Hexavalent chromium remediation based on the synergistic effect between chemoautotrophic bacteria and sulfide minerals.

Hexavalent chromium (Cr(VI)) is an environmental concern due to the carcinogenic and mutagenic effect on living organisms. Sulfide minerals based Cr(VI) reduction is an economical and efficient strategy for Cr(VI) remediation. In this study, Cr(VI) reduction through the synergistic effect between chemoautotrophic bacteria and sulfide mineral is systematically investigated. Sulfide minerals dissolution and Cr(VI) reduction performance highly depends on mineral acid soluble property. Cr(VI) reduction capacity of pyrrhotite, pyrite, marcasite and sphalerite was 50, 104, 104 and 44 mg/g (Cr(VI)/mineral) respectively in the biotic system. Acidithiobacillus ferrooxidans (A. ferrooxidans) significantly enhanced pyrite and marcasite based Cr(VI) reduction kinetic and capacity. Proton consumption, iron coprecipitation and the biological activity deficiency in the abiotic system significantly inhibited Cr(VI) reduction. Elemental sulfur and secondary iron mineral as the main composition of the passivation layer inhibited sustainable Cr(VI) reduction. A. ferrooxidans facilitated acid nonsoluble mineral dissolution and surface passivation layer removal, and promoted Cr(VI) reduction. Acid nonsoluble sulfide mineral disulfide bond rapture, S°/Sn2- oxidization, and Fe(III)/Cr(III) dissolution were accelerated by A. ferrooxidans, which facilitated Cr(VI) reduction reactive sites regeneration. Our study demonstrated that chemoautotrophic bacterial accelerated Cr(VI) reduction reaction through promoting acid nonsoluble sulfide mineral dissolution. This research is of environmental and practical significance to remediate redox sensitive contaminant based on the synergistic effect between sulfide minerals and chemoautotrophic A. ferrooxidans.

Synergistic effect between sulfide mineral and acidophilic bacteria significantly promoted Cr(VI) reduction.

Natural pyrite was an economical choice for efficient Cr(VI) remediation, while its deep utilization was inhibited due to the passivation effect. In this study, pyrite passivation/dissolution and active sites regeneration mechanism under the activity of acidophilic bacteria with different energy metabolism characteristic in Cr(VI) reduction have been investigated. The reduction capacity was in the order of Acidithiobacillus thiooxidans, Acidithiobacillus ferrooxidans(S), Acidithiobacillus ferrooxidans(Fe), Leptospirillum ferrooxidans and chemical control. The maximal reduction efficiency was achieved in A. thiooxidans system, which is 4.5 times higher than the L. ferrooxidans system. In chemical system, sulfur and Fe(III)/Cr(III)-oxyhydroxysulphate accumulation would result in pyrite passivation. A. thiooxidans attached on pyrite surface and exerted synergistic effect on pyrite corrosion coupled with Cr(VI). Sulfur oxidation promoted proton regeneration, pyrite lattice Fe(II) dissolution and active sites regeneration, which were beneficial to sustainable Cr(VI) reduction. Secondary iron mineral formation on pyrite was accelerated with the iron oxidation bacteria activity increasing. Excessive oxidation to surface sites Fe(II) and the accumulation of S0/Sn2- led to the passivation effect in L. ferrooxidans system. Cr(VI) acquired electron from Fe(II) and disulfide and resulted in the bond break between them. The combined effect of specific sulfur oxidizing bacteria activity and Cr(VI) oxidation efficient promoted pyrite dissolution and active sites regeneration. The interaction between acidophilic bacteria and pyrite significantly enhanced Cr(VI) reduction efficiency.

Mangrove sediments are environmental hotspots for pathogenic protists.

Mangrove sediments are unique ecosystems providing habitats for diverse organisms, especially microbial communities. However, little is known about the diversity and environmental risk of a critical group of microorganisms, the protists. To address this gap, we employed metagenome sequencing technologies to provide the first comprehensive view of the protistan community in the mangrove sediment. Our results surprisingly showed that parasitic protists dominated the protistan community in mangrove sediments, with an average abundance of 59.67%, one of the highest in all ecosystems on Earth. We also found that the relative abundance of protists decreased significantly (R = -0.21, p = 0.045) with latitude but increased with depths (R = 0.7099, p < 0.001). The parasitic communities were positively influenced by microbial (bacteria, fungi, and archaea) communities, including horizontal-scale and vertical-scale. In addition, sulfate and salinity had the most significant influence on the protistan community. Our findings provide new insights into our understanding of protistan variation in mangrove sediments, including abundance, composition, and possible functions, and indicate that mangrove sediments are hotspots for environmental pathogens, posing a potential risk to human health.

Unraveling the important role of comammox Nitrospira to nitrification in the coastal aquaculture system.

