Hematological, biochemical and ionoregulatory responses of Indian major carp Catla catla during chronic sublethal exposure to inorganic arsenic

In the present study the acute toxicity of arsenic trioxide in fingerlings of Catla catla an Indian major carp was evaluated with renewal bioassay method. The median lethal concentration of arsenic trioxide to the fish C. catla for 96 h was found to be 20.41 ppm (with 95% confidence limits). From this a non-lethal dose of (2.041 ppm; 1/10th of LC 50 96 value) was selected and fingerlings were exposed to 35 d and hematological, biochemical and ionoregulatory responses were studied at days 7, 14, 21, 28 and 35. Arsenic trioxide produced a significant increase in hemoglobin, hematocrit, WBC count, plasma GPT levels and reduction in RBC count, plasma sodium, chloride, potassium, glucose, protein, GOT, LDH levels as compared to the control group. Gill Na⁺/K⁺-ATPase activity was influenced by arsenic trioxide exposure. A biphasic response was noted in the value of MCH and MCV. However the MCHC level was not altered in arsenic trioxide treated fish throughout the study period. Results of the present investigation suggest that arsenic trioxide affects the hematological, biochemical and ionoregulatory parameters of fish and alterations of these parameters can be useful in environmental biomonitoring of arsenic contamination.     

Volatilization of polycyclic aromatic hydrocarbons from coal-tar-sealed pavement

The effects of the herbicide atrazine on the gill of the freshwater fish Prochilodus lineatus were evaluated after exposure of fish to 2, 10 and 25 μg L⁻¹ atrazine during 48 h (acute exposure) and 14 d (subchronic exposure). Ions and osmolality were measured in plasma and gill samples were taken to determine the Na⁺/K⁺-ATPase (NKA) and carbonic anhydrase (CA) activities and for morphological analysis. Plasma osmolality and Na⁺ and Cl⁻ ions changed depending on atrazine concentration, but atrazine exposure had no effect on the Na⁺/Cl⁻ ratio. NKA activity did not change after atrazine exposure, but CA activity decreased in fish exposed to 25 μg L⁻¹ for 14 d. Gill MRC density decreased after acute exposure but did not change in fish exposed to the subchronic treatment. The MRC density at the epithelial surface increased in fish exposed to 25 μg L⁻¹, and the MRC fractional area (MRCFA) increased in fish exposed to 10 μg L⁻¹. The changes in MRCs provide evidence of morphological adjustments to maintain ionic homeostasis in spite of the inhibition of CA activity at the highest atrazine concentration.     

Surfactant-based oil dispersant toxicity to developing nauplii of Artemia: effects on ATPase enzymatic system

The paper deals with the toxicity of a surfactant-based oil dispersant to the ATPase activities of two naupliar stages of Artemia (instar I & II). Both instars were exposed to sub-lethal and lethal concentrations derived from acute toxicity data. The chosen concentrations were near to LOECs and NOECs. An eightfold difference indicated between the instars was instar-exposure time dependent. The most prominent effects were the inhibition and the stimulation of Na⁺/K⁺-ATPase and Mg²⁺-ATPase activities, respectively. The cause of these effects was related to the dispersant components, the surfactants. The pattern stimulation/inhibition of Mg²⁺-ATPase and Na⁺/K⁺-ATPase activities could be used to indicate toxic stress by surfactant-based oil dispersants since previous studies with other contaminants have shown different ATPase activity patterns.     

Short and long-term effects of clofibric acid and diclofenac on certain biochemical and ionoregulatory responses in an Indian major carp, Cirrhinus mrigala

Extensive use of pharmaceuticals in human and veterinary medicine and aquaculture practices pose a serious threat to aquatic organisms. In the present investigation, Cirrhinus mrigala an Indian major carp was exposed to different concentrations of clofibric acid (CA) and diclofenac (DCF) and certain biochemical and ionoregulatory responses were assessed under short and long term exposures. During short-term (96 h) exposure period, plasma glucose and sodium (Na⁺) levels were increased at all concentrations (1, 10 and 100 μg L⁻¹) of CA and DCF treated fish. Plasma protein and chloride (Cl⁻) levels were found to be decreased at all concentrations of CA and DCF exposed fish comparatively to control groups. Meanwhile an increase in plasma potassium (K⁺) level was noted in fish exposed to CA treatments alone and in DCF treatments it was decreased. In long-term exposure (35 d), plasma Na⁺ and Cl⁻ levels were found to be significantly increased at all concentration of CA and DCF. However, a biphasic trend was observed in plasma glucose, protein and K⁺ levels. In both the treatments, a significant (P < 0.01 and P < 0.05) changes were observed in all parameters measured in fish exposed to different concentrations of CA and DCF. The results of the present investigation indicate that both the drugs caused significant changes in biochemical and ionoregulatory responses of fish at all concentrations. The alterations of these parameters can be useful in monitoring of pharmaceutical residues present in aquatic environment.     

