Expression and lateral transfer of nitrogen fixation genes in the gut of earthworms.

本文研究了一种植物--条远环蚓（Amynthas vittatus）对提高土壤氮素含量的贡献。将3条成年蚯蚓标本放入300 g添加1%羧甲基纤维素的土壤（灰低地土、粉质粘土）中，在22℃下避光培养32天。蚯蚓孵育后，土壤中全氮、铵氮、硝氮含量和好氧固氮菌数量均明显增加，而土壤中^<15>N（δ^<15>N）的自然丰度则下降。蚯蚓体内的氮含量在微宇宙中的孵化过程中没有减少。蚯蚓的引入显著提高了微宇宙的乙炔还原活性和<15>从大气中吸收氮<15>的能力，但土壤中氮的增加量远高于基于固氮活性的估算值。通过对蚯蚓生活过的土壤中NFB的分离和nifH的PCR-DGGE分析，研究了蚯蚓生活过的土壤中好氧固氮细菌的群落结构。生理生化特性和16 S rDNA序列分析表明，分离到的NFB分别属于Azorhizobium caulinodans、Azoehizillum brasilense、A.lipoferum、Klebsiella oxytoca、Pseudomonas sp.结果表明，蚯蚓对土壤中的氮素含量有明显的促进作用，蚯蚓对土壤中的氮素含量有明显的抑制作用，蚯蚓对土壤中的氮素含量有明显的促进作用，推测是由于蚯蚓从微观上增强了土壤的固氮活性。

The contribution of an earthworm species (Amynthas vittatus) to the increase of the nitrogen content of soil was examined. Three specimens of adult earthworms were introduced into 300 g of soil (Gray Lowland soil, silty clay) supplemented with 1% carboxymethyl cellulose in a container and incubated for 32 days at 22℃ in the dark. The contents of total-N, NH_4-N and NO_3-N, and the population of aerobic nitrogen-fixing bacteria in soil significantly increased after incubation with the earthworms, while the natural abundance of ^<15>N (δ^<15>N) in soil decreased. The amount of nitrogen in the earthworms did not decrease during the incubation in the microcosm. Both acetylene reduction activity of the microcosm and incorporation of ^<15>N to soil from atmospheric ^<15>N_2 were significantly enhanced by the introduction of the earthworms into soil, though the observed increment of nitrogen in soil was much higher than the estimated one based on the nitrogen-fixing activity.The community structure of aerobic nitrogen-fixing bacteria in the soils where the earthworms had lived was estimated by both isolating NFBs from the soils and the analysis of PCR-DGGE patterns of nifH from the soils. Physiological characteristics and 16S rDNA sequence analysis showed that the isolated NFBs were classified into Azorhizobium caulinodans, Azospirillum brasilense, A.lipoferum, Klebsiella oxytoca, Pseudomonas sp., Rhizobium etli, Rosemonas faurise, Sinorhizobium morelens, and Xanthobacter sp. The number and the pattern of DGGE bands of nifH from the soil incubated with earthworms were different from those of nifH from earthworm casts and the soil in which earthworms did not live.The results obtained in the present study indicated that the earthworms increased the nitrogen content of soil, presumably due to the enhancement of the nitrogen-fixing activity of the soil from the microcosm by the earthworms.