Delayed fluorescence in algal growth inhibition tests

A series of 72-hour growth inhibition tests with green alga Desmodesmus (Scenedesmus) subspicatus (ISO 8692) has been performed to test the delayed fluorescence (DF) parameters as possible endpoint measurements. Sensitivity to five toxicants with direct and indirect effects on photosynthesis was tested, and the median effective concentration (EC₅₀) values derived from the cell concentration, absorbance and DF were compared. The sensitivity of DF intensity (DFI) was comparable with the two endpoints suggested in ISO 8692 for all five toxicants: potassium dichromate, 3,5-dichlorophenol (3,5-DCP), DCMU, copper and cadmium. In the case of potassium dichromate and copper, DFI was more sensitive than the other endpoints in this study. The analysis of DF relaxation kinetics showed a specific response to the herbicide DCMU. Additionally, a 24-hour test was performed with the same toxicant concentrations (except copper), where DFI was measured 30 minutes, one hour and 24 hours after the exposure. The influence of toxicants on DFI after a 24-hour exposure was comparable with the effects after 72 hours. Only 3,5-DCP influenced DFI after 30 minutes. DF proved to be a simple, reliable and rapid measurement to assess toxicity in algal tests, which can to some extent differentiate among various toxicants.     

Landscape-level toxicant exposure mediates infection impacts on wildlife populations

Anthropogenic landscape modification such as urbanization can expose wildlife to toxicants, with profound behavioural and health effects. Toxicant exposure can alter the local transmission of wildlife diseases by reducing survival or altering immune defence. However, predicting the impacts of pathogens on wildlife across their ranges is complicated by heterogeneity in toxicant exposure across the landscape, especially if toxicants alter wildlife movement from toxicant-contaminated to uncontaminated habitats. We developed a mechanistic model to explore how toxicant effects on host health and movement propensity influence range-wide pathogen transmission, and zoonotic exposure risk, as an increasing fraction of the landscape is toxicant-contaminated. When toxicant-contaminated habitat is scarce on the landscape, costs to movement and survival from toxicant exposure can trap infected animals in contaminated habitat and reduce landscape-level transmission. Increasing the proportion of contaminated habitat causes host population declines from combined effects of toxicants and infection. The onset of host declines precedes an increase in the density of infected hosts in contaminated habitat and thus may serve as an early warning of increasing potential for zoonotic spillover in urbanizing landscapes. These results highlight how sublethal effects of toxicants can determine pathogen impacts on wildlife populations that may not manifest until landscape contamination is widespread.     

Use of environmental challenges in behavioral toxicology

The term environmental challenges encompasses variables that are either known or suspected to affect a baseline of behavior. Environmental challenges can be used to provide information that is important for characterizing the behavioral effects of prior exposure to a toxicant, as well as for revealing effects of the toxicant that may not otherwise be apparent in the baseline under investigation. The use of environmental challenges can be applied in studies of all known classes of behavior, and in each case is limited only to the extent that appropriate variables can be identified and manipulated. Use of environmental challenges may be particularly relevant for studies of schedule-controlled operant behavior because many of the controlling variables have already been well specified. The rationale for using environmental challenges to characterize the behavioral effects of toxicants is very similar to that for using pharmacological challenges; both represent promising new research strategies in behavioral toxicology.     

Alternative Stable States Driven by Density-Dependent Toxicity

Many populations are exposed to naturally occurring or synthetic toxicants. An increasing number of studies demonstrate that the toxicity of such compounds is not only dependent on the concentration or load, but also on the biomass or density of exposed organisms. At high biomass, organisms may be able to alleviate adverse effects of the toxicant by actively lowering ambient concentrations through either a joint detoxification mechanism or growth dilution. We show in a conceptual model that this mechanism may potentially lead to alternative stable states if the toxicant is lethal at low densities of organisms, whereas a high density is able to reduce the toxicant concentrations to sub-lethal levels. We show in an example that this effect may be relevant in real ecosystems. In an earlier published experimental laboratory study, we demonstrated that ammonia toxicity in eelgrass is highly dependent on the eelgrass shoot density. Here, we used the results of these experiments to construct a model describing the complex interactions between the temperate seagrass Zostera marina and potentially lethal ammonia. Analyses of the model show that alternative stable states are indeed present over wide ranges of key-parameter settings, suggesting that the mechanism might be important especially in sheltered, eutrophicated estuaries where mixing of the water layer is poor. We argue that the same mechanism could cause alternative stable states in other biological systems as well.     

