We characterized the sulfur geochemistry and microbial community structure of seven circumneutral wastewaters from two Canadian nickel mines collected in summer, winter, and spring, in 2014 and 2015. We also established and characterized sulfur oxidizing enrichments for these wastewater samples in two pH corrals of 7-5 and 5-3. Mine 1 exhibited lower contents of total soluble sulfur compounds and reactive soluble sulfur compounds (oxidation state < + VI) relative to Mine 2. Mine 1 also exhibited greater wastewater microbial community diversity with more unique sequences than Mine 2, resulting in clear NMDS differentiation and Bray-Curtis dissimilarity between the two mines' microbial communities.Proteobacteriadominated all wastewater samples and enrichment communities, ranging between 58-99% of the total of sequences retrieved from the corresponding samples. However, a shift in dominance occurred from primarilyAlphaproteobacteria(28-77%) in the circumneutral wastewater communities toGammaproteobacteria(> 80%) in the moderately acidic enrichment communities. A further pH dependent shift occurred fromHalothiobacillusspp. dominating the pH 7-5 enrichments toThiomonasspp. dominating the pH 5-3 enrichments. These results provide putative biological indicators for better prediction and management of sulfur processes and AMD onset in mining wastewaters.