Isolation And Identification Of Humic-reducing Bacteria In Marine Sediment And Its Application In Cr(Ⅵ) Reduction

Humic substances(HS), ubiquitous in the environment, are a kind of high-molecular-weight aromatic polymer with carbonyl structure. Recently, it has been reported that microorganisms in anaerobic conditions can involve HS into the respiratory metabolism and to have an important role in the biogeochemical circulation of elements and the biodegradation of pollutants. However, previous studies about this kind of microorganisms mostly focused on the terrestrial environment. In this paper, sediments from marine ecosystem was used as experimental materials. The distributions of the humic-reducing bacteria in northern South China Sea were preliminaryly investigated. Subsequently, the isolation and identification of the exactly humic-reducing bacteria were examined. Meanwhile, the interactional relationships between the HS and Cr(VI) reduction were also explored. The main results were listed as follows:(1) The investigatin to the distributions of the humic-reducing bacteria in northern South China Sea showed that the humic-reducing bacteria were ubiquitous in sediments of northern South China Sea, but the amount of the humic-reducing bacteria in each sampling sites were different. (2) Two humic-reducing bacteria, strain W3 and strain C3, were isolated from the MaiPo Mangrove sediment of Hong Kong. Based on the analysis of morphology, physiobiochemical characters and 16S rDNA gene sequence, the two strains were identified as Shewanella sp. and Bacillus sp. respectively. (3) The characters of humic reduction by strain W3 and strain C3 demonstrated that strain W3 preferred to use lactate and formate as electron donor. In addition, strain W3 could conserve energy to support growth from lactate oxidation coupled to humic reduction. Whereas, strain C3 could effectively reduce humic analog AQDS and AQS with lactate, formate and glucose as electron donor, but strain C3 was not able to benefit with cell growth from the reduction of humics. (4) The humic-reducing processes about the two strains were studied under various environmental conditions of different pH, salinity and temperature. The results indicated that the optimal initial pH for strain W3 and C3 reducing humics was pH 7-9. Compared with strain C3, the humic reduction by strain W3 can tolerate higher salinity concentration. The optimum temperature for humic reduction by strain W3 was relatively wide. Strain W3 can still have high rate of humic reduction at 40℃, whereas strain C3 can not. (5) Cr(VI) was toxic for strain C3, but the resistance capability of strain C3 to Cr(VI) was very strong. In presence of AQDS or AQS, Cr(VI) reduction by strain C3 was enhanced greatly. In general, AQDS was superior to AQS on Cr(VI) reduction, and the humic concentration also had great effect on Cr(VI) reduction.