On the mode of action of N-phenyl-2-naphthylamine in plants

Influence of endophytic and epiphytic nitrogen-fixing bacteria on the content of negative allelopathic compounds in root exudates of pea (<i>Pisum sativum</i> L.) seedlings

Substances that have a harmful effect on living organisms include N-phenyl-2-naphthalamine and phthalates, which are synthesized and widely used in the chemical industry. At the same time, N-phenyl-2-naphthylamine was found in the aerial parts and in the roots of some plant species, phthalates were found in many plant species and in bacteria. The aim of this research was to study the protective (antimicrobial) reaction of pea (Pisum sativum L.) seedlings of the Torsdag variety to the inoculation with bacteria Rhizobium leguminosarum bv. viceae (endosymbiont) and Azotobacter chroococcum (ectosymbiont) introduced into the aqueous medium of root growth were studied. Changes in the content of negative allelopathic compounds (pisatin, N-phenyl-2-naphthylamine, phthalates) in root exudates were the reaction indicators. After the inoculation, the seedlings grew for 24 h in the BINDER KBW-240 chamber at 21 °C, with lighting of 81 μM.m-2 . sec-1 and a 16/8 h day/night photoperiod. In ethyl acetate extracts from the aqueous medium where the seedling roots were immersed, the content of the compounds was determined by HPLC, while changes in the composition and ratio of phthalates were determined by GC-MS. Data indicating the different ability of both bacterial species to degrade N-phenyl-2-naphthylamine to phthalates and the dependence of this process activity in the bacteria studied on its concentration in the medium were presented. N-phenyl-2-naphthylamine differently but negatively affected the viability and growth of the bacteria used in the experiments. A different effect of rhizobia and azotobacter on the content of the above named compounds and on the ratio of types of phthalates in root exudates was elicited.

Comparative biotoxicity of N-Phenyl-1-naphthylamine and N-Phenyl-2-naphthylamine on cyanobacteria Microcystis aeruginosa

N-Phenyl-1-naphthylamine (P₁NA) and N-Phenyl-2-naphthylamine (P₂NA) are both widely used as antioxidant and plant secondary metabolites. In this study, growth, esterase, photosynthetic activity and cell membrane integrity were used as biomarkers to compare biotoxicity of P₁NA and P₂NA on Microcystis aeruginosa. According to the results, a dose-response relationship was observed only between P₁NA concentrations and growth inhibition. The EC₅₀ (48 h) of P₁NA calculated from growth inhibition was 16.62 μM, while that of P₂NA was not detected. When the esterase and photosynthetic activity were applied to evaluate the biotoxicity, it was found that a concentration of 20 μM P₁NA, P₂NA caused reduction of esterase activity and Fv/Fm of M. aeruginosa to 22.2 and 3.3%, 97.5 and 92.1%, respectively, after 48 h exposure. The percentage of membrane-damaged cells was increased as P₁NA exposure concentration increased, but that was not detected when exposure to P₂NA. The difference substituted position in the molecular structure of P₁NA and P₂NA leads to different toxicological properties and only P₁NA was found highly toxic to M. aeruginosa. The toxicity is due to that only P₁NA can be biotransformed to 1,4-naphthoquinone, which could induce overproduction of intracellular ROS as well as result in oxidative damage and growth inhibition of test organism.

Time-Dependent Effects in Algae for Chemicals with Different Adverse Outcome Pathways: A Novel Approach

Chemicals affect unicellular algae as a result of toxicokinetic and toxicodynamic processes. The internal concentration of chemicals in algae cells typically reaches equilibrium within minutes, while damage cumulatively increases over hours. The time gap between the steady state of internal exposure and damage development is thus suspected to span up to hours, mainly due to toxicodynamic processes. The quantification of rate-limited toxicodynamic processes, aggregated as a progressive effect from an initiating molecular event through biological key events toward the adverse outcome on algae growth inhibition, might discriminate between different adverse outcome pathways (AOPs). To support our hypothesis, we selected six chemicals according to different physicochemical properties and three distinctly dissimilar AOPs. The time courses of internal concentrations were linked to the observed affected Scenedesmus vacuolatus growth using toxicokinetic-toxicodynamic modeling. Effects on cell growth were explained by effect progression and not by the time to reach internal equilibrium concentration. Effect progression rates ranged over 6 orders of magnitude for all chemicals but varied by less than 1 order of magnitude within similar AOP (photosystem II inhibitors > reactive chemicals > lipid biosynthesis inhibitors), meaning that inhibitors of photosystem II advance an effect toward algae growth fastest compared to reactive chemicals and inhibitors of lipid biosynthesis.

