Interactive effects of earthworm Eisenia fetida and bean plant Phaseolus vulgaris L on the fate of soil selenium

Transcriptome analysis revealed accumulation-assimilation of selenium and physio-biochemical changes in alfalfa (Medicago sativa L.) leaves

BACKGROUND

Selenium (Se) is an increasing concern for investigators predominantly because of its consumption in the human body mainly from crops. As the fourth largest plant crop globally, alfalfa is one of the most important forages. Alfalfa was fertilized with selenium(IV) (Se(IV)) under field conditions to study the accumulation and assimilation of Se(IV) and to assess the impact of Se fertilization.

RESULTS

It was analyzed that the physio-biochemistry, Se species, combined with transcriptome after spraying Se(IV) at different times (0, 12, and 48 h). 9402 and 12 607 differentially expressed genes (DEGs) were identified at 12 h (versus 0 h) and 48 h (versus 12 h). DEG functional enrichments proposed two time-specific biological processes: Se(IV) accumulation was the primary process at 0-12 h, and its assimilation mainly occurred during 12-48 h. This was further proved by the separation of various Se speciation at different times. It showed that Se-supplementation also affected the soluble protein, soluble sugar, pigment contents and antioxidant capacity. Selenium-biofortification could improve the stress resistance of alfalfa by enhancing antioxidant system to scavenge reactive oxygen species (e.g. hydrogen peroxide) and boosting carbohydrate metabolism.

CONCLUSION

By integrating physio-biochemistry, Se-related metabolites, and transcriptome under Se(IV) treatment, this study provides data to guide further work on Se-fortification in alfalfa. © 2022 Society of Chemical Industry.

Bioremediation of selenium-contaminated soil using earthworm Eisenia fetida: Effects of gut bacteria in feces on the soil microbiome

Selenium (Se) contamination in the soil poses a food safety risk to humans. The present study was to investigate the role of earthworm Eisenia fetida in soil Se remediation. When exposed to selenite at 4 mg Se/kg, E. fetida efficiently concentrated Se in tissues (24.53 mg Se/kg dry weight), however, only accounting for a minor portion of the added Se. Microbial analysis shows 12 out of 15 functional genera became more abundant in the worm-inhabited soil when exposed to Se, suggesting E. fetida contributed to Se remediation mainly by introducing Se-reducing bacteria to the soil via feces, which were dominated by the genera Pseudomonas (∼62.65%) and Aeromonas (∼29.99%), whose abundance was also significantly boosted in the worm-inhabited soil. However, when isolated from worm feces at 200 mg Se/L, Pseudomonas strains only displayed a high tolerance to Se rather than removal capacity. In contrast, among 4 isolated Aeromonas strains, A. caviae rapidly removing 85.74% of the added selenite, mainly through accumulation (67.38%), while A. hydrophila and A. veronii were more effective at volatilizing Se (27.77% and 24.54%, respectively), and A. media performed best by reducing Se by ∼49.00% under anaerobic conditions. Overall, our findings have highlighted the importance of E. fetida as a key contributor of functional bacteria to the soil microbiome, building a strong foundation for the development of an earthworm-soil system for Se bioremediation.

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.

Valorization of selenium-enriched sludge and duckweed generated from wastewater as micronutrient biofertilizer

Selenium (Se) is an essential trace element for humans and animals with a narrow window between deficiency and toxicity levels. Application of conventional chemical Se fertilizers to increase the Se content of crops in Se deficient areas could result in environmental contamination due to the fast leaching of inorganic Se. Slow-release Se-enriched biofertilizers produced from wastewater treatment may therefore be beneficial. In this study, the potential of Se-enriched biomaterials (sludge and duckweed) as slow-release Se biofertilizers was evaluated through pot experiments with and without planted green beans (Phaseolus vulgaris). The Se concentration in the bean tissues was 1.1-3.1 times higher when soils were amended with Se-enriched sludge as compared to Se-enriched duckweed. The results proved that the Se released from Se-enriched biomaterials was efficiently transformed to health-beneficial selenoamino acids (e.g., Se-methionine, 76-89%) after being taken up by beans. The Se-enriched sludge, containing mainly elemental Se, is considered as the preferred slow-release Se biofertilizer and an effective Se source to produce Se-enriched crops for Se-deficient populations, as shown by the higher Se bioavailability and lower organic carbon content. This study could offer a theoretical reference to choose an environmental-friendly and sustainable alternative to conventional mineral Se fertilizers for biofortification, avoiding the problem of Se losses by leaching from chemical Se fertilizers while recovering resources from wastewater. This could contribute to the driver for a future circular economy.