Reduction of pollution impacts through the control of toxicant release rate must be site- and season-specific

The marine environment is commonly used for the deliberate disposal of industrial, mining and metabolic wastes. Managers will benefit from experimental work that identifies ways of reducing environmental impacts by varying the frequency and intensity of toxicant release. Using a field dosing technique we investigated the effects of three frequencies of pulse copper pollution event, and two intensities of copper dose, on developing assemblages of sessile marine invertebrates. The resulting impacts could then be compared in assemblages exposed to the same amount of toxicant via different disturbance regimes. The experiment was replicated simultaneously at three sites within Port Philip Bay, Victoria, Australia. Pulse pollution events altered assemblage composition through a direct negative effect on densities of large solitary ascidians. In response to the removal of the spatially dominant solitary ascidians, there were increases in recruitment of many different phyla, and in the abundance of older individuals of some serpulid and bryozoan taxa. Biodiversity, as measured by total species number and Shannon's H′, did not reflect the dramatic structural changes apparent within assemblages. If pulse pollution events had either a negative or positive effect on a species' density, then that effect was accentuated by increasing the intensity (strength) or the frequency of the pollution disturbance. Within populations, however, adult mortality might benefit new recruits of the same species through the freeing up of settlement space. In this case, the effects of copper pulses were evident as changes in the population size structure rather than the overall density of an organism. There were variable responses to manipulating the output rate of the toxicant between sites that appear to be driven by the rate of recovery of the dominant space occupiers. At one site there was a negligible effect of disturbance intensity and impacts could be minimized through reducing only the frequency of toxicant release. At two other sites both the intensity and frequency of disturbance determined the pollution effect and minimizing impacts was feasible only through an overall reduction in the amount of toxicant released. The management and reduction of pollution impacts through the control of toxicant release will require site- and season-specific modifications.     

Nematode community structure in the brush-tailed rock-wallaby, Petrogale penicillata: Implications of captive breeding and the translocation of wildlife

Despite an increasing appreciation of the disease risks associated with wild-life translocations, the effects which captive breeding programs exert on parasite communities remain understudied. This may be attributed, in part, to the current lack of rapid and cost-effective techniques for comparing parasite assemblages between host populations. Terminal restriction fragment length polymorphism (T-RFLP) analysis of the rDNA region encompassing the internal transcribed spacers (ITS-1 and ITS-2) and 5.8S rRNA gene was used to characterise bursate nematode communities (suborder Strongylida) across two captive and two non-captive colonies of the threatened brush-tailed rock-wallaby, Petrogale penicillata. A clone library was constructed and a restriction enzyme selected to differentiate the predominant operational taxonomic units (OTUs) by the unique peak profiles they generated. The prevalence, intensity of infection and comparative structure of strongylid assemblages was evaluated for each of the host colonies. Compared to wild conspecifics, captive wallabies exhibited a reduced prevalence of infection and significantly lower faecal egg counts. T-RFLP revealed that a high proportion of the OTUs co-occurred across three of the four study locations. Despite this, the composition of strongylid assemblages was significantly different between the colonies, even when host translocation events had occurred. These results suggest that captive breeding programs may exert a profound impact on parasitic helminth assemblages. Developing efficient techniques for characterising community dynamics in potentially pathogenic organisms is critical to the long term success of species recovery efforts worldwide.     