Pollution-Induced Community Tolerance To Diagnose Hazardous Chemicals in Multiple Contaminated Aquatic Systems

Aquatic ecosystems are often contaminated with large numbers of chemicals, which cannot be sufficiently addressed by chemical target analyses. Effect-directed analysis (EDA) enables the identification of toxicants in complex contaminated environmental samples. This study suggests pollution-induced community tolerance (PICT) as a confirmation tool for EDA to identify contaminants which actually impact on local communities. The effects of three phytotoxic compounds local periphyton communities, cultivated at a reference (R-site) and a polluted site (P-site), were assessed to confirm the findings of a former EDA study on sediments. The sensitivities of R- and P-communities to prometryn, tributyltin (TBT) and N-phenyl-2-naphthylamine (PNA) were quantified in short-term toxicity tests and exposure concentrations were determined. Prometryn and PNA concentrations were significantly higher at the P-site, whereas TBT concentrations were in the same range at both sites. Periphyton communities differed in biomass, but algal class composition and diatom diversity were similar. Community tolerance of P-communities was significantly enhanced for prometryn, but not for PNA and TBT, confirming site-specific effects on local periphyton for prometryn only. Thus, PICT enables in situ effect confirmation of phytotoxic compounds at the community level and seems to be suitable to support confirmation and enhance ecological realism of EDA.

A toxicokinetic study of specifically acting and reactive organic chemicals for the prediction of internal effect concentrations in Scenedesmus vacuolatus

The toxic potency of chemicals is determined by using the internal effect concentration by accounting for differences in toxicokinetic processes and mechanisms of toxic action. The present study examines toxicokinetics of specifically acting and reactive chemicals in the green algae Scenedesmus vacuolatus by using an indirect method. Concentration depletion in the exposure medium was measured for chemicals of lower (log KOW  < 3: isoproturon, metazachlor, paraquat) and moderate (log KOW 4-5: irgarol, triclosan, N-phenyl-2-naphthylamine) hydrophobicity at 7 to 8 time points over 240 min or 360 min. Uptake and overall elimination rates were estimated by fitting a toxicokinetic model to the observed concentration depletions. The equilibrium of exposure concentrations was reached within minutes to hours or was even not observed within the exposure time. The kinetics of bioconcentration cannot be explained by the chemical's hydrophobicity only, but influential factors such as ionization of chemicals, the ion trapping mechanism, or the potential susceptibility for biotransformation are discussed. Internal effect concentrations associated with 50% inhibition of S. vacuolatus reproduction were predicted by linking the bioconcentration kinetics to the effect concentrations and ranged from 0.0480 mmol/kg wet weight to 7.61 mmol/kg wet weight for specifically acting and reactive chemicals. Knowing the time-course of the internal effect concentration may promote an understanding of toxicity processes such as delayed toxicity, carry-over toxicity, or mixture toxicity in future studies.

Effect propagation in a toxicokinetic/toxicodynamic model explains delayed effects on the growth of unicellular green algae Scenedesmus vacuolatus

Ecotoxicological standard tests assess toxic effects by exposing an organism to high concentrations over defined periods of time. To evaluate toxicity under field conditions such as fluctuating and pulsed exposures, process-based toxicokinetic/toxicodynamic (TK/TD) models may be used for extrapolation from the existing evidence. A TK/TD model was developed that simulates the effect on growth of the green algae Scenedesmus vacuolatus continuously exposed to the model chemicals norflurazon, triclosan, and N-phenyl-2-naphthylamine. A pharmacological time-response model describing the effects of anticancer treatments on cancer cell growth was adapted and modified to model the affected growth of synchronized algae cells. The TK/TD model simulates the temporal effect course by linking the ambient concentration of a chemical to the observable adverse effect via an internal concentration and a sequence of biological events in the organism. The parameters of the toxicodynamic model are related to the growth characteristics of algae cells, a no effect concentration, the chemical efficacy as well as the ability of recovery and repair, and the delay during damage propagation. The TK/TD model fits well to the observed algae growth. The effect propagation through cumulative cell damage explained the observed delayed responses better than just the toxicokinetics. The TK/TD model could facilitate the link between several effect levels within damage propagation, which prospectively may be helpful to model adverse outcome pathways and time-dependent mixture effects.

