Sulfate-Reducing Bacteria (SRB) and Their Optimal Growth Conditions: pH and Temperature

Alkalinity

Most SRB thrive in a pH range between 5 and 9 [95], [217], [218]. Their growth and sulfate reduction activity are significantly hindered when the pH drops below 5 or rises above 9 [95], [217]. Sharma et al. [219] observed that free ammonia inhibits sulfate reduction under alkaline conditions. Under acidic conditions, strong acidity inhibits SRB activity. Furthermore, hydrogen sulfide and acetic acid, byproducts of SRB metabolism, can severely inhibit other microorganisms, including sulfur-producing bacteria and fermenting bacteria [220].

Acid-resistant SRB have been identified and employed in the treatment of acid mine drainage (AMD) via a sulfur reduction process [221]. Increasing TOC concentrations stimulate sulfur reduction and enrich sulfate-reducing bacteria in extremely acidic environments (pH ≤ 3.5) where Desulfitobacterium sp. CEB3 thrives. Eosinophilic sulfate-reducing bacteria have been successfully used for desulfurization of AMD at pH 3, achieving efficient removal (99%) of copper and zinc [222].

Buffering is widely utilized to stabilize pH within a weakly acidic or neutral range, which is favorable for microbial sulfate reduction. This ensures stable sulfide production and efficient metal removal [179], [223], [224]. Adding alkaline waste can increase pH values. Costa et al. [19] successfully used calcite tailings as a neutralization buffer to elevate pH from 2.5 to 8.4. The addition of calcite tailings not only neutralizes pH but also enhances biological sulfate reduction efficiency [186]. Importantly, SRB can produce alkaline metabolic compounds that can be recycled to balance pH in AMD without the need for additional reagents, significantly reducing operating costs in pilot-scale systems [206].

Examples of SRB with unique characteristics include:

• A sulfate-reducing bacterium with exceptional growth capacity in moderately acidic conditions.
• A novel consortium of acidophilic sulfidogenic bacteria capable of selectively removing transition metals from acidic mine waters.

Temperature

Most SRB are mesophilic or thermophilic anaerobes, with optimal growth temperatures between 28–32 °C. SRB activity declines significantly at low temperatures [225]. As temperatures decrease, the fluidity of cell membranes and the stability of proteins are reduced, leading to decreased activity of membrane proteins and reduced RNA translation efficiency [226]. Lower temperatures also reduce the permeability of cell membranes to substrates, resulting in lower substrate affinity. To overcome this, cold-resistant SRB produce more specific membrane components, such as more unsaturated fatty acids and short-chain fatty acids compared to mesophilic membranes, which facilitate substrate transport at low temperatures [227].

In the FGD-SANI process, organic matter removal efficiency reaches 94% when the temperature drops to around 10 °C. However, when the water temperature plummets to 2 °C, sulfide concentrations rapidly decrease, with a simultaneous accumulation of thiosulfate. This indicates that the reduction of thiosulfate to sulfide is more temperature-sensitive than the reduction of sulfate to thiosulfate at 2 °C [144]. Laura et al. [228] studied the sulfate activity of two SRB strains at temperatures ranging from 9 °C to 30 °C. The cold-tolerant enriched culture exhibited a sulfate reduction rate of up to 6.8 mmol·L−1·d−1 at 9 °C, while the mesophilic enrichment showed an 80% decrease in activity, reaching 2.6 mmol·L−1·d−1. Remarkably, SRB sulfate activity can be restored through temperature revitalization.

High temperatures also negatively impact microbial cell physiology and viability. Heat-induced stress acts as a barrier within and outside the cell, causing alterations in the physical properties of cell membranes. As temperatures increase, the disorder of membrane lipoproteins and membrane permeability increase, potentially leading to cell membrane rupture [229].