The microbes purify high concentration of metaboric acid water on the microbial mats of Matsushiro Hot Springs, Nagano Prefecture, Japan

Abstract

Matsushiro Hot springs in Nagano Prefecture, Japan is known as one of the hot springs in which the concentration of boron (B) is quite high. The hot spring water drainage ditch was covered with dark brown microbial mats. Before the drainage meets the river, the microbial mats deposit is more than 2m in depth. The microbial mats deposit has two layers. In the upper parts, dark brown microbial mats covered the surface. Underneath are black microbial mats which make up a portion of the two layer structure. At the junction of the drainage and river water, only black microbial mats occurred. The aim of this study was to reduce the boron concentration of the drainage water with microbial mats at Matsushiro Hot Springs. ASS analysis showed that microbial mats collected from five different points contain 854mg B/kg, 8628mg Fe/kg, 120mg As/kg, and 912mg Mg/kg on average. ED-XRF analysis showed that all microbial mats contained more than 50wt% of Ca. XRD analysis and FT-IR spectra of the microbial mats indicated the presence of calcite. Observations by optical and electron microscopy showed that cyanobacteria inhabited dark brown microbial mats, whereas cyanobacteria and diatoms inhabited in black mierobial mats. Cyanobacteria use an enzyme named carbonate anhydrase (CA). HCO3- dissolved in the hot spring water was broken down into CO2 and OH- by CA. That OH- made temporary alkaline conditions around the cyanobacteria. Such alkaline conditions facilitate the break down of HBO2 into H+ and BO2-. BO2- can form many chemical compounds. The concentration of boron described in this study might have profound implications for bioremediation of boron contaminated sites.