Sulfate availability drives the reductive transformation of schwertmannite by co-cultured iron- and sulfate-reducing bacteria.

Schwertmannite (Sch) is a highly bioavailable iron-hydroxysulfate mineral commonly found in acid mine drainage contaminated environment rich in sulfate (SO). Microbial-mediated Sch transformation has been well-studied, however, the understanding of how SO availability affects the microbial-mediated Sch transformation and the secondary minerals influence microbes is relatively limited. This study examined the effect of SO availability on the iron-reducing bacteria (FeRB) and SO-reducing bacteria (SRB) consortium-mediated Sch transformation and the resulting secondary minerals in turn on bacteria. Increased SO accelerated the onset of microbial SO reduction, which significantly accelerated Sch reduction transformation. The extent of intermediate products such as lepidocrocite (22.1 % ~ 76.3 %, all treatments) and goethite (15.3 %, 10 mM SO, 5 d) formed by Sch transformation depended on SO concentrations. Vivianite, siderite and iron‑sulfur minerals (e.g., FeS and FeS) were the dominant secondary minerals, in which the relative content of vivianite and siderite decreased while iron‑sulfur minerals increased with increasing SO concentration. Correspondingly, the abundance of FeRB and SRB was negatively and positively correlated with SO concentration, respectively; 1 mM SO promoted the cymA and omcA expression of FeRB, but 10 mM SO lowerd the cymA and omcA expression compared to the 1 mM SO; the dsr expression of SRB related linearly to the SO concentration. These secondary minerals accumulated on the cell surface to form cell encrustations, which limited the growth and gene expression of FeRB and SRB, and even inhibited the activity of SRB in the 10 mM SO treatment group. The 10 mM SO treatment group with low-intensity ultrasound effectively restored the SRB activity for reducing SO by disintegrating the cell-mineral aggregation, further indicating that cell encrustations limited the microbial metabolism. The results highlight the critical role that SO availability can play in controlling microbial transformation of mineral, and the influence of secondary minerals on microbial metabolism.