Effects of dioxin exposure in Eisenia andrei: integration of biomarker data by an Expert System to rank the development of pollutant-induced stress syndrome in earthworms

A battery of biomarkers has recently been developed in the earthworm Eisenia andrei. In this study, different biomarkers (i.e. Ca²⁺-ATPase activity, lysosomal membrane stability-LMS, lysosomal lipofuscin and neutral lipid content) were utilized to evaluate the alterations in the physiological status of animals, induced by exposure for 3 d to different sublethal concentrations of TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) (1.5 × 10⁻³, 1.5 × 10⁻², 1.5 × 10⁻¹ ng mL⁻¹) utilizing the paper contact toxicity test. Lysosome/cytoplasm volume ratio and DNA damage were also evaluated as a biomarker at the tissue level and as a biomarker of genotoxicity, respectively. Moreover, the NR retention time assay conditions were optimized for the determination of in vivo LMS in earthworm coelomocytes. The results demonstrate that LMS and Ca²⁺-ATPase activity were early warning biomarkers able to detect the effects of minimal amounts of TCDD and that biomarkers evaluated at the tissue level are important for following the evolution of the stress syndrome in earthworms. To evaluate the health status of the animals, an Earthworm Expert System (EES) for biomarker data integration and interpretation was developed. The EES proved to be a suitable tool able to rank, objectively, the different levels of the stress syndrome in E. andrei induced by the different concentrations of TCDD.     

Comparative study of the accumulation and detoxification of Cu (essential metal) and Hg (nonessential metal) in the digestive gland and gills of mussels Mytilus galloprovincialis, using analytical and histochemical techniques

The aim of the present study is the comparative examination of accumulation and detoxification of Cu and Hg in digestive gland and gills of mussels Mytilus galloprovincialis, using atomic absorption spectrophotometry and autometallography. Mussels were exposed to 0.08 mg L⁻¹ Cu, 0.08 mg L⁻¹ Hg, as well as to a mixture of 0.08 mg L⁻¹ Hg and 0.08 mg L⁻¹ Cu for 11 d. After the experimental exposure, animals were kept under laboratory conditions for a detoxification period of 7 d. An antagonistic effect of Cu against to Hg accumulation was noted in the digestive gland of mussels after the experimental exposure, as well as after the detoxification period, supporting the protective role of Cu against to Hg toxicity in this tissue. Digestive gland was suggested as a main organ for Hg accumulation and gills as a target position for Cu accumulation. Additionally, lower time was evaluated for Hg detoxification in the digestive gland and gills of mussels, in relation to those addressed for Cu detoxification in the same tissues. The evaluation of black silver deposits (BSD) extent performed in digestive gland and gills was suggested as a less sensitive approach, in relation to atomic absorption spectrophotometry (AAS), to indentify the concentration of heavy metals in tissues of mussels. The toxic effects of Hg, Cu and a mixture of them on lysosomal system of the digestive cells are also discussed.     

Importance of natural sedimentation in the fate of microcystins

Sedimentation processes of microcystins (MCs), cyanobacterial toxins, were studied in three reservoirs located in Central Spain in which the cyanobacterial community was dominated by the genus Microcystis. MCs were detected in the sediment traps deployed in all reservoirs. In Santillana reservoir, MCs were identified in sediment traps even though they could not be found in the pelagial samples. In the other reservoirs studied, sedimentation rates for MC-containing particles during the bloom period ranged from 0.43 to 2.53 mg m⁻² d⁻¹. Interestingly, this very high sedimentation of toxic biomass is not exclusively related to decaying blooms or autumnal sedimentation due to a drop in water temperature. Instead, it seems that MC-containing colonies may be settling constantly during the bloom period and we were able to estimate that during such a Microcystis dominated bloom, around 4.5% of pelagial MCs may be involved in sedimentation. Further, these settling colonies seem to maintain good cell integrity and MCs seem not to be excreted massively. A certain loss of toxin content along the vertical settling may be attributed to minor losses due to cell lysis or to variations in MC cell quota explained by reduced production or internal consumption. Our results for the first time establish specific settling rates for MC-containing particles in freshwaters and definitely identify sedimentation as a major destination for these toxins. These data may contribute to improve managing strategies concerning risks associated with MCs.     

