N.M. van Straalen

Population consequences of cadmium toxicity in soil microarthropods

Chronic toxicity experiments were performed using the collembolan Orchesella cincta (L.) and the oribatid mite Platynothrus peltifer (Koch), which were exposed to various levels of cadmium in the food (green algae). Experimental results were combined with life-history information to obtain realistic estimates of the intrinsic rate of population increase and its sensitivity to Cd stress. Chronic LC50 values for dietary exposure to Cd were estimated as 1.60 mumol/g for O. cincta and 3.18 mumol/g for P. peltifer. No-observed-effect concentrations (NOECs) for growth and reproduction were 0.042 mumol/g for O. cincta and 0.026 mumol/g for P. peltifer. The main effects of Cd were, however, different in each species. In O. cincta, Cd affected primarily female growth, without a direct effect on reproduction; in P. peltifer, the effect was primarily on reproduction. Uptake of Cd was higher in P. peltifer than in O. cincta, and caused a loss of zinc in the former species. As a consequence of their differing physiological responses to Cd, mites and collembolans also reacted differently in their population growth rates. The capacity for population increase of mites appeared to be rather sensitive to Cd, while collembolans were able, to some extent, to maintain their capacity for increase, in spite of toxic effects at the individual level. These results may contribute to developing ecotoxicological theory for the population consequences of toxic action in species with diverging life histories. Soil microarthropods may be suitable test organisms for evaluating the risk of chemicals to the soil ecosystem.

Comparison of cadmium kinetics in four soil arthropod species

Soil organisms may play an important role in the transfer of metals through the ecosystem in metal-polluted environments. To evaluate this role, four representative species were compared through modelling their toxico-kinetics when exposed to cadmium in chronic laboratory experiments. Considerable differences in consumption and assimilation of food and cadmium were found between the species. Cadmium assimilation correlated with food assimilation. Cadmium excretion and equilibrium concentrations also differed considerably between the species. The differences in equilibrium concentrations were comparable to the differences between these species in the field. Cadmium assimilation efficiencies were high in the predatorsNotiophilus biguttatus andNeobisium muscorum, and were lower in the saprotrophsOrchesella cincta andPlatynothrus peltifer. Excretion constants were high in the insectsN. biguttatus andO. cincta and low in the arachnidsN. muscorum andP. peltifer. There was no direct relationship between assimilation efficiency and excretion ability. The differences in cadmium assimilation efficiencies reflect differences in trophic level and most probably differences in nutrient demand, which may be determined taxonomically. The influence of excretion ability on the equilibrium concentration is larger than that of assimilation efficiency. Species with a high equilibrium concentration combine low excretion ability with either low or high assimilation. Together with compartment modelling, the study of uptake and excretion of cadmium by individual species provides a suitable tool for predicting the amount in the different soil invertebrates and for calculating pollutant fluxes.

Stress induced fitness reduction is not determined by the most sensitive life-cycle trait.

1. It is widely assumed that stressors such as toxicants affect organisms by impairment of those life-cycle variables that are most sensitive to these toxicants. We tested this premise by contrasting a fitness assessment with the most sensitive life-cycle variable approach using cadmium and the nematode Plectus acuminatus as a case study. 2. Based on complete life-cycle experiments, a deterministic model was constructed relating changes in juvenile and adult variables, including a Weibull survival distribution, to fitness, which was defined as the intrinsic rate of population increase. 3. Based on a sensitivity analysis of the model it was indicated that impairment of the most sensitive trait, the reproductive period which was reduced by 45%, did not have any effect on fitness. However, a prolongation by cadmium of the juvenile period by 7.5%, the least sensitive trait, resulted in a fitness decrease of 5%. 4. Application of this approach to the evaluation of critical effect levels for cadmium implies that a hazard assessment based on the most sensitive life-cycle trait leads to erroneous predictions of the biological impact that toxicants cause.

Spatial heterogeneity of element and litter turnover in a Bornean rain forest.

The spatial heterogeneity of element fluxes was quantified by meas- uring litterfall, throughfall and litter decomposition for 1 y in 30 randomly located sampling areas in a lowland dipterocarp rain forest. The idea tested was that turnover of elements is more variable than turnover of dry matter in a forest with extremely high tree species diversity. In spite of the low fertility of the soil (an ultisol), total litter production (leaves, trash, and wood <2 cm in diameter) was high (1105 g m -2 y -1 ) with inputs to the forest floor of carbon, nitrogen, phos- phorus, calcium, magnesium, potassium, manganese and iron of 550, 15.3, 0.47, 6.26, 2.49, 4.75, 0.95 and 0.14 g m -2 y -1 respectively. Throughfall was 81% of the annual rainfall and transferred 22.2, 1.37, 0.14, 1.07, 0.67, 0.39, 7.92, <0.06, and <0.06 g m -2 y -1 of organic carbon, nitrogen (all forms), phosphorus, sulphur, cal- cium, magnesium, potassium, manganese and iron, respectively. Average turnover rates of nutrients in litter were highest for potassium and decreased in the sequence calcium, magnesium, carbon, nitrogen and phosphorus. Concentrations of phosphorus, nitrogen and potassium in litterfall, litter mass and topsoil were closely correlated with each other. Concentrations of calcium and manganese were positively correlated with each other and with elevation. Variations in leaf chem- istry and total litterfall caused the spatial heterogeneity of element input to the forest floor to have a coefficient of variation of 30 - 70%, depending on the ele- ment. Due to the strong positive correlation between element fluxes and pools, the spatial variability of turnover rates (CV c. 20%) was lower than that of element input. Turnover rates for K varied by a factor of 4, and for Ca by a factor of 2.8 when the different sites were compared. The results strongly suggest that the most important factor determining spatial heterogeneity of organic matter and element dynamics on the forest floor is the site-specific amount of leaf fall, rather than spatially variable decomposition rates. 1 Corresponding author.

