Fast chromium removal by Shewanella sp.: an enzymatic mechanism depending on serine protease

Abstract

Environmental pollutions with heavy metals pose serious health and ecological risks. Sabzevar in the northeast of Iran has natural chromic mines and then chromium-polluted soils and groundwater. In the present work, the metal-tolerant bacterial strain KR2 was identified as Shewanella sp. following 16S rDNA gene sequence analysis. Bioremediation ability of isolated bacterial from agricultural soils that irrigated by groundwater, Shewanella sp., was evaluated for uptaking of chromium with varying Cr(VI) concentrations from 50 to 500 ppm in aerobic conditions (pH 7.0, 37 °C). The Shewanella sp. strain KR2 showed an obvious heavy metal tolerant in the wide range of heavy metals including: Cr6+, pb2+, and Hg2+. After 20 min, the maximum removal of chromium (89%) was obtained by Shewanella sp. in a medium containing 500 ppm Cr(VI). Our results showed that KR2 is able to tolerate high concentration of Cr(VI) through at least two different mechanisms: serine protease overexpression immediately after immersion in high concentration of chromium, and after a long time, an unknown mechanism causes in bacterial deformation, that confirmed with the SEM technique. It is concluded that Shewanella sp. KR2 potentially could be a suitable candidate for heavy metals removal from polluted environment.