Earthworm gut: An overlooked niche for anaerobic ammonium oxidation in agricultural soil

The interaction between biochar and earthworms: Revealing the potential ecological risks of biochar application and the feasibility of their co-application

Biochar's interaction with soil-dwelling organisms, particularly earthworms, is crucial in ensuring the effective and secure utilization of biochar in the soil. This review introduces the application of biochar in soil, summarizes how earthworms respond to biochar-amended soil and the underlying factors that can influence their response, discusses the synergistic and antagonistic impacts of earthworm activity on the efficacy of biochar, and considers the feasibility of applying them together. A review of existing research has identified uncertainty in the effect of biochar exposure on earthworms, with biochar derived from animal wastes, produced at higher pyrolysis temperatures, and used at higher doses of biochar having more negative effects on earthworms. Habitat modification, toxicity release, particle effects, and contaminant immobilization are underlying factors in how biochar affects earthworm indicators. While biochar in contaminated soils may alleviate the stress of pollutants on earthworms by decreasing their bioaccumulation, this remedial effect is not always effective. Additionally, earthworm bioturbation can enhance the migration, fragmentation, and oxidation of biochar, while also stimulating extracellular enzymes that convert biochar into 'vermichar'. Earthworms and biochar can synergize well to improve soil fertility and remediate soil organic pollution, yet exhibit contrasting roles in soil C sequestration and immobilizing heavy metals in soil. These findings highlight both the advantages and risks of their co-application. Therefore, when considering the use of biochar alone or with earthworms, it is crucial to thoroughly assess its potential ecotoxicity on earthworms and other soil organisms, as well as the influence of bioturbation, such as that caused by earthworms, on the effectiveness of biochar.

Earthworm gut bacteria facilitate cadmium immobilization through the formation of CdS nanoparticles

Gut bacteria of earthworm Amynthas hupeiensis exhibit significant potential for the in-situ remediation of cadmium (Cd)-contaminated soil. However, the mechanisms by which these gut bacteria immobilize and tolerate Cd remain elusive. The composition of the gut bacterial community was characterized by high-throughput sequencing. Cd-tolerant bacteria were isolated from the gut, and their roles in Cd immobilization, as well as their tolerance mechanisms, were explored through chemical characterization and transcriptome analysis. The predominant taxa in the gut bacterial community included unclassified Enterobacteriaceae, Citrobacter, and Bacillus, which were distinctly different from those in the surrounding soil. Notably, the most Cd-tolerant gut bacterium, Citrobacter freundii DS strain, immobilized 63.61% of Cd2+ within 96 h through extracellular biosorption and intracellular bioaccumulation of biosynthetic CdS nanoparticles, and modulation of solution pH and NH4+ concentration. Moreover, the characteristic signals of CdS were also observed in the gut content of A. hupeiensis when the sterilized Cd-contaminated soil was inoculated with C. freundii. The primary pathways involved in the response of C. freundii to Cd stress included the regulation of ABC transporters, bacterial chemotaxis, cell motility, oxidative phosphorylation, and two-component system. In conclusion, C. freundii facilitates Cd immobilization both in vitro and in vivo, thereby enhancing the host earthworm's adaptation to Cd-contaminated soil.

Endosymbiotic bacteria within the nematode-trapping fungus Arthrobotrys musiformis and their potential roles in nitrogen cycling

Endosymbiotic bacteria (ESB) have important effects on their hosts, contributing to its growth, reproduction and biological functions. Although the effects of exogenous bacteria on the trap formation of nematode-trapping fungi (NTF) have been revealed, the effects of ESB on NTF remain unknown. In this study, we investigated the species diversity of ESB in the NTF Arthrobotrys musiformis using high-throughput sequencing and culture-dependent approaches, and compared bacterial profiles to assess the effects of strain source and culture media on A. musiformis. PICRUSt2 and FAPROTAX were used to predict bacterial function. Our study revealed that bacterial communities in A. musiformis displayed high diversity and heterogeneity, with Proteobacteria, Firmicutes, Bacteroidetes and Actinobacteria as the dominant phyla. The ESB between A. musiformis groups isolated from different habitats and cultured in the same medium were more similar to each other than the other groups isolated from the same habitat but cultured in different media. Function analysis predicted a broad and diverse functional repertoire of ESB in A. musiformis, and unveiled that ESB have the potential to function in five modules of the nitrogen metabolism. We isolated nitrogen-fixing and denitrifying bacteria from the ESB and demonstrated their effects on trap formation of A. musiformis. Among seven bacteria that we tested, three bacterial species Bacillus licheniformis, Achromobacter xylosoxidans and Stenotrophomonas maltophilia were found to be efficient in inducing trap formation. In conclusion, this study revealed extensive ESB diversity within NTF and demonstrated that these bacteria likely play important roles in nitrogen cycling, including nematode trap formation.

