Differential Engagement of Fermentative Taxa in Gut Contents of the Earthworm Lumbricus terrestris

Effects of cadmium and pyrene on earthworm-associated bacterial communities: Unveiling new perspectives for soil pollution management

Earthworms are considered to be excellent bioindicators of soil pollution. In recent years, there has been increasing interest in examining the effects of soil pollution on earthworm-associated microbiomes, with a particular focus on the gut microbiomes. However, relatively little effort has been invested in comprehensively investigating other microbiomes associated with earthworms and their responses to soil pollution. To fill this gap, we systematically studied the effects of Cd, pyrene, and combined pollution on the bacterial community in different vermicompartments, i.e., burrow wall, gut, and cast, in both epigeic Eisenia fetida and anecic Metaphire guillelmi, using a 2D-terraria incubator and high-throughput sequencing techniques. The results showed that bacterial alpha diversity followed the order of burrow wall > cast > gut, and this did not vary with soil pollution or earthworm ecotypes. Moreover, the dominant phyla in the vermicompartments were similar across different pollution treatments. Principal coordinate analysis (PCoA) revealed that the bacterial communities in different vermicompartments and ecotypes of earthworm were separated from each other, whereas they were grouped together in polluted treatments and unpolluted conditions. These results imply that even in polluted soil, vermicompartment and earthworm ecotypes remain the most significant factors affecting earthworm-associated microbiomes. However, the impacts of soil pollution on the bacterial composition in each vermicompartment were still evident. A comprehensive analysis revealed that the gut bacterial communities are more sensitive to soil contamination than casts and burrow wall in different ecotypes. Additionally, linear discriminant analysis of effect size (LefSe) identified several bacteria in Gemmatimonadota, the Firmicutes phylum in the burrow walls, and Patescibacteria (phyla) in the gut as potential biomarkers for pyrene contamination in soil. This research provides a comprehensive understanding of the effects of soil pollution on earthworm-associated microbiomes, thereby enhancing our understanding of earthworm ecotoxicology and soil pollution management.

Integrated microbiome and multi-omics analysis reveal the molecular mechanisms of Eisenia fetida in response to biochar-derived dissolved and particulate matters

At present, most ecotoxicological studies are still confined to focusing on the harmful effects of biochar itself on soil fauna. However, the potential ecotoxicity of different components separated from biochar to terrestrial invertebrates remains poorly understood. In this study, the dissolved matter (DM) and particulate matter (PM) were separated from biochar (BC) and then introduced into the soil-earthworm system to investigate the response mechanism of earthworms at the molecular level. The results showed that BC and DM exposure caused an increase in the abundance of Proteobacteria in the cast bacterial community, suggesting the dysbiosis of intestinal microbiota. It was also observed that the cast bacterial communities were more sensitive to DM exposure than PM exposure. Transcriptomic analysis showed that BC and DM exposure induced significant enrichment of functional pathways related to infectious and neuropathic diseases. Metabolomic profiling manifested that DM exposure caused metabolic dysfunction, antioxidant and detoxification abilities recession. Furthermore, significant differences in the responses of earthworms at transcriptomic and metabolic levels confirmed that DM exhibited greater ecotoxicity than PM. This study highlighted the significant contributions of dissolved matter to the ecotoxicity of biochar from the perspective of transcriptomic and metabolomic profiles.

Dynamics of bacterial community in the foregut and hindgut of earthworms with the nutrition supplied by kitchen waste during vermicomposting

Earthworm gut microbiota is vital in degrading bio-waste during vermicomposting. However, microbial dynamics in earthworm gut during this process are unclear. Thus, the aim is to firstly report the bacterial dynamics in both foregut and hindgut of earthworms over a 28 days' timeframe of vermicomposting by Eisenia foetida with the nutrition supplied by kitchen waste. Results showed that except the changing of the bacterial diversity, composition and structure, dynamics of the foregut and hindgut bacteria also differed during vermicomposting which related to the changes of nutrient provision. Day 3 was a turning point. The abundant bacteria of the top 20 % genera nearly did not overlap between the foregut and hindgut. In the end of vermicomposting, a remarkable stable bacterial structure appeared in the hindgut compared to somewhat muddled one in the foregut. Understanding the dynamics of earthworm gut microbiota enables the improvements to regulate the efficiency of organic waste vermicomposting.

