Coupling between Nitrogen Fixation and Tetrachlorobiphenyl Dechlorination in a Rhizobium-Legume Symbiosis.

Legume-rhizobium symbioses have the potential to remediate soils contaminated with chlorinated organic compounds. Here, the model symbiosis between Medicago sativa and Sinorhizobium meliloti was used to explore the relationships between symbiotic nitrogen fixation and transformation of tetrachlorobiphenyl PCB 77 within this association. 45-day-old seedlings in vermiculite were pretreated with 5 mg L-1 PCB 77 for 5 days. In PCB-supplemented nodules, addition of the nitrogenase enhancer molybdate significantly stimulated dechlorination by 7.2-fold and reduced tissue accumulation of PCB 77 (roots by 96% and nodules by 93%). Conversely, dechlorination decreased in plants exposed to a nitrogenase inhibitor (nitrate) or harboring nitrogenase-deficient symbionts (nifA mutant) by 29% and 72%, respectively. A range of dechlorinated products (biphenyl, methylbiphenyls, hydroxylbiphenyls, and trichlorobiphenyl derivatives) were detected within nodules and roots under nitrogen-fixing conditions. Levels of nitrogenase-derived hydrogen and leghemoglobin expression correlated positively with nodular dechlorination rates, suggesting a more reducing environment promotes PCB dechlorination. Our findings demonstrate for the first time that symbiotic nitrogen fixation acts as a driving force for tetrachlorobiphenyl dechlorination. In turn, this opens new possibilities for using rhizobia to enhance phytoremediation of halogenated organic compounds.

Phytoremediation of oilfield sludge after prepared bed bioremediation treatment.

A field-scale experiment was carried out to compare the effectiveness of five plant species in the remediation of oily sludge. Alfalfa, tall fescue, and soybean substantially increased the removal rate of oil and grease (O&G) after 120 days of remediation. Of these, soybean treatment showed the highest removal rate of 34.2% compared with only 13.7% in the unplanted control plot. In addition to assisting with phytoremediation, soybean can also be used for energy production, for example in the production of biodiesel. Furthermore, microbial counts and community level physiological profiling using Biolog sole carbon source utilization tests were used to investigate the effect of plants on the microbial community of oily sludge. Plants selectively increased microbial counts in the rhizosphere and O&G concentrations of the sludge were negatively correlated with counts of TPH (total petroleum hydrocarbon) degraders. The acute biotoxicity of the sludge was also tested by the bacterial luminescence method and alfalfa treatments decreased the biological toxicity of the sludge compared with the unplanted control.

Bioremediation of oily sludge-contaminated soil by stimulating indigenous microbes.

In situ bioremediation of oily sludge-contaminated soil by biostimulation of indigenous microbes through adding manure was conducted at the Shengli oilfield in northern China. After bioremediation for 360 days, total petroleum hydrocarbon (TPH) content was reduced by 58.2% in the treated plots compared with only 15.6% in the control plot. Moreover, bioremediation significantly improved the physicochemical properties of the soil in the treated plot. Soil microbial counts and community-level physiological profiling were also examined. Manure addition increased TPH degraders and polycyclic aromatic hydrocarbon (PAH) degraders in the contaminated soil by one to two orders of magnitude. The activity and biodiversity of soil microbial communities also increased markedly in the treated plot compared with that of the control. Finally, biotoxicity was used to evaluate the soils and a sharp increase in the EC50 of the soil after bioremediation was observed, indicating that bioremediation had reduced the toxicity of the soil.

Prepared bed bioremediation of oily sludge in an oilfield in northern China.

Field-scale bioremediation of oily sludge in prepared beds was studied at Shengli oilfield in northern China. The influence of manure, coarse sand, sawdust, a specialized microbial preparation and greenhouse conditions on the efficiency of removal of oil and grease was evaluated. After bioremediation for 230d, oil and grease content fell by 32-42gkg(-1)dry sludge in treated plots, indicating removal of 27-46% compared with only 15% in the control plot. Addition of manure, coarse sand, sawdust and greenhouse conditions significantly (p<0.05) increased the amount removed. Moreover, the physico-chemical properties of the sludge in all treated plots improved significantly after bioremediation. Microbial biomass in sludge and community-level physiological profiling examined using BIOLOG microplates was also studied. Total petroleum hydrocarbon degraders and polycyclic aromatic hydrocarbon degraders increased in all treated oily sludge. The activity of sludge microbial communities increased markedly in the treated plots compared with the control. Canonical correspondence analysis showed that differences in substrate utilization patterns were highly correlated (p<0.05) with sludge hydrolyzable N and oil and grease content. The biological toxicity of the oily sludge was lower following bioremediation in most of the treated plots as evaluated using Photobacterium phosphoreum T3.