Application of nd-SPME to determine freely dissolved concentrations in the presence of green algae and algae-water partition coefficients

Biological tests are essential for toxicity assessment of chemicals to aquatic organisms. Since awareness is increasing that freely dissolved concentrations in test media may change during exposure and deviate significantly from nominal concentrations there is a need for concentration monitoring in biotests. Biotests are increasingly miniaturized with effect amounts of toxicants that are close to or below quantification limits of conventional analytical methods. Thus, the suitability of pre-equilibrium non-depletive (nd) solid phase microextraction (SPME) as an analytical method for small test volumes in the presence of green algae Scenedesmus vacuolatus was investigated. The method was validated for several compounds with different physical-chemical properties and effect concentrations. Limits of quantification were at least about 10 times smaller than the EC(50) of the compounds determined in earlier studies in a cell multiplication inhibition tests with Scenedesmus vacuolatus. Fouling of the fibres due to attached algae cells could be excluded and the method was found to be well suited to measure free concentrations in the algae test. The nd-SPME-method was applied to determine partitioning coefficients between algae and the surrounding aqueous phase and can be used to determine real exposure concentrations in a cell multiplication inhibition test with green algae.

Composition and biodegradation of a synthetic oil spilled on the perennial ice cover of Lake Fryxell, Antarctica

A helicopter crashed in January 2003 on the 5 m-thick perennial ice cover of Lake Fryxell, spilling synthetic turbine oil Aeroshell 500. Molecular compositions of the oils were analyzed by gas chromatography-mass spectrometry and compared to the composition of contaminants in ice, meltwater, and sediments collected a year after the accident. Aeroshell 500 is based on C20-C33 Pentaerythritol triesters (PET) with C5-C10 fatty acids susbstituents and contain a number of antioxidant additives, such as tricresyl phosphates. Biodegradation of this oil in the ice cover occurs when sediments are present PETs with short fatty acids substituents are preferentially degraded, whereas long chain fatty acids seem to hinder esters from hydrolysis by esterase derived from the microbial assemblage. It remains to be seen if the microbial ecosystem can degrade tricresyl phosphates. These more recalcitrant PET species and tricresyl phosphates are likely to persist and comprise the contaminants that may eventually cross the ice cover to reach the pristine lake water.

Allelochemical stress causes oxidative damage and inhibition of photosynthesis in Chlorella vulgaris

This study investigated the effects of N-phenyl-2-naphthylamine, an effective allelochemical on aquatic unicellular algae Chlorella vulgaris at physiological gene transcription level. Exposure to 2.5 mg L(-1) of N-phenyl-2-naphthylamine increased the activities of the antioxidant enzymes, superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), which were 2.47, 3.24, and 4.27 times higher than that of the control, however, exposure to 4.0 mg L(-1) N-phenyl-2-naphthylamine decreased the activities of these antioxidant enzymes. An increase in malondialdehyde content and a decrease in chlorophyll content following exposure to N-phenyl-2-naphthylamine suggested that the alga was severely damaged and that cell growth was greatly inhibited. Electron microscopy showed that the plasma membrane was detached from the cell wall, the nucleus was condensed, and the structure of chloroplasts was disrupted, in response to N-phenyl-2-naphthylamine exposure. Real-time PCR showed that N-phenyl-2-naphthylamine reduced the transcript abundance of psaB and psbC to 3% and 1% of the control, respectively. These results demonstrated that N-phenyl-2-naphthylamine not only inhibited photosynthesis, but also triggered the synthesis of reactive oxygen species (ROS) to disrupt the subcellular structure of this aquatic organism.

QSAR analysis and specific endpoints for classifying the physiological modes of action of biocides in synchronous green algae

We propose the use of additional physiological endpoints in the 24h growth inhibition test with synchronous cultures of Scenedesmus vacuolatus for the classification of physiological modes of toxic action of chemicals in green algae. The classification scheme is illustrated on the example of one baseline toxicant (3-nitroaniline) and five biocides (irgarol, diuron, Sea-Nine, tributyltin (TBT) and norflurazon). The well-established endpoint of inhibition of reproduction is used for an analysis of the degree of specificity of toxicity by comparing the experimental data with predictions from a quantitative structure-activity relationship (QSAR) for baseline toxicity (narcosis). For those compounds with a toxic ratio greater than 10, i.e. a 10 times higher effect in reproduction than predicted by baseline toxicity, additionally the physiological endpoints inhibition of photosynthesis, cell division and cell volume growth were experimentally assessed. Depending on the relative sensitivity of the different endpoints the chemicals were classified into five different classes of modes of toxic action using a flow chart that was developed in the present study. The advantage of the novel classification scheme is the simplicity of the experimental approach. For the determination of the inhibition of reproduction, the cell size and numbers are quantified with a particle analyzer. This information can be used to derive also the physiological endpoints of cell volume growth and inhibition of cell division. The only additional measurement is the inhibition of the photosynthesis efficiency, which can be easily performed using the non-invasive saturation pulse method and pulse-modulated chlorophyll fluorometry with the Tox-Y-PAM instrument. This mechanistic approach offers a great future potential in ecotoxicology for the physiological mode of action classification of chemicals in algae, which should be a crucial step considered in the risk assessment of chemicals.

