水稻根际兼性厌氧催娩克氏杆菌和阴沟肠杆菌的固氮与氢代谢

兼性厌氧细菌Enterobacter cloacae菌株E-26和Klebsiella oxytoca菌株NG-13的氢酶与固氮酶同时形成。固氮的最佳碳源为蔗糖、葡萄糖和丙酮酸,此外延胡索酸和苹果酸也能支持固氮。支持固氮的碳源也支持放氢,两者动力学基本一致。40％乙炔预处理后,吸氢活性下跌,放氢量未增加;NH_4~ 抑制固氮酶,但未导致放氢量降低;可能E-26菌株的放氢主要依赖于氢酶。菌株E-26和NG-13的吸氢反应,既能以O_2为电子受体,也能以延胡索酸、硝酸、MB为电子受体。但仅延胡索酸为电子受体时,E-26菌的固氮活性被分子H_2促进,它的氢吸收利用与固氮相偶联;而在CO_2和NH_4~ 代谢与H_2利用之间并无明显相关性,吸氢活性不被CO_2和NH_4~ 促进。

Hydrogen Metabolism and Nitrogen Fixation in Rice Associated Anaerobic N_2-Fixing Bacteria Enterobacter cloacae E-26 and Klebsiella oxytoca NG-13

Hydrogenates are involved in the improvement of the efficiency of N_2 fixation in both free-living and symbiotic aerobic microorganisms. H_2-recycling process has been postulated to be beneficial to symbiotic N_2 fixation. However, the role of hydrogenate in the recycling of hydrogen in maintaining a high efficiency of nitrogen fixation in rice associated N_2-fixing bacteria are still unclear (Table 1). The conditions necessary for coordinated derepression of nitrogenase and uptake hydrogenase activities in rice-associated anaerobic nitrogenfixing bacteria Entrobacter doacae E-26 and Klebsiella oxytoca NG-13 were studied. A number of carbon sources were screened for their ability to support nitrogenase, and then for hydrogenate activity (Table 2). There was a positive correlation between the level of nitrogenase activity and the corresponding hydrogenate activity among various carbon substrates (Table 3). The kinetics of nitrogen fixation was found to be similar to that of hydrogen evolution supported by carbon substrates (Figs, 1,2).Pretreatment with 40% acetylene caused a decrease of uptake hydrogenate (Fig. 4) but no effect on H_2 evolution was observed, and the inhibition of nitrogenase by ammonia did not affect H_2 evolution (Fig. 3). It was found that strain E-26 and NG-13 could catalyze the reduction of various substances physiologically active at electron acceptors, such as fumarate, nitrate, methylene blue, O_2, etc. (Table 4). A variety of terminal electron pathway in H_2 oxidation in these strains was suggested. However, only with fumarate as electron acceptor nitrogen fixation in E-26 strain was enhanced by H_2(Fig. 6). No significant correlation between CO_2 or NH_3 metabolism and H_2 uptake was observed (Fig. 5). The beneficial effect of interspecific hydrogen transfer in rice fields on nitrogen fixation efficiency was discussed.