Acidic pH as the deterministic factor shaping microbial community dynamics and interactions in molasses-fed sulfate-reducing bioreactors

Sulfate-reducing bacteria (SRB) are widely used in treating high-sulfate wastewater, but their performance is inhibited by low pH conditions. This study investigated prokaryotic community responses to pH decrements from 5.5 to 2.5 in anaerobic bioreactors using molasses as the carbon source. Effective removal of hydrogen ions (effluent pH > 6.17) and heavy metals (>98%) was achieved at influent pH 3.0–5.5, with performance declining and dropping sharply at pH 2.5. Acid-tolerant SRB genera, including Desulfatirhabdium, Desulfovibrio, and Desulfosporosinus, proliferated below pH 4.0. Microbial network complexity and negative interactions peaked at pH 3.5, while increased positive interactions between SRB and other taxa at pH 3.0 suggested cooperative adaptation to acidity stress. At pH 2.5, elevated modularity and a higher proportion of deterministic processes indicated strong selective pressure. Influent pH and network topological factors were closely correlated with microbial community structure, ultimately influencing bioreactor performance. Overall, this study reveals shifts in microbial assembly mechanisms and interaction patterns in response to declining pH, providing insights to improve SRB-based treatment of acidic wastewater by promoting acid-tolerant SRB groups and fostering cooperative microbial interactions.