Tissue S/N ratios and stable isotopes (δS and δN) of epilithic mosses (Haplocladium microphyllum) for showing air pollution in urban cities in Southern China

In urban cities in Southern China, the tissue S/N ratios of epilithic mosses (Haplocladium microphyllum), varied widely from 0.11 to 0.19, are strongly related to some atmospheric chemical parameters (e.g. rainwater SO₄²⁻/NH₄⁺ ratios, each people SO₂ emission). If tissue S/N ratios in the healthy moss species tend to maintain a constant ratio of 0.15 in unpolluted area, our study cities can be divided into two classes: class I (S/N > 0.15, S excess) and class II (S/N < 0.15, N excess), possibly indicative of stronger industrial activity and higher density of population, respectively. Mosses in all these cities obtained S and N from rainwater at a similar ratio. Sulphur and N isotope ratios in mosses are found significantly linearly correlated with local coal δ³⁴S and NH₄⁺-N wet deposition, respectively, indicating that local coal and animal NH₃ are the major atmospheric S and N sources.     

Development of a combined isotopic and mass-balance approach to determine dissolved organic carbon sources in eutrophic reservoirs

A combined mass-balance and stable isotope approach was set up to identify and quantify dissolved organic carbon (DOC) sources in a DOC-rich (9 mg L⁻¹) eutrophic reservoir located in Western France and used for drinking water supply (so-called Rophemel reservoir). The mass-balance approach consisted in measuring the flux of allochthonous DOC on a daily basis, and in comparing it with the effective (measured) DOC concentration of the reservoir. The isotopic approach consisted, for its part, in measuring the carbon isotope ratios (δ¹³C values) of both allochthonous and autochthonous DOC sources, and comparing these values with the δ¹³C values of the reservoir DOC. Results from both approaches were consistent pointing out for a DOC of 100% allochthonous origin. In particular, the δ¹³C values of the DOC recovered in the reservoir (−28.5 ± 0.2‰; n = 22) during the algal bloom season (May-September) showed no trace of an autochthonous contribution (δ¹³C in algae = −30.1 ± 0.3‰; n = 2) being indistinguishable from the δ¹³C values of allochthonous DOC from inflowing rivers (−28.6 ± 0.1‰; n = 8). These results demonstrate that eutrophication is not responsible for the high DOC concentrations observed in the Rophemel reservoir and that limiting eutrophication of this reservoir will not reduce the potential formation of disinfection by-products during water treatment. The methodology developed in this study based on a complementary isotopic and mass-balance approach provides a powerful tool, suitable to identify and quantify DOC sources in eutrophic, DOC-contaminated reservoirs.     

Seasonal and spatial variations of Po and Pb activity concentrations in Mytilus galloprovincialis from Croatian coast of the Adriatic Sea

Results of 2 years monitoring of ²¹⁰Po and ²¹⁰Pb activity concentrations in soft tissue of the species Mytilus galloprovincialis from Croatian part of the Adriatic coast are presented. The samples were collected at thirteen coastal stations (some of which are also a part of the Mediterranean Mussel Watch Project) in spring and autumn of 2010 and 2011. The collected mussels were ranging between 4 cm and 6 cm in shell length. After sample pre-treatment lead and polonium were radiochemically separated on Sr resin. ²¹⁰Po was determined by alpha-particle spectrometry and ²¹⁰Pb was determined, via ²¹⁰Bi, by a low-level gas proportional counter. The results of ²¹⁰Po activity concentrations were found to vary between (104 ± 11) and (1421 ± 81) Bq kg⁻¹ dry weight while ²¹⁰Pb activity concentrations were much lower and in range (8.2 ± 5.3)–(94.1 ± 29.8) Bq kg⁻¹ dry weight. Higher ²¹⁰Po and ²¹⁰Pb activity concentrations were determined in spring period. The inter-site differences seen in their activity concentrations can be due to natural background levels of sites. The ²¹⁰Po/²¹⁰Pb activity concentration ratios in all cases exceeded unity for all mussel samples and ranged between 4.0 and 47.9.     