Larvae of an invasive scarab increase greenhouse gas emissions from soils and recruit gut mycobiota involved in C and N transformations

Background

Soil-derived prokaryotic gut communities of the Japanese beetle Popillia japonica Newman (JB) larval gut include heterotrophic, ammonia-oxidizing, and methanogenic microbes potentially capable of promoting greenhouse gas (GHG) emissions. However, no research has directly explored GHG emissions or the eukaryotic microbiota associated with the larval gut of this invasive species. In particular, fungi are frequently associated with the insect gut where they produce digestive enzymes and aid in nutrient acquisition. Using a series of laboratory and field experiments, this study aimed to (1) assess the impact of JB larvae on soil GHG emissions; (2) characterize gut mycobiota associated with these larvae; and (3) examine how soil biological and physicochemical characteristics influence variation in both GHG emissions and the composition of larval gut mycobiota.

Methods

Manipulative laboratory experiments consisted of microcosms containing increasing densities of JB larvae alone or in clean (uninfested) soil. Field experiments included 10 locations across Indiana and Wisconsin where gas samples from soils, as well as JB and their associated soil were collected to analyze soil GHG emissions, and mycobiota (ITS survey), respectively.

Results

In laboratory trials, emission rates of CO₂, CH₄, and N₂O from infested soil were ≥ 6.3× higher per larva than emissions from JB larvae alone whereas CO₂ emission rates from soils previously infested by JB larvae were 1.3× higher than emissions from JB larvae alone. In the field, JB larval density was a significant predictor of CO₂ emissions from infested soils, and both CO₂ and CH₄ emissions were higher in previously infested soils. We found that geographic location had the greatest influence on variation in larval gut mycobiota, although the effects of compartment (i.e., soil, midgut and hindgut) were also significant. There was substantial overlap in the composition and prevalence of the core fungal mycobiota across compartments with prominent fungal taxa being associated with cellulose degradation and prokaryotic methane production/consumption. Soil physicochemical characteristics such as organic matter, cation exchange capacity, sand, and water holding capacity, were also correlated with both soil GHG emission, and fungal a-diversity within the JB larval gut. Conclusions: Results indicate JB larvae promote GHG emissions from the soil directly through metabolic activities, and indirectly by creating soil conditions that favor GHG-associated microbial activity. Fungal communities associated with the JB larval gut are primarily influenced by adaptation to local soils, with many prominent members of that consortium potentially contributing to C and N transformations capable of influencing GHG emissions from infested soil.

Effects of Three Pesticides on the Earthworm Lumbricus terrestris Gut Microbiota

Earthworms play a vital role in the terrestrial ecosystem functioning and maintenance of soil fertility. However, many pesticides, for example, imidacloprid, benomyl, and metribuzin that are world-widely used in agriculture, may be potentially dangerous to earthworms. At the same time, standard tests for pesticides acute and chronic toxicity do not reflect all aspects of their negative impact and might not be enough sensitive for effective assessment. In this paper, we studied the effects of non-lethal concentrations of imidacloprid, benomyl, and metribuzin on the gut bacterial community of Lumbricus terrestris using high-throughput sequencing approach. We found that pesticides reduced the total bacterial diversity in the earthworm's gut even at the recommended application rate. Under the applied pesticides, the structure of the gut prokaryotic community underwent changes in the relative abundance of the phyla Proteobacteria, Actinobacteria, Acidobacteria, Planctomyces, Verrucomicrobia, and Cyanobacteria, as well as the genera Haliangium, Gaiella, Paenisporosarcina, Oryzihumus, Candidatus Udaeobacter, and Aquisphaera. Moreover, the pesticides affected the abundance of Verminephrobacter-the earthworms' nephridia specific symbionts. In general, the negative impact of pesticides on bacterial biodiversity was significant even under pesticides content, which was much lower than their acute and chronic toxicity values for the earthworms. These results highlighted the fact that the earthworm's gut microbial community is highly sensitive to soil contamination with pesticides. Therefore, such examination should be considered in the pesticide risk assessment protocols.

