Litter quality modulates changes in bacterial and fungal communities during the gut transit of earthworm species of different ecological groups

Abstract

Earthworms are keystone animals stimulating litter decomposition and nutrient cycling. However, earthworms comprise diverse species which live in different soil layers and consume different types of food. Microorganisms in the gut of earthworms are likely to contribute significantly to their ability to digest organic matter, but this may vary among earthworm species. Here, we analyse the effect of food (litter) quality on gut microbiota and their changes during the gut passage (from foregut to hindgut) of earthworms of different ecological groups. The endogeic (soil living) species Aporrectodea caliginosa and the anecic (litter feeding) species Lumbricus terrestris were fed with high- (rape leaves) and low-quality litter (wheat straw) in a microcosm experiment for 18 weeks. Irrespective of earthworm species, alpha diversity of bacterial and fungal communities changed little during the gut passage, with the composition and diversity of microbial communities in the gut generally resembling those in soil more than in litter. In addition, the low-quality litter supported higher alpha diversity and more complex communities than high-quality litter. Further, gut microbial communities of the anecic L. terrestris changed less during gut passage than those of the endogeic A. caliginosa, especially when fed low-quality litter. Our findings indicate that earthworm gut microbial communities are predominantly shaped by the soil they ingest, but are modulated by the quality of litter they feed on and earthworm ecological group. Overall, the results suggest that earthworms primarily influence soil microbiota by mixing and spreading microorganisms from different microhabitats through bioturbation rather than by digesting microorganisms.

Keywords: NGS sequencing; earthworm ecological groups; gut passage; intestinal microbes; microbial diversity; resources quality.

Graphical Abstract

Figure 1

Bacterial (A, B, C, D) and fungal (E, F, G, H) alpha diversity in the foregut, midgut, and hindgut of two earthworm species [Aporrectodea caliginosa (left panels) and Lumbricus terrestris (right panels)], soil and two litter types used as food substrate (rape leaves or wheat straw) as indicated by the ACE and Shannon index (mean ± SE). Different letters indicate significant differences between means (Tukey's HSD test, P < .05). Asterisks indicate significant differences between litter types nested within earthworm species (^*P < .05 and ^***P < .001; Tukey's HSD test).

Figure 2

PCoA plot showing beta-diversity of bacteria (A) and fungi (D) in the foregut, midgut, hindgut, soil, and litter in different litter and earthworm species treatments, i.e. Aporrectodea caliginosa with rape leaves (solid circles), A. caliginosa with wheat straw (empty circles), Lumbricus terrestris with rape leaves (solid triangles) and L. terrestris with wheat straw (empty triangles) based on Bray-Curtis distances. Relative abundance of bacterial (B, C) and fungal phyla (E, F) in the foregut, midgut and hindgut of the two earthworm species, soil and two litter types used as food substrate (rape leaves or wheat straw).

Figure 3

Bar plots on clustering coefficient (A) and weighted degree (B) of networks based on bacterial and fungal OTUs (pooled for the two earthworm species studied, i.e. Aporrectodea caliginosa and Lumbricus terrestris) in rape leaves and wheat straw treatments (means ± SE). Box plots on clustering coefficient (C) and weighted degree (D) of networks based on combined bacterial and fungal OTUs (pooled for the two earthworm species studied) in rape leaves and wheat straw treatments (means ± SE). Asterisks indicate significant differences, ^***P < .001, ns P > .05 (Tukey's HSD test).