Spatial and temporal variability of ²¹⁰Po and ²¹⁰Pb in mussels (Mytilus galloprovincialis) at the Turkish coast of the Aegean Sea

In this study, the activity concentrations of ²¹⁰Po and ²¹⁰Pb were determined in mussel samples (Mytilus galloprovincialis) collected from the Turkish coast of the Aegean Sea. The samples were collected seasonally for a period of two years (2004-2006) at six coastal stations (Çanakkale, Dikili, Foça, Çe?me, Didim, Bodrum). Mussels were separated into several groups according to their size (1-4, 4-6, >6 cm). The results showed that ²¹⁰Po concentrations in mussels varied between 53 ± 4 and 1960 ± 60 Bq kg⁻¹ dw. The highest activity ²¹⁰Po concentrations were determined in winter samples of mussels with a shell length of 4-6 cm from Didim. In general, it was observed that the ²¹⁰Pb concentration levels in mussels were lower than ²¹⁰Po concentrations. The ²¹⁰Po/²¹⁰Pb activity concentration ratios exceeded unity for all mussel samples and averaged 26.0. The inter-site differences seen in ²¹⁰Po concentrations can be due to both the natural background levels of sites and industrial activities.     

Can Hediste diversicolor (Nereidae, Polychaete) be considered a good candidate in evaluating PAH contamination? A multimarker approach

The aim of this study was to develop a preliminary characterization of the biological responses of Hediste diversicolor to polycyclic aromatic hydrocarbons (PAHs) under controlled laboratory conditions. In order to test the effects of PAH exposure, a battery of biomarkers was applied to the polychaetes by exposing them to sublethal concentrations of benzo[a]pyrene (0.1 and 0.5 mg L⁻¹) for 10 d under laboratory conditions. The battery of biomarkers tested included oxidative stress biomarkers (glutathione content, enzymatic activities of catalase, glutathione S-transferases, glutathione reductase, glutathione peroxidases), total oxyradical scavenging capacity (TOSC) toward peroxyl and hydroxyl radicals and activity of acyl CoA oxidase (AOX) as a marker of peroxisome proliferation measured in the entire body; lipofuscin and neutral lipid accumulations and levels of Ca²⁺-ATPase activity analyzed in the intestinal epithelium; lysosomal membrane stability and genotoxic effects measured as DNA strand breaks and frequency of micronuclei in coelomocytes. Chemical analyses were also carried out to verify the polychaete’s benzo[a]pyrene (B[a]P) bioaccumulation levels after the exposure period. The results obtained indicate that B[a]P caused significant changes in most of the parameters measured in H. diversicolor. Biological responses to the organic compound were particularly significant for the biomarkers measured in the intestinal epithelium and in coelomocytes, emphasizing that these tissues were more affected during our experimental conditions. Considering the key trophic role of this benthic species in estuarine and coastal ecosystems, this study confirmed that H. diversicolor is an appropriate bioindicator of organic contamination.     

The impacts of bisphenol A (BPA) on abalone (Haliotis diversicolor supertexta) embryonic development

The effects of bisphenol A (BPA) on abalone (Haliotis diversicolor supertexta) embryonic development were investigated by exposing the fertilized eggs to four different concentrations of BPA (0.05, 0.2, 2 and 10 μg mL⁻¹). Toxicity endpoints including the embryo development parameters, the physiological features and the expression profile of several reference genes (prohormone convertase 1, PC1; cyclin B, CB; and cyclin-dependent kinase 1, CDK1) were assessed. The results showed that BPA could markedly reduce embryo hatchability, increase developmental malformation, and suppress the metamorphosis behavior of larvae. The possible toxicological mechanisms hidden behind of these effects (i.e. disturbing the embryogenesis) might result from three aspects: (1) BPA disturbance the cellular ionic homeostasis and osmoregulation of abalone embryos by changing the Na⁺-K⁺-ATPase and Ca²⁺-Mg²⁺-ATPase levels; (2) BPA induced oxidative damage of embryos by significantly alterating the peroxidase (POD) activities and the malondialdehyde (MDA) production; and (3) the RT-PCR analysis further demonstrated that BPA perturbed the cellular endocrine regulation and cell cycle progression by down-regulating the PC1 gene, as well as over-expressing the CB and CDK1 genes. This is the first comprehensive study on the developmental toxicity of BPA to the marine abalone at morphological, physiological and molecular levels. The results in this study also indicated that the embryo tests can contribute to the ecological risk assessment of the endocrine disruptors in marine environment.     