Recent developments in and intercomparisons of acute and chronic bioassays and bioindicators

The ultimate goal of toxicity testing is to monitor or predict the effects of single compounds, elements or mixtures on the long-term health of individual organisms, populations, communities and ecosystems. Unfortunately, one does not always have all of the information required to determine the long-term or chronic effects of toxicants on the survival, growth or reproduction of aquatic organisms. For this reason, the chronic effects of toxicants are often inferred or estimated from observations made during short-term or or acute field or laboratory studies, which may be conducted at greater concentrations of toxicant. The observations made in the short-term studies are then related to the chronic effects by some statistical relationship. There are basically two approaches: 1) The long-term effects on a parameter, such as survival (lethality.) are predicted from observations on the same parameter, during short-term exposures; 2) Alternatively, the response of one parameter to long-term exposures of a toxicant can be predicted from the short-term responses of a different parameter. In this report we present several different examples of both types of methods for estimating chronic responses from information on more short-term responses and discuss the rationale, advantages and disadvantages of each. We also report on two biochemical indicators; energetic substrates and RNA/DNA ratio. These indicators both act as sensitive, integrative measures of sublethal effects of contaminants during both acute and chronic exposures.     

Effects of Environmental Toxicants on Metabolic Activity of Natural Microbial Communities

Two methods of measuring microbial activity were used to study the effects of toxicants on natural microbial communities. The methods were compared for suitability for toxicity testing, sensitivity, and adaptability to field applications. This study included measurements of the incorporation of ¹⁴C-labeled acetate into microbial lipids and microbial glucosidase activity. Activities were measured per unit biomass, determined as lipid phosphate. The effects of various organic and inorganic toxicants on various natural microbial communities were studied. Both methods were useful in detecting toxicity, and their comparative sensitivities varied with the system studied. In one system, the methods showed approximately the same sensitivities in testing the effects of metals, but the acetate incorporation method was more sensitive in detecting the toxicity of organic compounds. The incorporation method was used to study the effects of a point source of pollution on the microbiota of a receiving stream. Toxic doses were found to be two orders of magnitude higher in sediments than in water taken from the same site, indicating chelation or adsorption of the toxicant by the sediment. The microbiota taken from below a point source outfall was 2 to 100 times more resistant to the toxicants tested than was that taken from above the outfall. Downstream filtrates in most cases had an inhibitory effect on the natural microbiota taken from above the pollution source. The microbial methods were compared with commonly used bioassay methods, using higher organisms, and were found to be similar in ability to detect comparative toxicities of compounds, but were less sensitive than methods which use standard media because of the influences of environmental factors.     

The use of meiofauna in freshwater sediment assessments: Structural and functional responses of meiobenthic communities to metal and organics contamination

Because soft sediments are often hotspots of chemical contamination, their assessment can aid in identifying the causes of environmental stress and the implementation of measures to improve the health of the respective ecosystems. Achieving a “good ecological status” of surface waters, as required by the European Water Framework Directive, strongly depends on recognition of the chemical status of sediments. Meiobenthic organisms are important, but widely neglected components of the ecologically relevant fauna of a wide variety of ecosystems. In the present study, microcosms containing freshwater sediments were used to investigate the effects of eight different metals and polycyclic aromatic hydrocarbons (PAHs), in single and mixed applications, on natural meiofaunal assemblages. Structural (abundance and biomass) and functional (secondary production) parameters of the investigated assemblages were measured as ecologically relevant endpoints. Their sensitivity in revealing both the differential effects and the responses of meiofaunal taxa was evaluated to assess the general suitability of meiofauna and, in particular, of individual tested taxa, as bioindicators of soft sediment contamination. Structural parameters were found to be more valuable indicators than functional measurements, with more pronounced effects observed on the taxon level than on total meiofauna. Among the meiofaunal taxa considered in this study, nematodes were of particular utility as early indicators of chemical stress in freshwater soft sediments. Overall, this study provides new insights into the impact of toxicants on soft freshwater sediments and demonstrates the suitability of meiofaunal communities, especially nematodes, in assessing contamination of this ecosystem.     

