Microbial reduction of schwertmannite by co-cultured iron- and sulfate-reducing bacteria

Schwertmannite (Sch) is an iron-hydroxysulfate mineral commonly found in acid mine drainage contaminated envi-ronment. The transformation mechanism of Sch mediated by pure cultured iron-reducing bacteria (FeRB) or sulfate -reducing bacteria (SRB) has been studied. However, FeRB and SRB widely coexist in the environment, the mechanism of Sch transformation by the consortia of FeRB and SRB is still unclear. This study investigated the Sch reduction by co -cultured Shewanella oneidensis (FeRB) and Desulfosporosinus meridiei (SRB). The results showed that co-culture of FeRB and SRB could accelerate the reductive dissolution of Sch, but not synergistically, and there were two distinct phases in the reduction of Sch mediated by FeRB and SRB: an initial phase in which FeRB predominated and Fe3+ in Sch was reduced, accompanied with the release of SO42-, and the detected secondary minerals were mainly vivianite; the sec-ond phase in which SRB predominated and mediated the reduction of SO42-, producing minerals including mackinawite and siderite in addition to vivianite. Compared to pure culture, the abundance of FeRB and SRB in the consortia decreased, and more minerals aggregated inside and outside the cell; correspondingly, the transcription levels of genes (cymA, omcA, and mtrCBA) related to Fe3+ reduction in co-culture was down-regulated, while the tran-scription levels of SO42--reducing genes (sat, aprAB, dsr(C)) was generally up-regulated. These phenomena suggested that secondary minerals produced in co-culture limited but did not inhibit bacterial growth, and the presence of SRB was detrimental to dissimilatory Fe3+ reduction, while existed FeRB was in favor of dissimilatory SO42- reduction. SRB mediated SO42- reduction by up-regulating the expression of SO42- reduction-related genes when its abundance was limited, which may be a strategy to cope with external coercion. These findings allow for a better understanding of the process and mechanism of microbial mediated reduction of Sch in the environment.