Effect of Se-Enriched Irrigation Water on the Biomass Production and Elemental Composition of Green Bean, Cabbage, Potato and Tomato

Additional Selenium (Se) intake may be recommended in areas of Se deficiency to prevent various human diseases. One possibility for this is biofortification. In this experiment, the effect of irrigation water containing 100 and 500 µg L⁻¹ Se, in the form of Na₂SeO₄, on green bean, cabbage, potato and tomato was investigated in a greenhouse pot experiment with sand, silty sand and silt soils. The chlorophyll content index was usually improved by Se and was significantly higher in potato in sand and silty sand and in tomato in silty sand and silt soils. The Se content of edible plant parts increased 63-fold in the 100 µg L⁻¹ Se treatment and almost 400-fold in the 500 µg L⁻¹ Se treatment, averaged over the four species and the three soils. Irrigation water with a Se content of 100 µg L⁻¹ may be suitable for the production of functional food in the case of green beans, potatoes and tomatoes. However, due to its greater Se accumulation, cabbage should only be irrigated with a lower Se concentration. The use of Se-enriched irrigation water might be a suitable method for Se biofortification without a significant reduction in plant biomass production and without a remarkable modification of other macro- and microelement contents.

Remediation of selenium-contaminated soil through combined use of earthworm Eisenia fetida and organic materials

Selenium (Se)-polluted soils pose serious threats to terrestrial ecosystems through food chains. This study evaluated the use of earthworm Eisenia fetida and organic materials for Se remediation. The greater mortality (6.7%) and weight loss (28.6%) were observed for earthworms exposed to selenate than selenite at 20 mg Se Kg^-1 over 21 d, while selenate was taken up 2.5-fold faster than selenite. Compared with peat moss, Se accumulation increased by 119% in selenite-exposed earthworms supplied with cow manure. Earthworm activity caused greater reduction in selenite (17.4%), with little impact on selenate-treated soil. X-ray absorption spectroscopy speciation analysis shows 87-91% of tissue Se was transformed to organo-Se, i.e., SeMet and SeCys, in earthworms exposed to either selenite or selenate, posing great risks to their predators. The study also found selenium increased bacterial diversity in earthworm casts, while greater relative abundances (~37.57%) of functional genera were obtained for selenite. Over 24 h, two bacteria strains, Bacillus cereus and Aeromonas encheleia, isolated from casts, rapidly reduced selenite by ~94%, compared to ~25% for selenate. Elemental Se was present only in strains (~27%), casts (~11%) and worm-inhabited soil (~2.7%) of selenite treatments, suggesting earthworm gut microbiota could buffer earthworms and other soil fauna from selenite toxicity.

Insights into the mechanisms underlying the remediation potential of earthworms in contaminated soil: A critical review of research progress and prospects

In recent years, soil pollution is a major global concern drawing worldwide attention. Earthworms can resist high concentrations of soil pollutants and play a vital role in removing them effectively. Vermiremediation, using earthworms to remove contaminants from soil or help to degrade non-recyclable chemicals, is proved to be an alternative, low-cost technology for treating contaminated soil. However, knowledge about the mechanisms and framework of the vermiremediation various organic and inorganic contaminants is still limited. Therefore, we reviewed the research progress of effects of soil contaminants on earthworms and potential of earthworm used for remediation soil contaminated with heavy metals, polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), polycyclic aromatic hydrocarbons (PAHs), pesticides, as well as crude oil. Especially, the possible processes, mechanisms, advantages and limitations, and how to boost the efficiency of vermiremediation are well addressed in this review. Finally, future prospects of vermiremediation soil contamination are listed to promote further studies and application of vermiremediation in contaminated soils.