Coupled anaerobic and aerobic microbial processes for Mn-carbonate precipitation: A realistic model of inorganic carbon pool formation

Mn carbonate is the main Mn-II mineral phase that precipitates in suboxic to anoxic environments. The coupled processes of Mn-IV oxide bioreduction and organic oxidation serve as dominant factors leading to Mn carbonate precipitation. This study examined the simultaneous respiration of oxygen and birnessite by a facultatively anaerobic bacterium, the Dietzia strain DQ12-45-1b (45-1b), and discussed the possible mechanism of rhodochrosite precipitation under general oxic environments. Compared to anaerobic experiments, the more rapid growth of 45-1b under aerobic conditions caused faster oxidation of acetate (1.0 x 10(3) mu Mh(-1)) and accumulation of HCO3- (5.5 x 10 2 mu Mh(-1)) within 72 h, which was coupled to a dramatic increase in pH from 7.0 to more than 9.2. By virtue of the higher biomass and bioactivity in the aerobic condition, the bioreduction of Mn IV was accelerated and it caused a higher accumulating rate of soluble reduced Mn (4.0 mu Mh(-1)) than that in the anaerobic condition (2.0 mu Mh(-1)). Those rates indicated that an anaerobic-aerobic sub-interface was present in the aerobic system, in which anaerobic and aerobic respiration co-occurred to give rise to sufficient Mn(II) and alkalinity, thus, increased the supersaturation index (SI) for rhodochrosite. The mineral intermediates and products were identified by time-course XRD, SEM, and Raman spectra. Manganite (MnOOH) was found as the transient intermediate, which suggested the stepwise one-electron transfer mechanism of birnessite reduction. The dialysis tube, lysed cells, dead cells and two-compartment experiments suggested that the living 45-1b not only carried out a direct extracellular electron transfer for birnessite reduction but also provided necessary nucleation sites for rhodochrosite precipitation. Furthermore, both the isotope experiments and Raman analysis showed that the carbon source in rhodochrosite was mainly C-13 isotope-labeled acetate, which corresponded well with the geological isotopic records. Finally, a conceptual model of Mn carbonate precipitation at oxic-suboxic/anoxic interfaces that could be possibly present in soil and sedimentary environments was proposed based on three prerequisites: (i) sufficient Mn(II) produced on an aerobic-anaerobic sub-interface, (ii) adequate alkalinity, and (iii) nucleation sites provided by cell surfaces. This model highlights the role of aerobic respiration in Mn(IV) reduction and Mn-carbonate formation, and may suggest a realistic way for inorganic carbon storage. (C) 2019 Elsevier Ltd. All rights reserved.