Increasing nitrogen (N) input to coastal ecosystems poses a serious environmental threat. It is important to understand the responses and feedback of N removal microbial communities, particularly nitrifiers including the newly recognized complete ammonia-oxidizers (comammox), to improve aquaculture sustainability. In this study, we conducted a holistic evaluation of the functional communities responsible for nitrification by quantifying and sequencing the key functional genes of comammox Nitrospira-amoA, AOA-amoA, AOB-amoA and Nitrospira-nxrB in fish ponds with different fish feeding levels and evaluated the contribution of nitrifiers in the nitrification process through experiments of mixing pure cultures. We found that higher fish feeding dramatically increased N-related concentration, affecting the nitrifying communities. Compared to AOA and AOB, comammox Nitrospira and NOB were more sensitive to environmental changes. Unexpectedly, we detected an equivalent abundance of comammox Nitrospira and AOB and observed an increase in the proportion of clade A in comammox Nitrospira with the increase in fish feeding. Furthermore, a simplified network and shift of keystone species from NOB to comammox Nitrospira were observed in higher fish-feeding ponds. Random forest analysis suggested that the comammox Nitrospira community played a critical role in the nitrification of eutrophic aquaculture ponds (40-70 µM). Through the additional experiment of mixing nitrifying pure cultures, we found that comammox Nitrospira is the primary contributor to the nitrification process at 200 µM ammonium. These results advance our understanding of nitrifying communities and highlight the importance of comammox Nitrospira in driving nitrification in eutrophic aquaculture systems.

The vertically-stratified resistomes in mangrove sediments was driven by the bacterial diversity.

Early evidence has elucidated that the spread of antibiotic (ARGs) and metal resistance genes (MRGs) are mainly attributed to the selection pressure in human-influenced environments. However, whether and how biotic and abiotic factors mediate the distribution of ARGs and MRGs in mangrove sediments under natural sedimentation is largely unclear. Here, we profiled the abundance and diversity of ARGs and MRGs and their relationships with sedimental microbiomes in 0-100 cm mangrove sediments. Our results identified multidrug-resistance and multimetal-resistance as the most abundant ARG and MRG classes, and their abundances generally decreased with the sediment depth. Instead of abiotic factors such as nutrients and antibiotics, the bacterial diversity was significantly negatively correlated with the abundance and diversity of resistomes. Also, the majority of resistance classes (e.g., multidrug and arsenic) were carried by more diverse bacterial hosts in deep layers with low abundances of resistance genes. Together, our results indicated that bacterial diversity was the most important biotic factor driving the vertical profile of ARGs and MRGs in the mangrove sediment. Given that there is a foreseeable increasing human impact on natural environments, this study emphasizes the important role of biodiversity in driving the abundance and diversity of ARGs and MRGs.

MCycDB: A curated database for comprehensively profiling methane cycling processes of environmental microbiomes.

Methane is a critical greenhouse gas with significant impacts on environmental and global change. However, CH4 cycling processes and coupling mechanisms with the biogeochemical cycling of carbon, nitrogen, sulfur and metals in the environment remain elusive. To fill such knowledge gaps, we constructed a manually curated methane cycling database (MCycDB) for comprehensive and accurate analysis of methane cycling microbial communities. MCycDB contains 298 methane cycling gene families covering 10 methane metabolism pathways with 610,208 representative sequences, and associated reference sequences from the NCBI RefSeq database with 48 phyla and 2,197 genera, and five phyla and 100 genera for bacteria and archaea, respectively. Also, homologous groups from public orthology databases were identified and included in MCycDB to reduce false positive assignments. We applied MCycDB to profile methane cycling gene families and associated taxonomic groups from various environments. Gene families involved in methanogenesis were abundant in hot spring sediment and less abundant in freshwater, whereas the ones involved in aerobic oxidation of methane were abundant in permafrost and peatland. This study demonstrates that MCycDB is a useful tool for studying microbially-driven methane cycling processes with high specificity, coverage and accuracy.

Development of a hydroxyl group-mediated biosynthetic schwertmannite as a persulfate activator for efficient degradation of RhB and Cr(VI) removal.

Recently, sulfate radical-based advanced oxidation processes (SR-AOPs) have shown broad potential for restoring the water environment. However, the application of SR-AOPs to simultaneously remove organic pollutants and Cr(VI) has rarely been reported. Herein, we developed a modified schwertmannite (Sch-PVA) synthesized via the mediation of Acidithiobacillus ferrooxidans in the introduction of polyvinyl alcohol (PVA). This modification significantly changed the morphology and structure of the schwertmannite (Sch). The specific surface area and the density of functional sites also increased. Sch-PVA significantly increased the persulfate (PDS) activation efficiency. Even in 100 mg L-1 rhodamine B (RhB) conditions, 96.3% of RhB was eliminated by 0.5 g L-1Sch-PVA and 6 mM PDS in 120 min. Moreover, excellent performance was exhibited over a wide pH range. The dissolution of the passivation layer facilitated the exposure of new adsorption and reduction sites, thereby enhancing the simultaneous removal of RhB and Cr(VI). Quenching experiments and electron spin resonance (ESR) measurements verified that sulfate and hydroxyl radicals were generated. The hydroxyl groups on the Sch-PVA surface played a key role in the bonding with and the activation of PDS. In conclusion, Sch-PVA provides new insights into the catalyst application for simultaneous removal of organic pollutants and Cr(VI).