Autochthonous tropical groundwater bacteria involved in manganese(ii) oxidation and removal

The presence of manganese (Mn) in drinking water causes aesthetic, operational, and health problems. Removal of Mn(ii) by the use of Mn oxidizing microorganisms in biofiltration processes has been studied leading to a more economical and environmentally friendly process than conventional treatments with chemical products. There are multiple reports in temperate regions, but little information under tropical conditions, concerning Mn(ii) removal by microorganisms. In this study, 13 autochthonous culturable manganese oxidizing bacteria isolated from water of two wells with ∼0.6 mg L-1 and ∼0.2 mg L-1 Mn(ii), respectively, were evaluated in vitro to determine their potential as inoculums for application in biofiltration processes. The oxidation capacity of Mn(ii) was evaluated by a qualitative test with Leucoberbelin blue I and the Mn(ii) removal was quantified by measurements of the residual Mn(ii) in a culture broth with an initial concentration of 5.9 mg L-1. The biofilm formation capacity of the strains was also determined by crystal violet staining and by quantification of polysaccharides of extracellular polymeric substances. Three strains showed the highest percentage of Mn(ii) removal in the culture in broth, two belonging to the genus Aeromonas (HAC-12 and RUE-1), with removal values of 42.5% and 40.1%, respectively, and one belonging to the genus Stenotrophomonas (S-3), with 40.3% removal. Besides, HAC-3B and RUE-6A showed the highest production of exopolysaccharides. These results allowed the selection of an Aeromonas strain (HAC-12) and one Stenotrophomonas strain (S-3) as possible candidates to be used as inoculums in biofilters for the removal of Mn(ii) from groundwater.