Synergistic Impacts of Arsenic and Antimony Co-contamination on Diazotrophic Communities

Phototrophic Nitrogen Fixation, a Neglected Biogeochemical Process in Mine Tailings?

Biological nitrogen fixation (BNF) has important ecological significance in mine tailing by contributing to the initial accumulation of nitrogen. In addition to chemolithotrophic and heterotrophic BNF, light may also fuel BNF in oligotrophic mine tailings. However, knowledge regarding the occurrence and ecological significance of this biogeochemical process in mine tailings remains ambiguous. The current study observed phototrophic BNF in enrichment cultures established from three primary successional stages (i.e., original tailings, biological crusts, and pioneer plants) of tailings. Notably, phototrophic BNF in tailings may be more active at vegetation stages (i.e., biological crusts and pioneering plants) than in bare tailings. DNA-stable isotope probing identified Roseomonas species as potential aerobic anoxygenic phototrophs responsible for phototrophic BNF. Furthermore, metagenomic binning as well as genome mining revealed that Roseomonas spp. contained essential genes involved in nitrogen fixation, anoxygenic photosynthesis, and carbon fixation, suggesting their genetic potential to mediate phototrophic BNF. A causal inference framework equipped with the structural causal model suggested that the enrichment of putative phototrophic diazotrophic Roseomonas may contribute to an elevated total nitrogen content during primary succession in these mine tailings. Collectively, our findings suggest that phototrophic diazotrophs may play important roles in nutrient accumulation and hold the potential to facilitate ecological succession in tailings.

Effects of nitrogen supply on hydrogen-oxidizing bacterial enrichment to produce microbial protein: Comparing nitrogen fixation and ammonium assimilation

To investigate the effects of nitrogen source supply on microbial protein (MP) production by hydrogen-oxidizing bacteria (HOB) under continuous feed gas provision, a sequencing batch culture comparison (N2 fixation versus ammonium assimilation) was performed. The results confirmed that even under basic cultivation conditions, N2-fixing HOB (NF-HOB) communities showed higher levels of CO2 and N2 fixation (190.45 mg/L Δ CODt and 11.75 mg/L Δ TNbiomass) than previously known, with the highest biomass yield being 0.153 g CDW/g COD-H2. Rich ammonium stimulated MP synthesis and the biomass accumulation of communities (increased by 7.4 ～ 14.3 times), presumably through the enhancement of H2 and CO2 absorption. The micro mechanism may involve encouraging the enrichment of species like Xanthobacter and Acinetobacter then raising the abundance of nitrogenase and glutamate synthase to facilitate the nitrogen assimilation. This would provide NF-HOB with ideas for optimizing their MP synthesis activity.

Use of Microbial Consortia in Bioremediation of Metalloid Polluted Environments

Metalloids are released into the environment due to the erosion of the rocks or anthropogenic activities, causing problems for human health in different world regions. Meanwhile, microorganisms with different mechanisms to tolerate and detoxify metalloid contaminants have an essential role in reducing risks. In this review, we first define metalloids and bioremediation methods and examine the ecology and biodiversity of microorganisms in areas contaminated with these metalloids. Then we studied the genes and proteins involved in the tolerance, transport, uptake, and reduction of these metalloids. Most of these studies focused on a single metalloid and co-contamination of multiple pollutants were poorly discussed in the literature. Furthermore, microbial communication within consortia was rarely explored. Finally, we summarized the microbial relationships between microorganisms in consortia and biofilms to remove one or more contaminants. Therefore, this review article contains valuable information about microbial consortia and their mechanisms in the bioremediation of metalloids.

Chemolithotrophic Biological Nitrogen Fixation Fueled by Antimonite Oxidation May Be Widespread in Sb-Contaminated Habitats