Bioassays with unicellular algae: deviations from exponential growth and its implications for toxicity test results

Growth assays with unicellular green algae are an established tool in ecotoxicological effect assessment for chemicals and environmental samples. From an ecological perspective it seems appropriate to use the growth rate as a process variable rather than a measure of biomass gain for calculating inhibitory effects of contaminants. The notion of simple exponential growth for the description of the population increase in undisturbed suspension cultures of unicellular green algae, however, seems to be an oversimplification. Experimental findings describe the increase in biomass, cell number, the development of cell volume distributions of populations, and the relationship between cell size and chlorophyll content for individual cells over one generation at a time resolution of 2-h intervals. It was observed that algal populations of Desmodesmus subspicatus show a time pattern of cell size growth; the average cell volume increases about sixfold, without corresponding increase in population size. This is followed by a distinct cell division phase with little gain in biomass. This synchronous growth behavior despite continuous illumination may be explained by the multiple fission characteristic of unicellular green algae which is an adaptation to cyclic light-dark changes in the environment. It might be controlled by an independent cell cycle clock. For routine regulatory testing fluorescence-based measurements rather than cell counting minimizes the confounding effect on toxicity determination. For investigations of time-dependent effects, e.g., by pulsed exposure, an alternative mechanistically based growth function for unicellular algae is proposed that accommodates for the observed growth pattern.

A fluorescence-based bioassay for aquatic macrophytes and its suitability for effect analysis of non-photosystem II inhibitors

BACKGROUND, GOALS AND SCOPE: During the last years the miniaturization of toxicity test systems for rapid and parallel measurements of large quantities of samples has often been discussed. For unicellular algae as well as for aquatic macrophytes, fluorescence-based miniaturized test systems have been introduced to analyze photosystem II (PSII) inhibitors. Nevertheless, high-throughput screening should also guarantee the effect detection of a broad range of toxicants in order to ensure routinely applicable, high-throughput measuring device experiments which can cover a broad range of toxicants and modes of action others than PSII inhibition. Thus, the aim of this study was to establish a fast and reproducible measuring system for non-PSII inhibitors for aquatic macrophyte species to overcome major limitations for use.

METHODS

A newly developed imaging pulse-amplitude-modulated chlorophyll fluorometer (I-PAM) was applied as an effect detector in short-term bioassays with the aquatic macrophyte species Lemna minor. This multiwell-plate based measuring device enabled the incubation and measurement of up to 24 samples in parallel. The chemicals paraquat-dichloride, alizarine and triclosan were chosen as representatives for the toxicant groups of non-PSII herbicides, polycyclic aromatic hydrocarbons (PAHs) and pharmaceuticals and personal care products (PPCPs), which are often detected in the aquatic environment. The I-PAM was used (i) to establish and validate the sensitivity of the test system to the three non-PSII inhibitors, (ii) to compare the test systems with standardized and established biotests for aquatic macrophytes, and (iii) to define necessary time scales in aquatic macrophyte testing. For validation of the fluorescence-based assay, the standard growth test with L. minor (ISO/DIS 20079) was performed in parallel for each chemical.

RESULTS

The results revealed that fluorescence-based measurements with the I-PAM allow rapid and parallel analysis of large amounts of aquatic macrophyte samples. The I-PAM enabled the recording of concentration-effect-curves with L. minor samples on a 24-well plate with single measurements. Fluorescence-based concentration-effect-curves could be detected for all three chemicals after only 1 h of incubation. After 4-5 h incubation time, the maximum inhibition of fluorescence showed an 80-100% effect for the chemicals tested. The EC50 after 24 h incubation were estimated to be 0.06 mg/L, 0.84 mg/L and 1.69 mg/L for paraquat-dichloride, alizarine and triclosan, respectively.

DISCUSSION

The results obtained with the I-PAM after 24 h for the herbicide paraquat-dichloride and the polycyclic aromatic hydrocarbon alizarine were in good accordance with median effective concentrations (EC50s) obtained by the standardized growth test for L. minor after 7 d incubation (0.09 mg/L and 0.79 mg/L for paraquat-dichloride and alizarine, respectively). Those results were in accordance with literature findings for the two chemicals. In contrast, fluorescence-based EC50 of the antimicrobial agent triclosan proved to be two orders of magnitude greater when compared to the standard growth test with 7 d incubation time (0.026 mg/L) as well as with literature findings.