Exposure to fomesafen alters the gut microbiota and the physiology of the earthworm Pheretima guillelmi

The application of herbicide fomesafen plays a crucial role in ensuring global soybean productivity in modern agriculture, but it results in both adverse effects on soil ecosystems and phytotoxicity to succeeding crops. Soil pollution due to herbicides has raised much concern worldwide. However, there has been little investigations concerning their effects on soil fauna, especially on the gut microbial communities of earthworms. In this study, the soil endogeic earthworm Pheretima guillelmi was incubated for 20 days in natural and fomesafen-polluted soils to investigate the effects of the herbicide on gut bacterial microbiota and the earthworm's physiological indices, including energy resource (protein) and antioxidant enzyme (superoxide dismutase, SOD) of earthworms in the soil ecosystem. A significantly different and smaller microbial community was presented in the earthworm's gut compared with the cast and the surrounding soil, with exposure to fomesafen further reducing the bacterial diversity and altering the gut community composition. This was observed as significant changes in the relative abundance of the phyla Actinobacteria, Firmicutes, and Proteobacteria as well as the genera Bacillus, Microvirga, Blastococcus, Nocardioides, and Gaiella. Moreover, exposure to fomesafen reduced earthworms' energy resources and activated the antioxidant system, with both effects being significantly correlated with the gut microbial diversity. These findings unravel the fact that exposure to the herbicide fomesafen may affect non-target soil fauna via changes in their microbiota and physiological indices, thereby contributing new knowledge regarding the adverse impacts of fomesafen on the terrestrial ecosystem.

Variations in bacterial taxonomic profiles and potential functions in response to the gut transit of earthworms (Eisenia fetida) feeding on cow manure

Earthworms play an important role in the organic matter decomposition in terrestrial ecosystems. Earthworms interact directly with the microorganisms to affect the organic matter decomposition via gut transit, i.e., the digestion and assimilation of organic matter in the foregut and midgut and its excretion by the hindgut. However, how the microbial community ingested by earthworms respond to the transit processes in different gut segments of earthworms is not clear. We used composted cow manure to feed earthworms and sampled vermicompost and the contents of foregut, midgut and hindgut for bacterial 16S rRNA gene sequencing analysis. We observed that earthworm gut transit decreased the abundances of the dominant phyla Proteobacteria and Bacteroidetes but increased Actinobacteria, Chloroflexi and Acidobacteria. The alpha diversity of bacterial community in midgut was the lowest of the different gut segments, and the bacterial community structure of the foregut was significantly different from the midgut and hindgut. The enrichment analysis results revealed different selective stimulatory and inhibitory effects on the ingested bacterial community in the different gut segments, which extended to vermicompost. The FAPROTAX data indicated that C and N metabolic microbes were enriched in the earthworm gut. Microbes involved in fermentation and methanogenesis were enriched in the hindgut, and denitrification microbes were enriched in the foregut. The N metabolism microbes in vermicompost were significantly enriched after the stimulation of earthworm gut transit (P < 0.05), and the pathogenic microbes of animals and plants were inhibited. Combined with the results of subsequent correlation and biochemical analyses, earthworm gut transit significantly altered the structure and function of the bacterial community to accelerate the degradation and mineralization of organic matter and the enrichment of phosphorus and potassium. Our study suggests that the gut transit process of earthworms plays an important role in regulating organic matter dynamics in terrestrial ecosystems.

How can fertilization regimes and durations shape earthworm gut microbiota in a long-term field experiment?

