Isolation and cadmium adsorption mechanisms of cadmium-resistant bacteria strains in digestive gland of scallops

In order to explore the relationship between the role of bacteria community and the high capacity for accumulation of Cd in the digestive tissues in scallops, two cadmium (Cd)-resistant bacterial strains (named A and B) were isolated from the digestive gland of scallop which were collected from the natural marine environment. Two bacterial strains were classified using 16S rDNA sequence analysis. And the ability of two bacterial strains for the accumulation of Cd and their adsorption characteristics of Cd were researched by the metal adsorption experiments. In addition, the combined technique of scanning electron microscopy (SEM) observation and transmission electron microscopy (TEM) observation was used to observe the cell morphology and inside structure of the bacterial strains under Cd exposure, and the cadmium precipitation and distribution outside and inside the cell were studied to explore the adsorption mechanism of Cd for two bacterial strains. Results showed that the two bacterial strains (A and B) could tolerate Cd²⁺ up to 100 mg/L and 80 mg/L in solid medium respectively. Based on 16S rDNA homology analysis, bacterial A sequences shared high similarities with Nitratireductor sp. and bacterial B was classified as Ruegeria sp. Their adsorption ability of 5 kinds of heavy metal ions (Cd²⁺, Cu²⁺, Mn²⁺, Zn²⁺ and Pb²⁺) were examined and both of them had higher adsorption of Cd than that of other four kinds of metals. In addition, bacterial strain A and B could accumulate Cd to 48.57 mg/g and 42.14 mg/g respectively in 50 mg/L liquid mediums and the accumulation factor for Cd was 971.4 and 842.8 respectively. Scanning electron microscopy (SEM) indicated that the amount of two strains both decreased and empty cavity occurred in some cells after Cd exposure. Obvious Cd precipitates appeared on A strain and more rough surface and irregular convex also appeared on B strain; therefore, extracellular precipitation might be an important way for two bacterial strains to resist and accumulate Cd.