Ca2+-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 MnCO3. In this study, we found Ca2+ plays a crucial role in controlling Mn(II) removal in the bacterium Aurantimonas sp. HBX-1. Under conditions with 6.8 mM Ca2+ and without adding Ca2+, 100 mu M Mn(II) was removed by 96.96 % and 38.28 % within 8 days, respectively. Xray photoelectron spectroscopy (XPS) showed that adding Ca2+ 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 Ca2+-supplemented condition, the ratio of MnOx (1 < x <= 2) to MnCO3 was 52% to 28.1 %, while under Ca2+-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 MnCO3. Additionally, animal heme peroxidase (AHP) in strain HBX-1 was found to be responsible for Mn(II) oxidation through superoxide production, with Ca2+ addition promoting its expression level. Given the widespread presence of Ca2+ in wastewater, its potential impact on the biogeochemical Mn(II) cycle driven by bacteria should be reconsidered.