The positive roles of earthworms on soil functionality has been extensively documented. The capacity of the earthworm gut microbiota on decomposition and nutrient cycling under long-term fertilization in field conditions has rarely been studied. Here, we report the structural, taxonomic, and functional responses of Eisenia foetida and Pheretima guillelmi gut microbiota to different fertilization regimes and durations using 16S rRNA gene-based Illumina sequencing and high-throughput quantitative PCR techniques. Our results revealed that the core gut microbiota, especially the fermentative bacteria were mainly sourced from the soil, but strongly stimulated with species-specificity, potential benefits for the host and soil health. The functional compositions of gut microbiota were altered by fertilization with fertilization duration being more influential than fertilization regimes. Moreover, the combination of organic and inorganic fertilization with the longer duration resulted in a higher richness and connectivity in the gut microbiota, and also their functional potential related to carbon (C), nitrogen, and phosphorus cycling, particularly the labile C decomposition, denitrification, and phosphate mobilization. We also found that long-term inorganic fertilization increased the abundance of pathogenic bacteria in the P. guillelmi gut. This study demonstrates that understanding earthworm gut microbiota can provide insights into how agricultural practices can potentially alter soil ecosystem functions through the interactions between soil and earthworm gut microbiotas.

Ecological role of earthworm intestinal bacteria in terrestrial environments: A review

Increasing evidence demonstrated the critical role the earthworm gut played in sustaining earthworm's metabolism and transformation of nutrients and pollutants in the environment. Being rich in nutrients, the earthworm gut is favorable for the colonization of (facultative) anaerobic bacteria, which bridge the host earthworm gut with adjacent terrestrial environment. Therefore, the status quo of earthworm gut research was primarily reviewed in this work. It was found that most studies focused on the bacterial composition and diversity of the earthworm gut, and their potential application in nutrient element and pollutant transformation, such as nitrification, methanogens, heavy metal detoxification, etc. Yet limited information was available about the specific mechanism of intestinal bacteria in nutrient and pollutant transformation. Therefore, in this work we highlighted the current problems and concluded the future prospect of worm's intestinal bacteria research. On one hand, high throughput sequencing and bioinformatics tools are critical to break the bottleneck in the intestinal bacteria research via clarifying the molecular mechanism involved in the transformation processes described above. In addition, a global dataset concerning worm gut bacteria will be needed to provide comprehensive information about intestinal bacteria pool, and act as a communication platform to further encourage the progress of worm gut research.

World within world: Intestinal bacteria combining physiological parameters to investigate the response of Metaphire guillelmi to tetracycline stress

Due to the abusive usage of antibiotics in animal husbandry, a large amount of residual antibiotics has been released into the environment, therein posing great threat against both environment security and public health. Therefore, it is of great significance to investigate the toxicity of antibiotics on the widely-applied bioindicator-earthworm. In this work, the physiological parameters and the intestinal bacteria community of Metaphire guillelmi were monitored simultaneously to evaluate their sensitivity to the tetracycline (TC) exposure. As expected, the antioxidant enzyme activity and coelomocyte apoptosis acted fairly well as biomarkers for the TC toxicity. In contrast, the intestinal bacteria of Metaphire guillelmi responded varyingly to different TC doses. When TC concentration increased from 0 to 35.7 μg cm^-2, the percentage of the Proteobacteria phylum declined significantly from 85.5% to 34.4%, while the proportions of the Firmicutes, Planctomycetes and Atinomycete phyla clearly increased (p < 0.05). Meanwhile, the levels of TC resistance genes tetA, tetC, and tetW increased with the increasing TC concentration, in contrast to the declined abundance in denitrifying genes nirS and nosZ (p < 0.05). By analyzing the correlation between the antioxidant enzyme activity and the dominant intestinal bacteria in the worm gut, it is interesting to found that the four dominant bacteria genera Mesorhizobium, Aliihoeflea, Romboutsia, and Nitrospira are the promising bioindicator of TC stress due to their sensitive response. This work shed novel light on evaluating the ecotoxicological risks posed by residual TC in environment by using a combination of physiological parameters and intestinal bacterial activity in earthworms.