Levels of ²¹⁰Po and ²¹⁰Pb in fish and molluscs in Slovenia and the related dose assessment to the population

²¹⁰Po and ²¹⁰Pb activity concentrations in fish from the Slovenian part of Adriatic Sea, in the vicinity of a former uranium mine at ?irovski vrh and from the Slovenian market were determined. In addition, ²¹⁰Po and ²¹⁰Pb activity concentrations in squid from the Slovenian market and in mussels from the Slovenian part of the Adriatic Sea were also determined. Fish, squid and mussel consumption in Slovenia was assessed from the data available from Eurostat and Food and Agriculture Organisation (FAO) and the data used for the corresponding dose calculation. Fish species with the highest activity concentrations were grilled to assess possible loss of ²¹⁰Po during the food preparation process. Samples were freeze dried and radiochemical separation of ²¹⁰Po and ²¹⁰Pb was performed. Measurements of ²¹⁰Po were performed by alpha spectrometry and ²¹⁰Pb by a low background gas-flow proportional counter. ²¹⁰Po activity concentrations in fish, squid and mussels were from 0.039 to 35.0 Bq kg⁻¹ fresh weight and ²¹⁰Pb activity concentrations were from 0.08 to 3.03 Bq kg⁻¹ fresh weight. Grilling of fish resulted in no significant loss of ²¹⁰Po at 90 °C. The assessed combined annual effective ingestion dose due to ²¹⁰Po and ²¹⁰Pb for fish, squid and mussels consumed in Slovenia is 47.6 μSv year⁻¹.     

人工湿地基质对 NH+4-N 吸附性能简

为研究建筑废物红砖和工业废物煤渣用作人工湿地脱氮基质的可行性,分别通过静态吸附实验和动态NH₄⁺-N去除效果实验进行考察。结果表明,红砖和煤渣对NH₄⁺-N最大静态吸附量分别为0.2533 mg/g和0.0533 mg/g,其吸附等温曲线均符合Freundlich型吸附方程,吸附常数分别为0.0419和0.0091;红砖煤渣组合对污水中NH₄⁺-N平均动态脱除率达到41.18%,高于红砖的37.63%和煤渣的30.92%。     

银钛共负载型光催化材料降解甲基橙废水简

以膨胀珍珠岩为载体,采用溶胶凝胶法对其进行负载,制备出不同类型的光催化材料（TiO₂-EP、Ag⁺-TiO₂-EP）,并在模拟日光条件下,研究其对甲基橙溶液的降解效果。结果表明,浸渍3次且担载0.04% Ag⁺的负载型TiO₂光催化活性最高,在光催化剂用量为0.3 g,20 mL初始浓度为10 mg/L甲基橙溶液光照4 h后降解率可达81.6%,且甲基橙的光催化降解服从一级动力学方程。回收3次后仍有较强的活性,其2 h降解率为24.8%。     

Salicylic acid involved in the regulation of nutrient elements uptake and oxidative stress in Vallisneria natans (Lour.) Hara under Pb stress

In this study, the alterations in nutrient elements content, reactive oxygen species level and antioxidant response were studied in leaves of Vallisneria natans (Lour.) Hara exposed to salicylic acid (SA, 10 or 100 μM), or Pb (50 μM) or their combinations for 4 d. No significant alterations in Mn and Ca content were observed but content of Cu, Zn, Fe and P decreased in plants exposed to SA alone. SA application inhibited the uptake of Pb and partially reversed Pb-induced the alterations in Mn, Ca and Fe content in leaves of V. natans exposed to 50 μM Pb. The decreased chlorophyll (a + b) and increased malondialdehyde and O²⁻ and H₂O₂ content were detected in plants exposed to 100 μM SA, 50 μM Pb, 10 μM SA + 50 μM Pb or 100 μM SA + 50 μM Pb. Application SA partially inhibited Pb-induced the increase of malondialdehyde, O²⁻ and H₂O₂ content. 100 μM SA decreased the activity of NADH oxidase and the content of non-protein thiols, carotenoids and ascorbic acid and increased the content of dehydroascorbate in plants treated with or without Pb. SA alone decreased the ascorbate peroxidase activity and increased the catalase and peroxidase activity, while SA application increased catalase activity but had no significant effect on ascorbate peroxidase and peroxidase activity in V. natans exposed to Pb. The results indicate that SA involves in the regulation of Pb uptake, nutrient balance and oxidative stress.     