Removal of sulfamethoxazole and antibiotic resistance genes in paddy soil by earthworms (Pheretima guillelmi): Intestinal detoxification and stimulation of indigenous soil bacteria

Vermiremediation, which use earthworms to remove contaminants from soil, has been proven to be an alternative, low-cost technology. However, the effects of earthworm activity, especially the degraders in earthworm intestines, on the fate of sulfamethoxazole (SMX), and the effects of intestinal bacteria on degrading bacteria in soil are unclear. In this study, the effects of earthworms on the fate of SMX and related antibiotic resistance genes (ARGs) were investigated. Special attention was paid to the impact of earthworms on SMX degradation efficiency, degradation products, related ARGs, and degraders in both soil and earthworm intestines; the effect of intestinal bacteria on soil bacteria associated with SMX was also studied. Earthworms significantly accelerated SMX degradation by both intestinal detoxification and the stimulation of indigenous soil bacteria. Compared with the treatment without earthworms, the treatment with earthworms reduced SMX residues by 25.1 %, 49.2 %, 35.7 %, 34.2 %, and 35.7 % on the 10th, 20th, 30th, 60th, and 90th days, respectively. Compared with those in soil (treated with earthworms), the SMX residues in wormcasts were further reduced by 12.2-29.0 % from the 2nd to the 20th day, producing some unique anaerobic degradation products that were distinct from those in the soil. In earthworm intestines, SMX degradation was enhanced by bacteria of the genera Microvirga, Sphingomonas, Methylobacterium, Bacillus, and Tumebacillus. All of these bacteria (except Bacillus spp.) entered and colonised the soil with wormcasts, further promoting SMX degradation. Additionally, earthworms removed a significant number of ARGs by increasing the fraction of potential SMX degraders and inhibiting the potential hosts of ARGs and int1. This study demonstrated that earthworms could remediate SMX-contaminated soil by enhancing the removal of SMX and ARGs.

Earthworms reduce the dissemination potential of antibiotic resistance genes by changing bacterial co-occurrence patterns in soil

Globally distributed earthworms affect compositions of soil compounds, microbial community structures, as well as antibiotic resistance genes (ARGs). Compared to their surroundings, earthworm gut is a simpler environment which could filter out microbes when soil passes through it. However, little is known about how earthworms affect the dissemination of ARGs in soil, and the understanding of the relationship between microbe-microbe interactions and ARGs is still lacking. Here, we designed a microcosm experiment with earthworm addition, and determined bacterial and fungal community compositions based on amplicon sequencing. We also examined mobile genetic elements (MGEs) and ARGs in earthworm gut and soils using high-throughput qPCR. The results showed significant differences of bacterial, fungal and ARG patterns between gut and soil. Earthworms indirectly impacted the patterns of ARGs in soils by affecting bacterial communities and soil properties, which play key roles in the distribution of ARGs and MGEs. The absolute abundances of MGEs in earthworm gut were significantly lower than those in soils, and earthworms reduce the absolute abundance of MGEs in soils. Earthworms changed the microbial co-occurrence patterns, and reduced bacterial connectivity, which were significantly and positively correlated with MGE abundance. These results highlight the importance of earthworm on the distribution and dissemination of ARGs in soils.

Earthworms as candidates for remediation of potentially toxic elements contaminated soils and mitigating the environmental and human health risks: A review

Global concerns towards potentially toxic elements (PTEs) are steadily increasing due to the significant threats that PTEs pose to human health and environmental quality. This calls for immediate, effective and efficient remediation solutions. Earthworms, the 'ecosystem engineers', can modify and improve soil health and enhance plant productivity. Recently, considerable attention has been paid to the potential of earthworms, alone or combined with other soil organisms and/or soil amendments, to remediate PTEs contaminated soils. However, the use of earthworms in the remediation of PTEs contaminated soil (i.e., vermiremediation) has not been thoroughly reviewed to date. Therefore, this review discusses and provides comprehensive insights into the suitability of earthworms as potential candidates for bioremediation of PTEs contaminated soils and mitigating environmental and human health risks. Specifically, we reviewed and discussed: i) the occurrence and abundance of earthworms in PTEs contaminated soils; ii) the influence of PTEs on earthworm communities in contaminated soils; iii) factors affecting earthworm PTEs accumulation and elimination, and iv) the dynamics and fate of PTEs in earthworm amended soils. The technical feasibility, knowledge gaps, and practical challenges have been worked out and critically discussed. Therefore, this review could provide a reference and guidance for bio-restoration of PTEs contaminated soils and shall also help developing innovative and applicable solutions for controlling PTEs bioavailability for the remediation of contaminated soils and the mitigation of the environment and human risks.

The impact of invertebrate decomposers on plants and soil

Soil invertebrates make significant contributions to the recycling of dead plant material across the globe. However, studies focussed on the consequences of decomposition for plant communities largely ignore soil fauna across all ecosystems, because microbes are often considered the primary agents of decay. Here, we explore the role of invertebrates as not simply facilitators of microbial decomposition, but as true decomposers, able to break down dead organic matter with their own endogenic enzymes, with direct and indirect impacts on the soil environment and plants. We recommend a holistic view of decomposition, highlighting how invertebrates and microbes act in synergy to degrade organic matter, providing ecological services that underpin plant growth and survival.