Exploration of an Extracellular Polymeric Substance from Earthworm Gut Bacterium (Bacillus licheniformis) for Bioflocculation and Heavy Metal Removal Potential

The present study shows the potential of an extracellular polymeric substance (EPS) produced by Bacillus licheniformis strain KX657843 isolated from earthworm (Metaphire posthuma) gut in the sorption of Cu(II) and Zn(II) and in flocculation. After harvesting bacterial cells from sucrose supplemented denitrifying culture medium, the EPS was extracted following ethanolic extraction method. The Fourier Transform Infrared Spectroscopy (FTIR) and 1H and 13C Nuclear Magnetic Resonance (NMR) of EPS revealed its functional groups, electronegative constituents, unsaturated carbon, and carbonyl groups. The negatively charged functional groups of carbohydrates and protein moiety of the EPS endowed it with heavy metal binding capacity through electrostatic interactions. The highest flocculation activity (83%) of EPS was observed at 4 mg L−1 and pH 11. The metal sorption by EPS increased with increasing pH. At pH 8, the EPS was able to remove 86 and 81% Cu(II) and Zn(II), respectively, from a 25 mg L−1 metal solution. 94.8% of both the metals at 25 mg L−1 metal solutions were removed by EPS at EPS concentration of 100 mg L−1. From Langmuir isotherm model, the maximum sorption capacities of EPS were calculated to be 58.82 mg g−1 for Cu(II) and 52.45 mg g−1 for Zn(II). The bacterial EPS showed encouraging flocculating and metal sorption properties. The potential to remove Cu(II) and Zn(II) implies that the EPS obtained from the earthworm gut bacteria can be used as an effective agent for environmental remediation of heavy metals and in bioflocculation.

Enteric bacteria from the earthworm (Metaphire posthuma) promote plant growth and remediate toxic trace elements

This work aimed at elucidating the role of bacteria present in the gut of the earthworm Metaphire posthuma in plant growth promotion and toxic trace elements (TTEs) bioremediation. We isolated and identified three bacterial strains Bacillus safensis (MF 589718), Bacillus flexus (MF 589717) and Staphylococcus haemolyticus (MF 589719) among which the Bacillus strains appeared to be significantly more potent than the Staphylococcus strain (P < 0.05) in promoting plant growth and removing TTE (Cr(VI), Cu(II) and Zn(II)) from aqueous media. These strains exhibited several plant growth promoting traits (e.g., indole acetic acid (IAA), gibberellic acid (GA) and ammonium ion production, 1-aminocyclopropane- 1-carboxylic acid (ACC) deaminase activity, and phosphate solubilizing potential). In a pot trial, the gut isolates improved Vigna radiata seed germination, and enhanced the leaf area (30-79%), total chlorophyll content (26-67%) and overall root-shoot biomass (32-83%) as compared to the control. Bacillus safensis and Bacillus flexus were equipotent in removing Cr(VI) (40.5 and 40.3%) from aqueous media; the former triumphed for Zn(II) removal (52.8%), while the latter performed better for Cu(II) removal (43.5%). The gut isolates successfully solubilized phosphate even in TTE-contaminated conditions. The results demonstrate that the earthworm's enteric bacteria possess inherent plant growth promoting, TTE resistance and phosphate solubilization (even under TTE stress) properties which can be further explored for their application in sustainable crop production and environmental management.

Amino Acids and Ribose: Drivers of Protein and RNA Fermentation by Ingested Bacteria of a Primitive Gut Ecosystem

Animal health is linked to gut ecosystems whose primary function is normally the digestion of dietary matter. Earthworms are representative of one of the oldest known animal lineages and, despite their primitive nature, have unique environmental impact by virtue of their dietary consumption of their habitat, i.e., soil-associated matter. A resident gut community is a hallmark of many gut ecosystems of evolutionarily more advanced animals, but the alimentary canal of earthworms is dominated by ingested transient soil microbes. Protein and RNA are (i) the primary organic components of microbial cells that are subject to lysis during gut passage and (ii) fermentable dietary substrates in the alimentary canal. This study examined the gut-associated fermentation of constituents of these biopolymers to determine how their fermentation is integrated to the microbiological dynamics of the gut and might contribute to earthworm-linked transformations of organic matter in the terrestrial biosphere.