Variability in the impact of an introduced predator (Asterias amurensis: Asteroidea) on soft-sediment assemblages

The introduction and establishment of the predatory asteroid Asterias amurensis in coastal waters of southern Australia is considered a major threat to benthic marine assemblages and commercial bivalve species. We compare the impact of the seastar in experiments on three soft-sediment assemblages. The experiments were carried out immediately beyond the current range of the seastar in southeast Tasmania. This allows us to assess the repeatability, and hence predictability, of the type and magnitude of the impact of A. amurensis on soft-sediment assemblages. Responses to manipulations at the species level were dissimilar across the three experiments, reflecting marked initial differences in the three assemblages at both the species and functional group levels. However, at the functional group level, there were notable similarities in the impact of the seastar on the different assemblages. When potential prey taxa were separated into functional groups reflecting ecological availability (e.g. surface dwelling vs. deep burrowing bivalves), seastar effects were largely restricted to the surface bivalves. While the effect of seastar predation on surface bivalves was common in all three assemblages, the magnitude of the impact varied both among and within assemblages. Variability in the impact of A. amurensis at both levels appeared to be largely due to differences in relative availability of prey species. Overall, the results of these experiments indicate that while the impact of the seastar is broadly predictable at the functional group level, the exact nature of seastar effects is likely to be site- and time-specific given the inherent natural variability in soft-sediment assemblages and the seastar's responses to them.     

Continuous culture algal bioassays for organic pollutants in aquatic ecosystems

Short-term responses of phytoplankton to organic pollutants are highly transitory. Time-course studies of non-steady state cells in continuous culture showed varying growth or photosynthetic responses such as enchancement, inhibition, adaptation (or development of resistance) or rebound, depending on the direction of changes in the intracellular toxicant concentration and the duration of exposure. However, steady-state cells in a two-stage chemostat system exhibited an increased tolerance to toxicants and subtle physiological effects such as photosynthetic enhancement which was accompanied by a considerable leakage of photosynthesates. It is important to understand such steady-state responses for the prediction and assessment of ecological impact by organic pollution on phytoplankton, since the time scale of changes in the toxicant/biomass ratio in most natural waters is long enough to approximate an equilibrium state.     

Are zebra mussels restricted by toxicant levels in the River Meuse? a review

This paper questions if chemical barriers prevent the occurrence of benthic invertebrates in the river Meuse. To this purpose an ecotoxicological analysis is presented, using published observations on the zebra musselDreissena polymorpha, a tolerant species that maintains populations in the river. Zebra mussels collected or exposed at the Belgian-Dutch border contained high levels of several groups of toxicants, and mixture toxicity is likely to occur. A recently developed bio-assay, using the filtration rate of the zebra mussel, demonstrated strong inhibitory effects of water from the river Meuse. To determine which (combination of) toxicants cause such effects, laboratory experiments with toxicant mixtures were carried out. It was demonstrated that in a mixture of five metals (Cu, Zn, Ni, Cd, Pb) the metals contributed to the toxicity of the mixture below the No Observed Effect Concentrations (NOEC) for these metals when tested individually. The average concentration of Cu, Zn and possibly Pb in Meuse water exceed the NOEC values for filtration rate. Thus, it seems likely that joint effects of different (groups of) toxicants in the river Meuse cause the overall toxicity of the water for the zebra mussel, explaining the marginal populations at the test site, while other river species are absent there. It is suggested that toxicants in the river Meuse restrict the recolonisation of the river by invertebrate species more sensitive than the zebra mussel. This will be studied using invertebrates of different sensitivities (molluscs, arthropods), enabling the assessment of toxicity at different stages of water quality improvement of the river Meuse.     

Do Larvae and Ovipositing Chironomus riparius (Diptera: Chironomidae) Females Avoid Copper-Contaminated Environments?

