Investigating Nitrosomonas europaea Stress Biomarkers in Batch, Continuous Culture, and Biofilm Reactors

Evaluating acute toxicity in enriched nitrifying cultures: Lessons learned

Toxicological batch assays are essential to assess a compound's acute effect on microorganisms. This methodology is frequently employed to evaluate the effect of contaminants in sensitive microbial communities from wastewater treatment plants (WWTPs), such as autotrophic nitrifying populations. However, despite nitrifying batch assays being commonly mentioned in the literature, their experimental design criteria are rarely reported or overlooked. Here, we found that slight deviations in culture preparations and conditions impacted bacterial community performance and could skew assay results. From pre-experimental trials and experience, we determined how mishandling and treatment of cultures could affect nitrification activity. While media and biomass preparations are needed to establish baseline conditions (e.g., biomass washing), we found extensive centrifugation selectively destabilised nitrification activities. Further, it is paramount that the air supply is adjusted to minimise nitrite build-up in the culture and maintain suitable aeration levels without sparging ammonia. DMSO and acetone up to 0.03% (v/v) were suitable organic solvents with minimal impact on nitrification activity. In the nitrification assays with allylthiourea (ATU), dilute cultures exhibited more significant inhibition than concentrated cultures. So there were biomass-related effects; however, these differences minimally impacted the EC₅₀ values. Using different nutrient-media compositions had a minimal effect; however, switching mineral media for the toxicity test from the original cultivation media is not recommended because it reduced the original biomass nitrification capacity. Our results demonstrated that these factors substantially impact the performance of the nitrifying inoculum used in acute bioassays, and consequently, affect the response of AOB-NOB populations during the toxicant exposure. These are not highlighted in operation standards, and unfortunately, they can have significant consequential impacts on the determinations of toxicological endpoints. Moreover, the practical procedures tested here could support other authors in developing testing methodologies, adding quality checks in the experimental framework with minimal waste of time and resources.

Pharmaceuticals and personal care products' (PPCPs) impact on enriched nitrifying cultures

The impact of pharmaceutical and personal care products (PPCPs) on the performance of biological wastewater treatment plants (WWTPs) has been widely studied using whole-community approaches. These contaminants affect the capacity of microbial communities to transform nutrients; however, most have neither honed their examination on the nitrifying communities directly nor considered the impact on individual populations. In this study, six PPCPs commonly found in WWTPs, including a stimulant (caffeine), an antimicrobial agent (triclosan), an insect repellent ingredient (N,N-diethyl-m-toluamide (DEET)) and antibiotics (ampicillin, colistin and ofloxacin), were selected to assess their short-term toxic effect on enriched nitrifying cultures: Nitrosomonas sp. and Nitrobacter sp. The results showed that triclosan exhibited the greatest inhibition on nitrification with EC₅₀ of 89.1 μg L^-1. From the selected antibiotics, colistin significantly affected the overall nitrification with the lowest EC₅₀ of 1 mg L^-1, and a more pronounced inhibitory effect on ammonia-oxidizing bacteria (AOB) compared to nitrite-oxidizing bacteria (NOB). The EC₅₀ of ampicillin and ofloxacin was 23.7 and 12.7 mg L^-1, respectively. Additionally, experimental data suggested that nitrifying bacteria were insensitive to the presence of caffeine. In the case of DEET, moderate inhibition of nitrification (<40%) was observed at 10 mg L^-1. These findings contribute to the understanding of the response of nitrifying communities in presence of PPCPs, which play an essential role in biological nitrification in WWTPs. Knowing specific community responses helps develop mitigation measures to improve system resilience.