Earthworms are among the most primitive animals and are of fundamental importance to the turnover of organic matter in the terrestrial biosphere. These invertebrates ingest materials that are colonized by microbes, some of which are subject to disruption by the crop/gizzard or other lytic events during gut passage. Protein and RNA are dominant polymers of disrupted microbial cells, and these biopolymers facilitate robust fermentations by surviving ingested bacteria. To further resolve these fermentations, amino acids and ribose (as fermentable constituents of protein and RNA, respectively) were evaluated as potential drivers of fermentation in gut content of the model earthworm Lumbricus terrestris (taxa were examined with 16S rRNA-based analyses). Of eight amino acids tested, glutamate, aspartate, and threonine were most stimulatory and yielded dissimilar fermentations facilitated by contrasting taxa (e.g., glutamate stimulated the Fusobacteriaceae and yielded H₂ and formate, whereas aspartate stimulated the Aeromonadaceae and yielded succinate and propionate). A marginal Stickland fermentation was associated with the Peptostreptococcaceae and Lachnospiraceae. Ribose fermentation yielded a complex product profile facilitated primarily by the Aeromonadaceae. The transient nature of succinate was linked to its decarboxylation to propionate and the Fusobacteriaceae, whereas the transient nature of formate was linked to formate-hydrogen lyase activity and the Peptostreptococcaceae. These findings reinforce the likelihood that (i) the animal host and hosted fermentative bacteria compete for the constituents of protein and RNA in the alimentary canal and (ii) diverse gut fermenters engaged in the fermentation of these constituents produce products that can be utilized by earthworms.

IMPORTANCE Animal health is linked to gut ecosystems whose primary function is normally the digestion of dietary matter. Earthworms are representative of one of the oldest known animal lineages and, despite their primitive nature, have unique environmental impact by virtue of their dietary consumption of their habitat, i.e., soil-associated matter. A resident gut community is a hallmark of many gut ecosystems of evolutionarily more advanced animals, but the alimentary canal of earthworms is dominated by ingested transient soil microbes. Protein and RNA are (i) the primary organic components of microbial cells that are subject to lysis during gut passage and (ii) fermentable dietary substrates in the alimentary canal. This study examined the gut-associated fermentation of constituents of these biopolymers to determine how their fermentation is integrated to the microbiological dynamics of the gut and might contribute to earthworm-linked transformations of organic matter in the terrestrial biosphere.

Metaphire guillelmi gut as hospitable micro-environment for the potential transmission of antibiotic resistance genes

Earthworm gut played an important role in the transformation of various contaminants in the soil environments. With the increasing application of organic fertilizer recently, the ingestion of antibiotics, antibiotic resistance bacteria (ARB), and antibiotic resistance genes (ARGs) made the earthworm gut a potential favorable micro-environment for the transmission of ARGs in the soil. In this work, the conventional plate incubation and high-throughput sequencing methods were both employed to investigate the composition of the cultivable and overall ARB/ARGs in the Metaphire guillelmi earthworm gut. A total of 87 cultivable isolates that resisted tetracycline (TC) and/or sulfadiazine (SD) were obtained, most of which belonged to phylum Firmicutes, genus Bacillus. Meanwhile, the counts of isolates with TC-SD dual resistance were higher than those with sole SD or TC resistance. Moreover, higher ARB counts and diversity were detected in the earthworm gut by high-throughput sequencing technique than those by the classical plate cultivation. Overall, the combination of conventional cultivable bacteria isolation and high-throughput sequencing methods provided a comprehensive understanding of the ARB composition in the earthworm gut. The results demonstrate that the earthworm gut is a hospitable micro-environment for ARB colonization. The potential role of earthworm intestinal ARB and ARGs proliferation in soil environments warrants further research.

