## 3 Results and Discussion  
### Distribution of Cr(VI) Reduction Products and Possible Cr(VI) Reduction Mechanism  
Knowledge of the state, form, and location of Cr(VI) treated by _Ochroductrum_ sp. CUST210-1, such as adsorption state, complex formation, or precipitate form, is essential for the development of a suitable remediation process. In addition, the characterization of the product is helpful for understanding the reduction mechanism of Cr(VI) by _Ochroductrum_ sp. CUST210-1. Table 2 lists the distribution of various chromium species inside and outside the cells after the batch treatment. A small amount (3.05%) of CrO\({}_{4}\)\({}^{2}\)- or Cr\({}_{2}\)O\({}_{7}\)\({}^{2}\)- was adsorbed on the positively charged functional group of the cell wall, 8.49% as the Cr(III) form was coordinated with negatively charged functional groups such as carbonyl or amide groups, and 3.12% as the Cr(III) form existed in the solution as a free form; however, most of the Cr(VI) was deposited, inside or outside of the cells, as the Cr(OH)\({}_{3}\) form during the Cr(VI) reduction due to Cr(III) tending to form precipitates at pH > 5 [4]. Table 3 lists the Cr(III) distribution in Cr(III) species of Cr(VI) reduction products after the batch treatment. The results indicate that the exchangeable form of Cr(III), adsorbed Cr(III), organically bound Cr(III), and carbonate-bound Cr(III) on the cell surface accounted for 0.21%, 2.90%, 5.53%, and 0.13%, respectively. Cr(OH)\({}_{3}\) precipitate accounted for 88.01%. In brief, Cr(VI) compounds were usually adsorbed by the cell surface, and Cr(III) may exist in the form of organic-Cr(III) complexes or in the precipitate form of Cr(OH)\({}_{3(5)}\). We speculate that few complex salts occurred in the form of Cr(OH)\({}_{4}\)\({}^{-}\), Cr\({}_{2}\)O\({}_{2}\)(OH)\({}_{4}\)\({}^{2-}\), or Cr\({}_{3}\)O\({}_{4}\)(OH)\({}_{4}\)\({}^{3-}\) in the solution. Moreover, most Cr(OH)\({}_{3(5)}\) was located inside cells, which was similar to results reported for Cr(VI) reduction by _Bacillus amyloidosisatefaciens_, _Synechocystis_ sp., _Cellulosinicrobium funkei_, and _P. aeruginosa_[33; 34; 35; 36] but different from studies identifying extracellular deposits by _Orthobacterium anthropi_, _Vigribacillus_ sp., and _Shewanella oneidensis_[37; 38; 39].  
On the basis of the product analysis results, the following Cr(VI) reduction mechanism or Cr(VI) resistance-tolerance mechanism of _Orthobacterium_ sp. CUST210-1 may be inferred. (1) Partial Cr(VI) directly binds via electrostatic attraction to positively charged functional groups on the surface of the CUST210-1 strain (3.05%), or partial Cr(VI) is reduced to Cr(III) via membrane-associated chromate reductase or soluble chromate reductase (i.e., ChrR) and binds to negatively charged functional groups on the surface of the CUST210-1 strain (8.49%), (2) a portion of Cr(VI) is reduced to Cr(III) by ChrR and (i) remains in the solution as the Cr(OH)\({}_{4}\)\({}^{-}\) form or even as the multinuclear polymers Cr\({}_{2}\)O\({}_{2}\)(OH)\({}_{4}\)\({}^{2-}\) or Cr\({}_{3}\)O\({}_{4}\)(OH)\({}_{4}\)\({}^{3-}\) (3.12%) or (ii) is mainly formed as Cr(OH)\({}_{3(5)}\) outside the cell (26.70%), and (3) most Cr(VI) species enter the cell via the SO\({}_{4}\)\({}^{2-}\) channels then are reduced to Cr(III) via intracellular ChrR and deposited inside the cell as Cr(OH)\({}_{3(5)}\) (58.64%). An analogous mechanism has been discovered in other bacteria, such as P. aeruginosa and S. maltophilia [14; 36].