Nitrogen (N) deficiency in mining-contaminated habitats usually hinders plant growth and thus hampers tailing revegetation. Biological N fixation (BNF) is an essential biogeochemical process that contributes to the initial accumulation of N in oligotrophic mining-contaminated regions. Previous studies reported that chemolithotrophic rather than heterotrophic diazotrophs frequently dominated in the mining-contaminated regions. Chemolithotrophic diazotrophs may utilize elements abundant in such habitats (e.g., sulfur (S), arsenic (As), and antimony (Sb)) as electron donors to fix N₂. BNF fueled by the oxidation of S and As has been detected in previous studies. However, BNF fueled by Sb(III) oxidation (Sb-dependent BNF) has never been reported. The current study observed the presence of Sb-dependent BNF in slurries inoculated from Sb-contaminated habitats across the South China Sb belt, suggesting that Sb-dependent BNF may be widespread in this region. DNA-stable isotope probing identified bacteria associated with Rhodocyclaceae and Rhizobiaceae as putative microorganisms responsible for Sb-dependent BNF. Furthermore, metagenomic-binning demonstrated that Rhodocyclaceae and Rhizobiaceae contained essential genes involved in Sb(III) oxidation, N₂ fixation, and carbon fixation, suggesting their genetic potential for Sb-dependent BNF. In addition, meta-analysis indicated that these bacteria are widespread among Sb-contaminated habitats with different niche preferences: Rhodocyclaceae was enriched in river sediments and tailings, while Rhizobiaceae was enriched only in soils. This study may broaden our fundamental understanding of N fixation in Sb-mining regions.

Characterization of diazotrophic root endophytes in Chinese silvergrass (Miscanthus sinensis)

BACKGROUND

Phytoremediation is a potentially cost-effective way to remediate highly contaminated mine tailing sites. However, nutrient limitations, especially the deficiency of nitrogen (N), can hinder the growth of plants and impair the phytoremediation of mine tailings. Nevertheless, pioneer plants can successfully colonize mine tailings and exhibit potential for tailing phytoremediation. Diazotrophs, especially diazotrophic endophytes, can promote the growth of their host plants. This was tested in a mine-tailing habitat by a combination of field sampling, DNA-stable isotope probing (SIP) analysis, and pot experiments.

RESULTS

Bacteria belonging to the genera Herbaspirillum, Rhizobium, Devosia, Pseudomonas, Microbacterium, and Delftia are crucial endophytes for Chinese silvergrass (Miscanthus sinensis) grown in the tailing, the model pioneer plant selected in this study. Further, DNA-SIP using ¹⁵N₂ identified Pseudomonas, Rhizobium, and Exiguobacterium as putative diazotrophic endophytes of M. sinensis. Metagenomic-binning suggested that these bacteria contained essential genes for nitrogen fixation and plant growth promotion. Finally, two diazotrophic endophytes Rhizobium sp. G-14 and Pseudomonas sp. Y-5 were isolated from M. sinensis. Inoculation of another pioneer plant in mine tailings, Bidens pilosa, with diazotrophic endophytes resulted in successful plant colonization, significantly increased nitrogen fixation activity, and promotion of plant growth.

CONCLUSIONS

This study indicated that diazotrophic endophytes have the potential to promote the growth of pioneer plant B. pilosa in mine tailings. Video Abstract.

Primary Succession Changes the Composition and Functioning of the Protist Community on Mine Tailings, Especially Phototrophic Protists

Primary succession in mine tailings is a prerequisite for tailing vegetation. Microorganisms, including bacteria, fungi, and protists, play an important role in this process in the driving force for improving the nutritional status. Compared to bacteria and fungi, protist populations have rarely been investigated regarding their role in mine tailings, especially for those inhabiting tailings associated with primary succession. Protists are the primary consumers of fungi and bacteria, and their predatory actions promote the release of nutrients immobilized in the microbial biomass, as well as the uptake and turnover of nutrients, affecting the functions of the wider ecosystems. In this study, three different types of mine tailings associated with three successional stages (original tailings, biological crusts, and Miscanthus sinensis grasslands) were selected to characterize the protistan community diversity, structure, and function during primary succession. Some members classified as consumers dominated the network of microbial communities in the tailings, especially in the original bare land tailings. The keystone phototrophs of Chlorophyceae and Trebouxiophyceae showed the highest relative abundance in the biological crusts and grassland rhizosphere, respectively. In addition, the co-occurrences between protist and bacterial taxa demonstrated that the proportion of protistan phototrophs gradually increased during primary succession. Further, the metagenomic analysis of protistan metabolic potential showed that abundances of many functional genes associated with photosynthesis increased during the primary succession of tailings. Overall, these results suggest that the primary succession of mine tailings drives the changes observed in the protistan community, and in turn, the protistan phototrophs facilitate the primary succession of tailings. This research offers an initial insight into the changes in biodiversity, structure, and function of the protistan community during ecological succession on tailings.

