Bacterivorous protists inhibit nitrification and N2O emissions in cadmium polluted soils via negative feedback loops

Water regime alters microbial mechanisms of N2O emission in metal-contaminated paddy soils

Microorganisms are essential for soil nitrous oxide (N2O) emissions through participating in key nitrogen (N)-related processes. However, the effect of water regimes on the interactions between N2O emissions and microbial processes in metal-contaminated soils is unclear. Here, we conducted a soil microcosm experiment with two water management strategies (non-flooding and flooding) and six metal addition treatments including low (2 and 200 mg kg-1) and high (10 and 1000 mg kg-1) levels of individual and combined Cd and Cu. The effects of high levels of individual Cd and Cu contamination on soil N2O emissions varied depending on water regimes, showing antagonistic effects under non-flooding conditions and synergistic effects under flooding conditions. High levels of co-contamination significantly inhibited nitrification under both water regimes, primarily due to reduced abundance of Nitrosospira. In contrast, this co-contamination decreased the abundance of Ramlibacter, thereby inhibiting denitrification and dissimilatory nitrate reduction to ammonium (DNRA) under flooding conditions. The inhibition of these key microorganisms and their mediated N-cycle processes reduced soil N2O emissions under both water regimes. This reduction was greater under flooding conditions because more N-related processes were inhibited. Metagenomic binning further indicated that Nitrosospira carried nitrifying genes, while Ramlibacter contained genes involved in denitrification, assimilatory nitrate reduction to ammonium (ANRA), and DNRA. These findings implied that both microorganisms had potential to produce N2O. Overall, water management strategies and metal contamination altered the microbial processes of N2O emissions, highlighting the importance of appropriate water management in mitigating greenhouse gas emissions from metal-contaminated paddy soils in southern China.

Assessment of Metals Toxicity Using a Nitrifying Bacteria Bioassay Kit Based on Oxygen Consumption

The escalating concentrations of emerging contaminants in water systems and the possible environmental threats they emphasize the necessity for more sophisticated methods in the evaluation of water quality. Traditional bioassays raise ethical concerns, require intricate procedures, entail significant expenses, and only allow for endpoint measurements. The using of nitrifying bacteria in bioassays has resulted in increased sensitivity to a wide range of toxic substances, making them valuable for the identification of water pollution. This study introduces a novel nitrifying bacteria bioassay kit for detecting heavy metal contaminants in water. This bioassay is specifically designed for expedited analysis of oxygen consumption. This technique enables the identification of a range of toxic metals. Optimization studies indicated that 100 mg ammonia NH4+-N/L, and 1 mL acclimated culture were the ideal conditions facilitating the necessary volume of gas consumption for sensitive data generation. Determined EC50 values of the selected toxic metals were: chromium (Cr6+), 0.51 mg/L; silver (Ag+), 2.90 mg/L; copper (Cu2+), 2.90 mg/L; nickel (Ni2+), 3.60 mg/L; arsenic (As3+), 4.10 mg/L; cadmium (Cd2+), 5.56 mg/L; mercury (Hg2+), 8.06 mg/L; and lead (Pb2+), 19.3 mg/L. Metagenomics analysis found key species in the research included Nitrosomonas eutropha, Nitrosomonas oligotropha, Nitrosomonas europaea, Nitrobacter vulgaris, Nitrobacter winogradskyi, Nitrospira moscoviensis and Nitrospira lenta. In addition, this bioassay is ideal for field screening and real-time monitoring due to its simplicity and reliability. This bioassay provides a precise, economical, and effective substitute for more intricate and ethically problematic techniques, enhancing the effectiveness of water quality monitoring programs.