Studies on the avoidance behavior of aquatic organisms to contaminants have confirmed that such behavior can be relevant in field situations. However, almost all toxicity tests involve the forced exposure of organisms to toxicants. In particular, despite the importance of Chironomus riparius Meigen larvae in sediment toxicity testing, only a few studies on avoidance behavior have been performed. This study investigated the ability of different life stages of C. riparius, including ovipositing females, first-, second-, and fourth-instar larvae, to avoid copper-contaminated environments. Ovipositing females were given a choice between a control and copper solution (1.3 mg Cu l^{? 1}). First-instar larvae were provided with a choice between a control and a copper (2.0 mg Cu l^{? 1})-spiked sediment. Both second- and fourth-instars were exposed to a copper gradient (0.38–3.4 mg Cu l^{? 1}) in a flow-through system. None of the life stages avoided copper, even though the highest concentrations caused lethal effects on midges. The avoidance behavior of C. riparius is not a sensitive endpoint to assess copper sublethal toxicity.     

Feeding behavior as an index of copper stress in Daphnia magna and Brachionus calyciflorus

1. The effect of short-term exposure to the fungicide copper sulfate, on the feeding behavior of two freshwater zooplankters, the rotifer Brachionus calyciflorus and the cladocera Daphnia magna, was studied.2. Both species were exposed to sublethal levels (0.01, 0.02, 0.05 and 0.08 mg/l) of copper sulfate during 24 hr in order to determine the effect of this toxicant on filtration and ingestion rates.3. The experiments were performed using the unicellular algae Nannochloris oculata in a density of 5 × 10⁵ cells/ml, as food for both organisms.4. Rates of filtration and ingestion declined on B. calyciflorus and D. magna with increasing toxicant concentrations.5. The effective copper sulfate concentration at which feeding rate was reduced to 50% of that in controls (ec₅₀) was 0.043 and 0.090 mg/1 for B. calyciflorus and D. magna, respectively.     

Measurement of chronic toxicity using the opossum shrimpMysidopsis bahia

The chronic toxicity of silver and endosulfan to the opossum shrimpMysidopsis bahia was determined using continuous-flow bioassays. The 28-day bioassays measured survival, fecundity, and growth (length and weight measurements). Maximum acceptable toxicant concentrations (MATC) were estimated from measured toxicant concentrations. MATC values were similar using either brood size or growth as a criterion for sublethal effects. As an alternative to the determination of fecundity impairment, measurement of growth reduction in response to exposure to toxicants may provide a useful tool in the assessment of chronic toxicity inMysidopsis life-cycle bioassays.     

Background and overview of current sediment toxicity identification evaluation procedures

Laboratory bioassays can provide an integrated assessment of the potential toxicity of contaminated sediments to aquatic organisms; however, toxicity as a sole endpoint is not particularly useful in terms of identifying remedial options. To focus possible remediation (e.g., source control), it is essential to know which contaminants are responsible for toxicity. Unfortunately, contaminated sediments can contain literally thousands of potentially toxic compounds. Methods which rely solely on correlation to identify contaminants responsible for toxicity are limited in several aspects: (a) actual compounds causing toxicity might not be measured, (b) concentrations of potentially toxic compounds may covary, (c) it may be difficult to assess the bioavailability of contaminants measured in a sediment, and (d) interactions may not be accounted for among potential toxicants (e.g., additivity). Toxicity identification evaluation (TIE) procedures attempt to circumvent these problems by using toxicity-based fractionation procedures to implicate specific contaminants as causative toxicants. Phase I of TIE characterizes the general physio-chemical nature of sample toxicants. Phase II employs methods to measure toxicants via different analytical methods, and Phase III consists of techniques to confirm that the suspect toxicants identified in Phases I and II of the TIE actually are responsible for toxicity. These TIE procedures have been used to investigate the toxicity of a variety of samples, including sediments. Herein we present a brief conceptual overview of the TIE process, and discuss specific considerations associated with sediment TIE research. Points addressed include: (a) selection and preparation of appropriate test fractions, (b) use of benthic organisms for sediment TIE work, and (c) methods for the identification of common sediment contaminants.     