Effects of perfluorooctanoic acid (PFOA) on activated sludge microbial community under aerobic and anaerobic conditions

Per- and polyfluoroalkyl substances (PFASs) have received increasing attention due to their widespread presence in diverse environments including wastewater treatment plants (WWTPs) and their potential adverse health effects. Perfluorooctanoic acid (PFOA) is one of the most detected forms of PFASs in WWTPs. However, there is still a paucity of knowledge about the effect of PFASs on microorganisms of the key component of WWTP, activated sludge. In this study, lab-scale microcosm experiments were established to evaluate the influences of PFOA on activated sludge microbes under aerobic and anaerobic conditions. The diversity, structure, and microbe-microbe interaction of microbial community were determined by 16S rRNA gene amplicon sequencing and co-occurrence network analysis. After 90 days of exposure to PFOA, activated sludge microbial richness decreased under both aerobic and anaerobic conditions. Specifically, under aerobic condition, Rhodopseudomonas (mean relative abundance 3.6%), Flavobacterium (2.4%), and Ignavibacterium (6.6%) were enriched in PFOA-spiked activated sludge compared with that in the unspiked sludge (2.6%, 0.1%, and 1.9%, respectively). By contrast, after 90 days of exposure to PFOA, Eubacterium (2.1%), Hyphomicrobium (1.8%), and Methyloversatilis (1.2%) were enriched under anaerobic condition, and more abundant than that in the control sludge (0.4%, 1.5%, and 0.6%, respectively). These genera were the potential PFOA-resistant members. In addition, Azospirillum and Sporomusa were the most connected taxa in PFOA-aerobic and PFOA-anaerobic networks, respectively. Prediction of the functional gene showed that PFOA inhibited some gene expression of sludge microbes, such as transcription, amino acid transport and metabolism, and energy production and conversion. In summary, continued exposure to PFOA induced substantial shifts of the sludge bacterial diversity and composition under both aerobic and anaerobic conditions.

Keystone taxa and functional analysis in arsenic and antimony co-contaminated rice terraces

Both arsenic (As) and antimony (Sb) are primary environmental contaminants that often co-exist at contaminated sites. Though the microbial community compositions of As- and Sb-contaminated sites have been previously described, the changes in microbial community interactions and community functions remain elusive. In the current study, several key metabolic processes, such as As/Sb detoxification and carbon fixation, were enriched under heavily contaminated conditions. Furthermore, the identified keystone taxa, which are associated with the families Nitrosomonadaceae, Pedosphaeraceae, Halieaceae, and Latescibacterota, demonstrated positive correlations with As and Sb concentrations, indicating that they may be resistant to As and Sb toxicities. Accordingly, arsenic resistance-related functions, along with several functions such as carbon fixation, were found to be enriched in heavily contaminated sites. The current study elucidated the key microbial populations in As- and Sb-contaminated rice terraces and may provide useful information for remediation purposes.

Serratia spp. Are Responsible for Nitrogen Fixation Fueled by As(III) Oxidation, a Novel Biogeochemical Process Identified in Mine Tailings

Biological nitrogen fixation (BNF) has important environmental implications in tailings by providing bioavailable nitrogen to these habitats and sustaining ecosystem functions. Previously, chemolithotrophic diazotrophs that dominate in mine tailings were shown to use reduced sulfur (S) as the electron donor. Tailings often contain high concentrations of As(III) that might function as an alternative electron donor to fuel BNF. Here, we tested this hypothesis and report on BNF fueled by As(III) oxidation as a novel biogeochemical process in addition to BNF fueled by S. Arsenic (As)-dependent BNF was detected in cultures inoculated from As-rich tailing samples derived from the Xikuangshan mining area in China, as suggested by nitrogenase activity assays, quantitative polymerase chain reaction, and ¹⁵N₂ enrichment incubations. As-dependent BNF was also active in eight other As-contaminated tailings and soils, suggesting that the potential for As-dependent BNF may be widespread in As-rich habitats. DNA-stable isotope probing identified Serratia spp. as the bacteria responsible for As-dependent BNF. Metagenomic binning indicated that the essential genes for As-dependent BNF [i.e., nitrogen fixation, As(III) oxidation, and carbon fixation] were present in Serratia-associated metagenome-assembled genomes. Over 20 Serratia genomes obtained from NCBI also contained essential genes for both As(III) oxidation and BNF (i.e., aioA and nifH), suggesting that As-dependent BNF may be a widespread metabolic trait in Serratia spp.