| With the rapid development of modern industry, agriculture and transportation, the soil pollution ofheavy metal gets worse and worse. The migrate of heavy metals in the soil along the food chain, not onlyaffects the growth of crops, reduces yields as well as quality of crops, but also is harm to human’s healththe lifes. Therefore, how to control soil pollution of heavy metal has become a serious problem. Themicrobial remediation technology of heavy metals in polluted soil is an efficient and potentialbioremediation method against heavy metals.Firstly, in this paper, two strains with higher resistance to chromium were obtained throughdomestication, selection and separation technology from soils contaminated seriously in Xinxiang andnamed as strain A and B, espectively. The highest concentrations of the two strains resistanting to Cr3+were450mg/L and500mg/L,respectively, and their concentrations resistanting to Cr6+were200mg/L and220mg/L, respectively. The moephlosy of strain A is as follows: short, round or nearly round, soft and slightlyshiny Gram-positive bacterium with rounded ends, spores, white bacterial colony, smooth and moistsurface, and irregular edges. its physiological and biochemical characteristics were similar to those ofBacillus.16S rDNA sequence analysis showed that the similarity of strain A and Bacillus cereus was up to99%, thus the strain A was identified as Bacillus cereus. The moephlosy of strain A is as follows: coloniesloose, cotton wool, cultivate early white, after become brown, covered the entire flat within2d, which wasobserved that spherical sporangium with rhizoids, spores spherical or near spherical, its shapes and sizesunder the microscope.18S rDNA sequence analysis showed that the similarity of strain B and Rhizopusoryzae was up to99%, thus the strain B was identified as Rhizopus oryzae.Secondly, the growth conditions of Bacillus cereus and Rhizopus oryzae were studied. And the resultsshowed that the optimal pH, temperatures and liquid medium volume of both Bacillus cereus and Rhizopusoryzae were pH7,30℃and80mL-100mL (in250mL bottle), respectively. When the initial concentrationof Cr3+were100,200,300,400and500mg/L, the two strains could grow normally. But, the higherconcentrations of Cr3+inhibiting the growth of the two strains. The biosorption results of Bacillus cereusand Rhizopus oryzae to Cr3+showed that with the increasing concentration of Cr3+, the adsorption rates oftwo strains to Cr3+decreased but the biological adsorption quantity increased gradually. The adsorption rate of two strains to Cr3+increased firstly and then decreased under different concentration of Cr3+, which wasthe same as that of biological adsorption quantity.Thirdly, the effects of Bacillus cereus and Rhizopus oryzae on the growth and development, chromiumcontents and physiological-biochemical indexes of wheat seedlings were investigated by water-cultureexperiment. The results showed that the growth of wheat seedlings was inhibited by Cr3+, and the inhibitioneffect enhanced with the Cr3+concentrations. After the addition of the bacterial suspension of the tworesistant strains to chromium, the length and dry weight of the leaves and roots in the wheat seedlings wereincreased, and indicating the inhibition to the growth of wheat seedlings was alleviated in different degrees.With the increasing concentrations of Cr3+, the chromium contents in wheat seedlings increased too, and thechromium contents in roots were more than those in leaves. Moreover, the chromium contents of wheatseedlings after the addition of the bacterial suspension of the two strains resisting to chromium were lowerthan the control treatments. Detailedly, the chromium contents of the leaves and roots in wheat seedlingswere reduced8.87~15.21%and2.76~11.32%, respectively by adding Bacillus cereus; and the chromiumcontents of the leaves and roots in wheat seedlings were dropped15.63~24.67%and1.84~18.20%,respectively by adding Rhizopus oryzae. With the increasing concentration of Cr3+, the contents ofmalondialdehyde (MDA) of the leaves of wheat seedlings increased, but the root activity, the contents ofchlorophyll a, chlorophyll b and total chlorophyll in the wheat seedlings leaves decreased, while theactivitie curves of superoxide dismutase (SOD) and peroxidase (POD) of the wheat seedlings leaves had anincreased firstly and then decreased trend. After the addition of the bacterial suspension of the two strainsresisting to chromium, the contents of MDA of the leaves of wheat seedlings decreased, the activity rootand the contents of chlorophyll increased significatly, and the activities of SOD and POD had been restoredto some extent.Finally, to study the alleviative effect of the strains in chromium-contaminated soil, the experimentswere carried out on in the greenhouse with wheat (Triticum aestivum L.) as a test plant and Bacillus cereusand Rhizopus oryzae as the test strains. In exogenous chromium contamination conditions, both the growthand chromium enrichment of wheat with different proportions of the two strains were studied, and at thebest ratio of the two strains, the growth and enrichment of chromium of wheat with different weights ofmixed agents were tested. Then, the best ratio and dosage of the mixed agents resisting to chromium in chromium-contaminated soil were discussed. Major findings were as follows(1) Compared with the control treatment, wheat seedlings grew better with different proportions ofBacillus cereus and Rhizopus oryzae; the content of available chromium in soil and the contents ofchromium in both overground and underground part of wheat plant all dropped in different degrees; but thetotal content of chromium in soil rised. When the ratio of Bacillus cereus and Rhizopus oryzae was1:3, thealleviative effect of the mixed agents on wheat seedlings was the best.(2) In the treatments of mixed agents of Bacillus cereus and Rhizopus oryzae, the height, the fresh anddry weight of wheat plant showed an increasing first and decreasing afterwards trend with the increasingquality of mixed agents; the contents of chromium in overground and underground part of wheat plant in alltreatments with mixed agents were obviously lower than that in the control, and42.15~46.91%and26.11~48.24%of reductions were obtained, respectively; the same result to the content of availablechromium in soil with a declination percent of2.1~9.0%. When inoculation quantity of the mixed agentsamounted to2.0g/kg, the declination reacheed the peak value and the effect on wheat seedlings growthachieved the best.(3) The growth of wheat seedlings were restricted by chromium contamination at different levels. Andas chromium concentration increased, the inhibition effects of chromium on wheat seedlings was more andmore obvious, and the available chromium in soil and the contents of chromium in overground andunderground part of wheat plant all increased correspondingly. Therefore, mixed agents could alleviate thesuppression of chromium on wheat and was help for the wheat seedlings growth, and they make chromiumcontent in soil and overground and underground part of the wheat plant reduce correspondingly; In addition,the pH in soil increases, which resulted in the decreased trend of the bioavailability of chromium. |
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