Mobile Incubator for Iron(III) Reduction in the Gut of the Soil-Feeding Earthworm Pheretima guillelmi and Interaction with Denitrification

Diets of soil-feeding earthworms contain abundant nitrate and iron(III) oxides, which are potential electron acceptors for mineralization of organic compounds. The earthworm gut provides an ideal habitat for ingested iron(III)-reducing microorganisms. However, little is known about iron(III) reduction and its interaction with other processes in the guts of earthworms. Here, we determined the dynamics of iron(III) and revealed its interaction with the turnover of organic acids and nitrate in the gut of the earthworm Pheretima guillelmi. Samples from gut contents combined with anoxic incubation were used for chemical analysis and 16S rRNA based Illumina sequencing. Chemical analysis showed that higher ratios of iron(II)/iron(III), nitrite/nitrate, and more abundant organic acids were contained in the in vivo gut of the earthworm P. guillelmi than those in the in situ soil. A higher rate of iron(III) reduction was detected in treatments of microcosmic incubation with gut contents (IG gut) than that with soil (IG soil), and nitrate reduction occurred earlier than iron(III) reduction in both treatments. Potential iron(III) reducers were dominated by fermentative genera Clostridium, Bacillus, and Desulfotomaculum in the treatment of IG gut, while they were dominated by dissimilatory iron(III)-reducing genera Geobacter in the treatment of IG soil. The iron(III)-reducing microbial community shared several genera with denitrifers in the treatment of IG gut, revealing a close link between iron(III) reduction and denitrification in the gut of earthworms. Collectively, our findings demonstrated that iron(III) reduction occurred along the gut and provided novel insights into the great contribution of earthworm gut microbiota on Fe and the associated C and N cycling in soil environments.

Protein- and RNA-Enhanced Fermentation by Gut Microbiota of the Earthworm Lumbricus terrestris

Earthworms are a dominant macrofauna in soil ecosystems and have determinative effects on soil fertility and plant growth. These invertebrates feed on ingested material, and gizzard-linked disruption of ingested fungal and bacterial cells is conceived to provide diverse biopolymers in the anoxic alimentary canals of earthworms. Fermentation in the gut is likely important to the utilization of ingested biopolymer-derived compounds by the earthworm. This study therefore examined the fermentative responses of gut content-associated microbes of the model earthworm Lumbricus terrestris to (i) microbial cell lysate (to simulate gizzard-disrupted cells) and (ii) dominant biopolymers of such biomass, protein, and RNA. The microbial cell lysate augmented the production of H₂, CO₂, and diverse fatty acids (e.g., formate, acetate, propionate, succinate, and butyrate) in anoxic gut content microcosms, indicating that the cell lysate triggered diverse fermentations. Protein and RNA also augmented diverse fermentations in anoxic microcosms of gut contents, each yielding a distinct product profile (e.g., RNA yielded H₂ and succinate, whereas protein did not). The combined product profile of protein and RNA treatments was similar to that of cell lysate treatments, and 16S rRNA-based analyses indicated that many taxa that responded to cell lysate were similar to taxa that responded to protein or RNA. In particular, protein stimulated Peptostreptococcaceae, Clostridiaceae, and Fusobacteriaceae, whereas RNA stimulated Aeromonadaceae. These findings demonstrate the capacity of gut-associated obligate anaerobes and facultative aerobes to catalyze biopolymer-driven fermentations and highlight the potential importance of protein and RNA as substrates linked to the overall turnover dynamics of organic carbon in the alimentary canal of the earthworm.

IMPORTANCE The subsurface lifestyle of earthworms makes them an unnoticed component of the terrestrial biosphere. However, the propensity of these invertebrates to consume their home, i.e., soil and litter, has long-term impacts on soil fertility, plant growth, and the cycling of elements. The alimentary canals of earthworms can contain up to 500 ml anoxic gut content per square meter of soil, and ingested soil may contain 10⁹ or more microbial cells per gram dry weight, considerations that illustrate that enormous numbers of soil microbes are subject to anoxia during gut passage. Feeding introduces diverse sources of biopolymers to the gut, and the gut fermentation of biopolymers could be important to the transformation of matter by the earthworm and its capacity to utilize fermentation-derived fatty acids. Thus, this study examined the capacity of microbes in earthworm gut contents to ferment protein and RNA, dominant biopolymers of cells that become disrupted during gut passage.