Ca-controlled Mn(II) removal process in Aurantimonas sp. HBX-1: Microbially-induced carbonate precipitation (MICP) versus Mn(II) oxidation.

The application of manganese-oxidizing bacteria (MnOB) to produce manganese oxides (MnOx) has been widely studied, but often overlooking the concurrent formation of MnCO. In this study, we found Ca plays a crucial role in controlling Mn(II) removal in the bacterium Aurantimonas sp. HBX-1. Under conditions with 6.8 mM Ca and without adding Ca, 100 μM Mn(II) was removed by 96.96 % and 38.28 % within 8 days, respectively. X-ray photoelectron spectroscopy (XPS) showed that adding Ca increased the average oxidation state (AOS) of the solid products from 2.05 to 2.37. X-ray absorption fine structure (XAFS) analysis revealed the product proportions as follows: under Ca-supplemented condition, the ratio of MnOx (1 < x ≤ 2) to MnCO₃ was 52 % to 28.1 %, while under Ca-free condition, the ratio shifted to 4.6 % for MnOx (1 < x ≤ 2) and 55.2 % for MnCO₃. Urease activity assay and proteomic analysis confirmed the expression of urease and carbonic anhydrase, leading to the formation of MnCO. Additionally, animal heme peroxidase (AHP) in strain HBX-1 was found to be responsible for Mn(II) oxidation through superoxide production, with Ca addition promoting its expression level. Given the widespread presence of Ca in wastewater, its potential impact on the biogeochemical Mn(II) cycle driven by bacteria should be reconsidered.