a microbial community dominated by sulfur oxidizers. After being applied to field and laboratory scale leaching experiments, the initial microbial community transitioned into communities unique to each experiment with clear shifts related to the duration of the experiment and temperature treatments. Both field and laboratory experiments supported microbial communities which were dominated by heterotrophs, with a less abundant community of acidophilic chemolithotrophs that persisted from the initial rock community under warm and cold temperatures. Metagenomic analysis also concluded that microogranisms living on the waste rock have the capability to function in cool temperatures. The findings of this work present a first glimpse into the microbial communities existing on low sulfide waste rock and how they influence the behavior of kinetic testing systems at relevant seasonal temperatures.