北台灣蚯蚓的功能群與其腸道土壤微生物相

Abstract

土壤中蘊育著高度的生物多樣性；這個多樣性中的生物藉由營養階層與複雜的土壤食物網彼此相連。其中，動物與微生物在森林生態系中參與凋落物分解與養分循環。地下大型無脊椎動物—蚯蚓，其鑽土行為會改變土壤的物理性質，取食落葉、有機質及微生物可加速凋落物的分解和養分循環，經由腸道排出的糞土亦會改變土壤微生物相。不同生態群的蚯蚓，其食性與鑽土行為不同，對土壤微生物的影響也不同。然而，近年來多數關注蚯蚓對土壤微生物影響的研究，多僅專注在單一物種，且集中在少數養殖物種，只有極少數的研究將重心放在野外的蚯蚓群聚。因此，本研究以台灣北部淺山地區四個樣區的蚯蚓群聚為研究對象，探討(1)蚯蚓取食偏好與周界土壤之間；(2)基於穩定同位素劃分生態棲位的蚯蚓類群之間；(3)蚯蚓腸道中不同部位之間的微生物菌相差異。我們以穩定同位素碳13、氮15探討蚯蚓的食性差異，並進一步探討生態群分類，以微生物16S核糖體核糖核酸基因結合次世代定序分析蚯蚓腸道土壤與周界土壤微生物菌相。總計採得13種蚯蚓，δ13C、δ15N數值越低顯示主要以落葉與表層有機質為食；數值越高則以土壤以及土壤裡的有機質為食。13個物種可劃分為四個生態群，分屬四個營養棲位，其中傳統上認定的底層型蚯蚓包含兩個營養棲位，推測可能是土壤中棲息深度的變異使得獲取食物資源方式不同。腸道菌相分析的結果與生態群的劃分一致，四個不同的生態群有不同的腸道菌相，並與穩定同位素的結果呈現相同的趨勢。周界土壤與蚯蚓腸道微生物菌相的相對豐富度具有顯著差異。其中，蚯蚓腸道中的優勢菌門為變形菌門、放線菌門、厚壁菌門等。蚯蚓腸道菌相組成顯示相同樣區的蚯蚓個體較相似，而非不同樣區的相同蚯蚓物種，顯示此微生物相地點上的專一性高於宿主物種專一性。

Soils harbor an unparalleled biological diversity that is interconnected via numerous trophic links and complex food webs. Among them, soil fauna and microbes are important players in decomposition and nutrient cycling. As ecosystem engineers, earthworms alter biological, chemical, and physical processes and play a vital role in soil functioning. Their borrowing behavior reshapes soil pores and modifies the air permeability and water infiltration; their grazing on leaf litter, soil organic matter, and microorganisms enhance decomposition rate and nutrient cycling; their ingestion of large amounts of material and casting upon the soil surface or in the soil impact microbial community composition and chemical properties. Earthworms of ecological groups have different feeding and burrowing behaviors, and are expected to affect soil microbes differently. However, most studies investigating the interaction between earthworms and microbes are focused on a couple of common, especially vermicomposting, species. Only a few studies looked into earthworm communities in natural habitats. In this study, we sampled earthworm communities in four secondary broadleaf forest sites in northern Taiwan to understand (1) how the microbial communities in the soil selectively ingested by earthworms differ from their surrounding soil habitat, (2) the differences of gut microbiomes between earthworm groups categorized based on their isotopic niches, (3) how microbial communities change after the ingested soils pass earthworm guts. We used carbon and nitrogen stable isotopes to investigate the feeding ecology of different earthworm species and ecological groups. Furthermore, we used next-generation sequencing of the 16S rRNA gene to analyze soil and earthworm gut microbiomes. A total of 13 species of earthworms were collected. δ13C and δ15N values show that, isotopically, less enriched species represent litter and surface soil feeders, while more enriched are mineral soil feeders. A total of four trophic niches representing four ecological groups are recognized. Endogeic (soil feeding) earthworms can be divided into two trophic niches, possibly reflecting resource use differences, both spatially and trophically. Earthworm gut microbiomes show a pattern similar to that of stable isotopes: the four ecological groups have distinct gut microbiomes, with anecic species and mesohumic endogeic species occupying the two ends of the spectrum. Bacterial community structures of earthworm gut were distinct from their surrounding soil. Among them, Proteobacteria, Actinobacteria, Firmicutes were the dominant bacterial phylum in the gut. The gut microbial communities in earthworms showed that the host individuals from the same location are more similar to each other than those of the same species from different locations, indicating earthworms may have regional specificity rather than host species specificity.