Uncovering microbial responses to sharp geochemical gradients in a terrace contaminated by acid mine drainage.

Acid mine drainage (AMD) is harmful to the environment and human health. Microorganisms-mineral interactions are responsible for AMD generation but can also remediate AMD contamination. Understanding the microbial response to AMD irrigation will reveal microbial survival strategies and provide approaches for AMD remediation. A terrace with sharp geochemical gradients caused by AMD flooding were selected to study the microbial response to changes in environmental parameters related to AMD contamination. AMD intrusion reduced soil microbial community diversity and further changed phylogenetic clustering patterns along the terrace gradient. We observed several genera seldom reported in AMD-related environments (i.e., Corynebacterium, Ochrobactrum, Natronomonas), suggesting flexible survival strategies such as nitrogen fixation, despite the poor nutritional environment. A co-occurrence network of heavily-contaminated fields was densely connected. The phyla Proteobacteria, Acidobacteria, Chloroflexi, and Euryarchaeota were all highly interconnected members, which may affect the formation of AMD. Detailed microbial response to different soil characterizations were highlighted by random forest model. Results revealed the top three parameters influencing the microbial diversity and interactions were pH, Fe(III), and sulfate. Various acidophilic Fe- and S-metabolizing bacteria were enriched in the lower fields, which were heavily contaminated by AMD, and more neutrophiles prevailed in the less-contaminated upper fields. Many indicator species in the lower fields were identified, including Desulfosporosinus, Thermogymnomonas, Corynebacterium, Shewanella, Acidiphilium, Ochrobactrum, Leptospirillum, and Allobaculum, representing acid-tolerant bacteria community in relevant environment. The detection of one known sulfate-reducing bacteria (i.e., Desulfosporosinus) suggested that biotic sulfate reduction may occur in acidic samples, which offers multiple advantages to AMD contamination treatment. Collectively, results suggested that the geochemical gradients substantially altered the soil microbiota and enriched the relevant microorganisms adapted to the different conditions. These findings provide mechanistic insights into the effects of contamination on the soil microbiota and establish a basis for in situ AMD bioremediation strategies.