CONCLUSION

Typically, aquatic macrophyte testing is very time consuming and relies on laborious experimental set-ups. The I-PAM measuring device enabled fast effect screening for the three chemicals tested. While established test systems for aquatic macrophytes need incubation times of > or = 7 d, the I-PAM can detect inhibitory effects much earlier (24 h), even if inhibition of chemicals is not specifically associated with PSII. Thus, the fluorescence-based bioassay with the I-PAM offers a promising approach for the miniaturization and high-throughput testing of chemicals with aquatic macrophytes. For the chemical triclosan, however, the short-term effect prediction with the I-PAM has been shown to be less sensitive than with long-term bioassays, which might be due to physicochemical substance properties such as lipophilicity.

RECOMMENDATIONS AND PERSPECTIVES

The results of this study show that the I-PAM represents a promising tool for decreasing the incubation times of aquatic macrophyte toxicity testing to about 24 h as a supplement to existing test batteries. The applicability of this I-PAM bioassay on emergent and submerged aquatic macrophyte species should be investigated in further studies. Regarding considerations that physicochemical properties of the tested substances might play an important role in microplate bioassays, the I-PAM bioassay should either be accompanied by evaluating physicochemical properties modeled from structural information prior to an experimental investigation, or by intensified chemical analyses to identify and determine nominal concentrations of the toxicants tested. The chemicals paraquat-dichloride, alizarine and triclosan were chosen as representatives for the toxicant groups of non-PSII herbicides, PAHs and PPCPs which are often detected in the aquatic environment. Nevertheless, in order to ensure a routinely applicable measuring device, experiments with a broader range of toxicants and samples of surface and/or waste waters are necessary.

Physicochemical substance properties as indicators for unreliable exposure in microplate-based bioassays

In the last years many efforts were made to transform standardized algal test protocols into low-cost microplate assays. While advantages were pointed out frequently, limitations are not systematically addressed, thus hindering a widespread utilisation. In this study a group of organic substances with a wide distribution of volatility (logKAW from -6.53 to -2.13) and lipophilicity (logKOW from 1.26 to 4.92) was investigated with respect to the influence of these physicochemical properties on their algal toxicity in different assays. Therefore the EC50 values were determined with a microplate assay based on ISO 8692 protocol and the results were compared with those of an established algal growth inhibition test conducted in air tight glass vessels. Using the ratio of the EC50 values, a clear connection between biological response and volatility as well as lipophilicity of test substances could be detected. Chemicals with a logKOW higher than 3 or a Henry coefficient logKAW higher than -4 were identified as less effective in the microplate assay than in the comparative assay. The loss in nominal concentration due to physicochemical properties could be shown to contribute to this using HPLC analysis. Consequently, when using microplate assay's one should be aware that lipophilic and volatile chemicals might be underestimated in their toxicity, which could be indicated from evaluating related physicochemical properties modelled from structural information prior to an experimental investigation.

Development and validation of a new fluorescence-based bioassay for aquatic macrophyte species

Bioassays with unicellular algae are frequently used as ecotoxicological test systems to evaluate the toxicity of contaminated environmental samples or chemicals. In contrast, aquatic macrophyte test systems are still rarely used as they are laborious to handle because species exhibit distinct ecological requirements. The aim of this study was to establish a fast and reproducible measuring system for aquatic macrophyte species to overcome those limitations for use. Thus, a newly developed pulse-amplitude modulated chlorophyll fluorometer (Imaging-PAM) was applied as an effect detection in short-term bioassays with aquatic macrophyte species. This multiwell-plate-based measuring device enables the incubation and measurement of up to 24 samples in parallel. The Imaging-PAM was used (i) to establish and validate the sensitivity of the test systems to three Photosystem II (PSII) inhibitors (atrazine, prometryn, isoproturon), (ii) to compare the test systems with established biotests for macrophytes and (iii) to define necessary time scales in aquatic macrophyte testing. The results showed that fluorescence-based measurements with the Imaging-PAM allow rapid and parallel analysis of large amounts of aquatic macrophyte samples and of toxicants effects of the PSII inhibitors tested on aquatic macrophytes. Measurements revealed a good correlation between obtained median effective concentrations (EC50s) for the new and the established biotest systems. Hence, the Imaging-PAM measuring device is a promising tool to allow fast chemical effect screening for high amounts of samples with little time and material and thus offers scope for high-throughput biotesting using aquatic macrophyte species.