Controls and Adaptive Management of Nitrification in Agricultural Soils

Agriculture is responsible for over half of the input of reactive nitrogen (N) to terrestrial systems; however improving N availability remains the primary management technique to increase crop yields in most regions. In the majority of agricultural soils, ammonium is rapidly converted to nitrate by nitrification, which increases the mobility of N through the soil matrix, strongly influencing N retention in the system. Decreasing nitrification through management is desirable to decrease N losses and increase N fertilizer use efficiency. We review the controlling factors on the rate and extent of nitrification in agricultural soils from temperate regions including substrate supply, environmental conditions, abundance and diversity of nitrifiers and plant and microbial interactions with nitrifiers. Approaches to the management of nitrification include those that control ammonium substrate availability and those that inhibit nitrifiers directly. Strategies for controlling ammonium substrate availability include timing of fertilization to coincide with rapid plant update, formulation of fertilizers for slow release or with inhibitors, keeping plant growing continuously to assimilate N, and intensify internal N cycling (immobilization). Another effective strategy is to inhibit nitrifiers directly with either synthetic or biological nitrification inhibitors. Commercial nitrification inhibitors are effective but their use is complicated by a changing climate and by organic management requirements. The interactions of the nitrifying organisms with plants or microbes producing biological nitrification inhibitors is a promising approach but just beginning to be critically examined. Climate smart agriculture will need to carefully consider optimized seasonal timing for these strategies to remain effective management tools.

Short-term effects of TiO2, CeO2, and ZnO nanoparticles on metabolic activities and gene expression of Nitrosomonas europaea

Nanosized TiO2 (n-TiO2), CeO2 (n-CeO2), and ZnO (n-ZnO) and bulk ZnO were chosen for a 4-h exposure study on a model ammonia oxidizing bacterium, Nitrosomonas europaea. n-ZnO displayed the most serious cytotoxicity while n-TiO2 was the least toxic one. The change of cell morphologies, the retardance of specific oxygen uptake rates and ammonia oxidation rates, and the depression of amoA gene expressions under NP stresses were generally observed when the cell densities and membrane integrities were not significantly impaired yet. The TEM imaging and the synchrotron X-ray fluorescence microscopy of the NPs impacted cells revealed the increase of the corresponding intracellular Ti, Ce or Zn contents and suggested the intracellular NP accumulation. The elevation of intracellular S contents accompanied with higher K contents implied the possible activation of thiol-containing glutathione and thioredoxin production for NP stress alleviation. The NP cytotoxicity was not always a function of NP concentration. The 200 mg L(-1) n-TiO2 or n-CeO2 impacted cells displayed the similar ammonia oxidation activities but higher amoA gene expression levels than the 20 mg L(-1) NPs impacted ones. Such phenomenon further indicated the possible establishment of an anti-toxicity mechanism in N. europaea at the genetic level to redeem the weakened AMO activities along with the NP aggregation effects.

Influence of Water Hardness on Silver Ion and Silver Nanoparticle Fate and Toxicity Toward Nitrosomonas europaea

This study investigated the influence of water hardness (Mg²⁺ and Ca²⁺) on the fate and toxicity of 20 nm citrate silver nanoparticles (AgNPs) and Ag⁺ toward Nitrosomonas europaea, a model ammonia-oxidizing bacterium. Nitrification inhibition of N. europaea by 1 ppm AgNPs and 0.5 ppm Ag⁺ was reduced from 80% and 83%, respectively, in the absence of Mg²⁺ to 2% and 33%, respectively, in the presence of 730 μM Mg²⁺. Introduction of Mg²⁺ resulted in the rapid aggregation of the AgNP suspensions and reduced the 3 h Ag⁺ dissolution rates from 30%, in the absence of Mg²⁺, to 9%, in the presence of 730 μM Mg²⁺. Reduced AgNP dissolution rates resulted in decreased concentrations of silver that were found adsorbed to N. europaea cells. Increasing AgNP concentrations in the presence of Mg²⁺ increased the observed inhibition of nitrification, but was always less than what was observed in the absence of Mg²⁺. The presence of Mg²⁺ also reduced the adsorption of Ag⁺ to cells, possibly due to multiple mechanisms, including a reduction in the negative surface charge of the N. europaea membrane and a competition between Mg²⁺ and Ag⁺ for membrane binding sites and transport into the cells. Ca²⁺ demonstrated similar protection mechanisms, as Ag⁺ toxicity was reduced and AgNP suspensions aggregated and decreased their dissolution rates. These results indicate that the toxicity of Ag⁺ and AgNPs to nitrifying bacteria in wastewater treatment would be less pronounced in systems with hard water.