| The Pearl River Delta region has many industrial agglomeration areas and tidal flat reclamation farmland,which is suffering heavy metal combined pollution.Among them,cadimum?Cd?pollution is the most serious and the long-term unreasonable application of Cd-contained phosphate fertilizer aggravated the load of soil Cd pollution.Plant growth promoting rhizobacteria has a good application prospect in promoting plant growth,reducing the application of phosphate and potash fertilizer and improving the efficiency of remediation of soil Cd contaminated plants.In the presentstudy,phosphorus-solubilizingbacteria?Escherichiacoli?and potassium-solubilizing bacteria?Rhizobium sp..?were investigated about their characteristics of organic acids secretion and their Cd-solubilizing mechanisms under effect of available phosphate.Pot experiments were applied to investigate the impact of these two rhizobacterias on the phytoremediation of Cd-polluted soils by Solanum nigrum.It will provide a theoretical base for phytoremediation coupled with plant growth-promoting rhizobacteria and the development of microorganisms agent.The main results and conclusions were listed as below:?1?An obvious difference in composition of extracellular secretion existed between Escherichia coli and Rhizobium sp..,and their secretion characteristics were influenced by strain species and microbial culture conditions.Escherichia coli secreted more acids,but less polysaccharide than Rhizobium sp..Under inorganic phosphate medium culture conditions,11 kinds of organic acids were identified from Rhizobium sp..culture fluid,and 21 kind of organic acids were identified from Escherichia coli culture fluid.Among them,propionic acid and succinic acid contents from Escherichia coli culture fluid were 9.23 and 2.46-fold of those from Rhizobium sp..culture fluid,while lactic acid and palmitic acid content from Rhizobium sp..culture fluid were 3.29 and 7.19-fold of those from Escherichia coli culture fluid.Under potassium silicate potassium-free medium culture conditions,15 kinds of organic acids were identified in Rhizobium sp..culture fluid,and 21 kinds of organic acids were identified in Escherichia coli culture fluid.Among them,butyric acid and D-gluconic acid content from Escherichia coli culture fluid were 60.66-and 5.85-fold of those from Rhizobium sp..culture fluid.Both Escherichia coli and Rhizobium sp..secret some organic acids capable of dissolving cadmium carbonate.?2?The effects of available phosphorus on cadmium solubilization and extracellular secretion composition of Escherichia coli and Rhizobium sp..showed that,available phosphorus could stimulate the growth of Escherichia coli and Rhizobium sp..and the secretion of organic acids significantly,resulting in reduced medium pH and improved cadmium-solubilizing efficiency.Adding 5 mg·L-1available phosphorus,the cadmium solubilization by Escherichia coli and Rhizobium sp..Increased 2.19 and 1.27-fold that of no available phosphorous.With the increased of phosphorus concentration,the content of organic acid showed a decreasing trend in general.The proliferation of microorganism is promoted,and it is more obviously restricted by insufficient carbon source.The secretion of microbial organic acids is reduced and the extracellular organic acids are used as carbon sources.Most of the identified organic acids in Rhizobium sp..culture fluid showed a decreasing trend,and only 5 kinds of organic acids increased.In Escherichia coli culture fluid,five kinds of organic acids increase first and then reduce the trend,while other 17 kinds of organic acids presented increased persistently.The deficiency of carbon source affected the secretion of organic acids and limited the cadmium solubilization by microbes.?3?The pot experiment results showed that the inoculation of Escherichia coli and Rhizobium sp..could stimulate the growth of Solanum nigrum root.Rhizobium sp..could promote the absorption of potassium,phosphorus,calcium and cadmium by Solanum nigrum,and the contents of stem and leaf increased by 46.718.84%and43.96%,respectively;Escherichia coli could promote the accumulation of potassium,phosphorus,calcium,magnesium and cadmium by Solanum nigrum,and the contents of shoot increased by 70.4%,44.65%,26.74%,42.32%and 50.78%,respectively.The effect of Escherichia coli is stronger than that of Rhizobium sp..The composition of root secretion showed that the content of organic acid in the control was higher than that inoculation of bacteria,and no carbohydrate was detected.It indicated that bacteria could utilize organic acid secreted by root as carbon source. |
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