Gut microbial diversity in Lumbricus rubellus

【Objective】 To elucidate the diversity characteristics of gut microbiota in artificially cultured Lumbricus rubellus， which could provide theoretical basis for subsequent investigation into the functions of earthworm gut microbiota and their environmental effects.【Method】 The intestinal contents （anatomical samples） and excretions （excretion samples） of Lumbricus rubellus fed with cattle manure were collected separately. After chloroform-methanol extraction， silica gel column separation， and methanol methylation treatment， the fatty acids were qualitatively and quantitatively analyzed using gas chromatography-mass spectrometry （GC-MS）. The relative abundances of different microbial groups were calculated based on phospholipid fatty acid （PLFA） biomarkers.【Result】 A total of 18 PLFAs were detected in both the anatomical samples and excretion samples of Lumbricus rubellus， namely： 14：0， i-15：0， 16：1ω7c， 16：0， i-17：0， a-15：0， 15：0， 10Me16：0， i-16：0， cy17：0， 17：0， 10Me18：0， 18：2ω6，9， 18：1ω9t， 18：1ω9c， 18：0， cy19：0， and 20：0. Among these， 10Me16：0 and 16：0 had relatively high contents， with 10Me16：0 accounting for 19.18%-19.24% of the total fatty acid content and 16：0 accounting for 17.48%-17.66%. There were subtle differences in the proportions of diffe-rent types of fatty acids between the anatomical samples and excretion samples of Lumbricus rubellus， with each type composed of specific PLFA species. No significant differences （P>0.05） were observed between anatomical and excretion samples in stress index （cyc17：0 + cyc19：0）/（16：1ω7c + 18：1ω7c）， Gram-positive/Gram-negative bacteria ratio （G+/G-）， or bacteria/fungi ratio. Microbial communities in both anatomical and excretion samples of Lumbricus rubellus were predominantly composed of Actinomycetes （24.49% and 24.47%） and Gram-positive bacteria （19.29% and 19.31%）， while the relative abundances of Gram-negative bacteria and fungi remained at relatively low levels. Moreover， the microbial community correlation patterns were consistent between the two sample groups： Gram-positive bacteria showed extremely significant positive correlation with Actinobacteria， with correlation coefficients （r） of 0.82 and 0.88 respectively； while Actinomycetes exhibited negative correlation with fungi， with r values of -0.57 and -0.29 respectively.【Conclusion】 Lumbricus rubellus exhibits a weak selective retention mechanism for microorganisms in its gut， with the microbial community predominantly composed of Actinomycetes and Gram-positive bacteria. Among them， Actinomycetes dominate the degradation of complex organic matter in cattle manure by secreting cellulases and ligninases， while Gram-positive bacteria participate in intermediate product metabolism and humification. A metabolic synergy exists between these two groups， and they maintain high functional activity even in excretions. The results establish the foundation for the efficient degradation of cattle manure by cellulose and hemicellulose in Lumbricus rubellus.