Xenotransplantation Models to Study the Effects of Toxicants on Human Fetal Tissues

Many diseases that manifest throughout the lifetime are influenced by factors affecting fetal development. Fetal exposure to xenobiotics, in particular, may influence the development of adult diseases. Established animal models provide systems for characterizing both developmental biology and developmental toxicology. However, animal model systems do not allow researchers to assess the mechanistic effects of toxicants on developing human tissue. Human fetal tissue xenotransplantation models have recently been implemented to provide human‐relevant mechanistic data on the many tissue‐level functions that may be affected by fetal exposure to toxicants. This review describes the development of human fetal tissue xenotransplant models for testis, prostate, lung, liver, and adipose tissue, aimed at studying the effects of xenobiotics on tissue development, including implications for testicular dysgenesis, prostate disease, lung disease, and metabolic syndrome. The mechanistic data obtained from these models can complement data from epidemiology, traditional animal models, and in vitro studies to quantify the risks of toxicant exposures during human development     

Vertical migration of infaunal invertebrates in response to dosing with secondary treated sewage effluent: a microcosm experiment

Short-term experiments testing behaviouralresponses to a toxicant are liable to besensitive indicators of the potential effectsof a pollutant and allow predictions abouthabitat preferences of organisms in the field.We have undertaken short-term microcosmexperiments with intact assemblages ofintertidal invertebrate infauna andinvestigated the responses of these assemblagesto dosing with a secondary treated sewageeffluent. Infaunal assemblages were taken fromsites that differed in their proximity toeffluent discharges and in recent exposure todisturbances. We observed a range of responsesto different dose treatments during theseexperiments with differential migration intothe water column by the corophiid amphipodCorophium insidiosum and changes in depthdistribution within the sediment by severalannelid taxa. We discuss these results withrespect to potential field responses followingexposure to treated sewage effluents andbiological monitoring programs.     

How do toxicants affect epidemiological dynamics?

Populations are formed of their constituent interacting individuals, each with their own respective within‐host biological processes. Infection not only spreads within the host organism but also spreads between individuals. Here we propose and study a multilevel model which links the within‐host statuses of immunity and parasite density to population epidemiology under sublethal and lethal toxicant exposure. We analyse this nested model in order to better understand how toxicants impact the spread of disease within populations. We demonstrate that outbreak of infection within a population is completely determined by the level of toxicant exposure, and that it is maximised by intermediate toxicant dosage. We classify the population epidemiology into five phases of increasing toxicant exposure and calculate the conditions under which disease will spread, showing that there exists a threshold toxicant level under which epidemics will not occur. In general, higher toxicant load results in either extinction of the population or outbreak of infection. The within‐host statuses of the individual host also determine the outcome of the epidemic at the population level. We discuss applications of our model in the context of environmental epidemiology, predicting that increased exposure to toxicants could result in greater risk of epidemics within ecological systems. We predict that reducing sublethal toxicant exposure below our predicted safe threshold could contribute to controlling population level disease and infection.     

Spatially variable effects of copper on sessile invertebrates across a marina

Heavy metals often occur at elevated concentrations in bays and estuaries surrounded by urbanised areas and can cause substantial ecological changes to marine communities. The toxic effect of heavy metals can be modified by a several physico-chemical processes in the marine environment. These processes can vary greatly over small scales, so it is likely that the ecological effects of heavy metals on marine biota also vary substantially over similar spatial scales. We used a manipulative field experiment to assess the spatial variation in the effect of copper on sessile invertebrates within a marina and tested whether this variation was influenced by variation in localised water flow. Effects of copper were not consistent over relatively small spatial scales (i.e. metres) for all sessile invertebrate species. Three of the twenty sessile invertebrates from our experimental assemblages showed significant small scale variation in the effect of copper at the scale of metres. For these taxa, the effect of copper varied in magnitude and direction. We had predicted that the effect of copper would diminish with increased water flow, but this did not occur for any taxa. We suggest that variations in the presence of organic or inorganic compounds or in pH within the marina may provide alternative explanations for the spatially variable effects detected in this study. Our study's findings highlight the need for future field assessments of the complex physical, chemical and biological influences on the ecological effects of heavy metals, particularly at small spatial scales. Such studies are the key to improving our prediction about the effects of heavy metals on marine organisms and aid in better characterisation of the risk of heavy metals in the marine environment.