Eu uptake by calcite: Preliminary results from coprecipitation experiments and observations with surface-sensitive techniques

A lack of information in databases for contamination risk assessment about the transport behaviour of the trivalent f-orbital elements in groundwater systems where calcite is at equilibrium motivated this study of Eu³⁺ uptake. The free drift technique was used to examine the effects of Eu³⁺ concentration, presence of Na⁺ or K⁺ and temperature, as well as calcite nucleation and precipitation kinetics, on the partitioning of calcite. Changes in surface composition and morphology resulting from exposure of single crystals of Iceland spar to Eu³⁺-bearing solutions were observed with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). First results confirm that calcite has high affinity for Eu³⁺. Rates of nucleation and precipitation strongly affect the extent of uptake but the presence of Na⁺ and K⁺ has no effect, suggesting formation of solid solution as CaCO₃–EuOHCO₃. Surface-sensitive techniques prove that Eu³⁺ is adsorbed to calcite even when the surface is dissolving and adsorption is not accompanied by precipitation of a separate Eu³⁺-solid phase. Adsorbed Eu modifies calcite's dissolution behaviour, roughening terraces and rounding step edges, and producing surface morphology where some surface sites appear blocked. Results imply that Eu³⁺ concentrations in natural calcites are limited by Eu³⁺ availability rather than by a lack of ability to fit into calcite's atomic structure. This behaviour can probably be expected for other trivalent rare Earth elements (REE), actinides and fission products whose behaviour is similar to that of Eu³⁺. These elements are likely to be incorporated within the calcite bulk in systems where it is precipitating and the demonstrated strong partitioning ensures some uptake even where calcite is at or under saturation.     

Histopathological effects of copper and lithium in the ramshorn snail, Marisa cornuarietis (Gastropoda, Prosobranchia)

The aim of this study was to determine and quantify effects of copper and lithium in tissues of early juveniles of the ramshorn snail, Marisa cornuarietis. For this purpose, hatchlings of M. cornuarietis were exposed for 7 days to a range of five different sublethal concentrations of copper (5, 10, 25, 50, and 75 μg Cu²⁺ L⁻¹) and lithium (50, 100, 200, 1000, and 5000 μg Li⁺ L⁻¹). Both metals changed the tissue structure of epidermis, hepatopancreas, and gills, varying between slight and strong reactions, depending on the copper and lithium concentration. The histopathological changes included alterations in epithelial and mucous cells of the epidermis, swelling of hepatopancreatic digestive cells, alterations in the number of basophilic cells, abnormal apices of digestive cells, irregularly shaped cilia and changes in the amount of mucus in the gills. The most sensible organ in M. cornuarietis indicating Cu or Li pollution is the hepatopancreas (LOECs were 10 μg Cu²⁺ L⁻¹, or 200 μg Li⁺ L⁻¹). In epidermis, mantle and gills relevant effects occurred with higher LOECs (50 μg Cu²⁺ L⁻¹, or 1000 μg Li⁺ L⁻¹). Base on LOECs, our results indicated that histopathological endpoints are high sensitivity to copper and lithium compared to endpoints for embryonic developmental toxicity.     

Influence of liberated silver from silver nanoparticles on nitrification inhibition of Nitrosomonas europaea

The ecotoxicity of silver nanoparticles (Ag-NPs) to wastewater biota, including ammonia oxidizing bacteria (AOB), is gaining increasing interest as the number of products containing Ag-NPs continues to rise exponentially and they are expected to accumulate in wastewater treatment plants. This research demonstrated that the addition order of Ag-NP and the media constituents had a profound influence on the stability of the Ag-NP suspension and the corresponding repeatability of results and sensitivity of Nitrosomonas europaea. N. europaea, a model AOB, was found to be extremely sensitive to ionic silver (Ag⁺) and two sizes of Ag-NPs (20 and 80 nm). Ag⁺ exposures resulted in the highest level of toxicity with smaller Ag-NPs (20 nm) being more toxic than larger Ag-NPs (80 nm). The increased sensitivity of N. europaea to smaller Ag-NPs was caused by their higher rates of dissolved silver (dAg) release, via dissolution, due to a greater surface area to volume ratio. dAg was shown to be responsible for the vast majority of the observed Ag-NP toxicity, as determined by abiotic Ag-NP dissolution tests. For the sizes of Ag-NP studied (20 and 80 nm), there appears to be a negligible nanoparticle-specific toxicity. This was further supported by similarities in inhibition mechanisms between Ag⁺ and Ag-NP, with both causing decreases in AMO activity and destabilization of the outer-membrane of N. europaea. Finally, equal concentrations of total silver were found to be tightly associated to both Ag⁺ and Ag-NP-exposed cells despite Ag-NP concentrations being five times greater, by mass, than Ag⁺ concentrations.