Early reproduction and increased reproductive allocation in metal-adapted populations of the terrestrial isopod Porcellio scaber

Organisms inhabiting metal-contaminated areas can be stressed by metal exposure and are possibly subject to selection, resulting in increased metal tolerance and changes in growth and/or reproduction characteristics. In a previous study it was found that in the terrestrial isopod Porcellio scaber, sampled from the vicinity of a zine smelter, the body size was small and the brood size was large compared to isopods from a reference area. To assess whether these differences were due to genetic differentiation between strains, isopods collected from a reference wood, a zinc smelter area and a lead mine were cultured on non-polluted food, while growth, reproduction and metal concentrations were studied in first and second laboratory generations. The isopods from the three populations differed in age and weight at first reproduction, although there were hardly any differences in growth. The females of the mine and the smelter population started to reproduce earlier, at a lower weight, which resulted in fewer young per female. However, reproductive allocation (=wight of young relative to the weight of the mother) was higher in mine and smelter isopods. We conclude that the isopods at the metal-contaminated sites have been selected for early reproduction and increased reproductive allocation. The results indicate that populations inhabiting metal-polluted sites have probably undergone evolutionary changes. This study showed that growth and reproduction characteristics of different populations under laboratory conditions may provide information on selection processes in the field.

Bioaccumulation of cadmium by the freshwater isopod Asellus aquaticus (L.) from aqueous and dietary sources

Experiments were conducted to determine the kinetics and relative importance of aqueous and dietary uptake of cadmium by the freshwater isopod Asellus aquaticus (L.). Test animals were exposed during 30 days to aqueous Cd in a continuous flow system (exposure levels: 0.2 - 10 microg litre(-1)) and kept on a diet of previously contaminated Elodea sp. (range of Cd concentrations: 2-350 microg g(-1), dry weight). Preceding semi-static experiments on dosage-control of the dietary factor revealed a rapid uptake of Cd by Elodea, with relatively high concentration factors (CF), which ranged from 4.8 to 5.5 (dry weight log (CF) after 16 days). For Asellus uptake from water appeared to be the predominant route. Highly significant bioconcentration of cadmium from water was observed in the animals, even at exposure levels below 1.0 microg litre(-1). In the various treatments, direct uptake from water accounted for 50-98% of the body burdens after 30 days exposure. The experimental results were described with a first order one-compartment bioaccumulation model. Model parameter estimates (mean +/- standard error) were obtained for rate constant of uptake (560 +/- 110 day(-1)), rate constant of elimination (0.032 +/- 0.017 day(-1)) and assimilation efficiency of Cd uptake from food (1.1 +/- 0.7%). The (dry weight) bioconcentration factor (BCF) and bioaccumulation factor (BAF) for extrapolated steady state conditions were estimated at 18 000 (BCF) and 0.08 (BAF). Experiments conducted at two different pH levels (5.9 versus 7.6) revealed no significant effects of pH on the uptake of aqueous Cd by the isopods. The results are discussed in relation to their potential significance to the field situation.

Genetic structure in Orchesella cincta (Collembola): strong subdivision of European populations inferred from mtDNA and AFLP markers.

Population genetic structure is determined both by current processes and historical events. Current processes include gene flow, which is largely influenced by the migration capacity of a species. Historical events are, for example, glaciation periods, which have had a major impact on the distribution of many species. Species with a low capacity or tendency to move about or disperse often exhibit clear spatial genetic structures, whereas mobile species mostly show less spatial genetic differentiation. In this paper we report on the genetic structure of a small, wingless arthropod species (Orchesella cincta: Collembola) in Europe. For this purpose we used mtDNA COII sequences and AFLP markers. We show that large genetic differences exist between populations of O. cincta, as expected from O. cinctas winglessness and sedentary lifestyle. Despite the fact that most variability was observed within populations (59%), a highly significant amount of AFLP variation (25%) was observed between populations from northwestern Europe, central Europe and Italy. This suggests that gene flow among regions is extremely low, which is additionally supported by the lack of shared mtDNA alleles between regions. Based on the genetic variation and sequence differences observed we conclude that the subdivision occurred long before the last glaciation periods. Although the populations still interbreed in the lab, we assume that in the long term the genetic isolation of these regions may lead to speciation processes.

Ecotoxicological Risk Assessment of Soil Fauna Recovery from Pesticide Application

For the registration procedures of pesticides, the level of exposure during the various activities in which pesticides are handled has to be estimated. Different worker exposure models have been put forward in Europe and North America. These models are discussed and compared to each other for their output in terms of estimated levels of potential inhalation and dermal exposure. It is concluded that the output varies with the model, largely for two reasons. First, the underlying databases are completely different, which is inherent to locally different equipment and farm sizes. Second, the statistic used in the estimation of a level of exposure from a database varies from a geometric mean to the 90-percentile. The estimated levels of exposure may be used in a first step of the risk assessment. It is recommended that the relevant parts of all four models be considered to increase the representativity of the predicted level of exposure.Ecotoxicological risk assessment provides a measure for adverse ecological effects of chemicals as a function of their concentration in the environment. The risk is commonly expressed as the ratio between the predicted environmental concentration (PEC) and the predicted no-effect concentration (PNEC) (Norton et al. 1992; Van Leeuwen and Hermens 1995). Another approach is to use statistical distributions for PECs and PNECs and to derive maximum acceptable concentrations from the risk associated with the probability of PEC being greater than PNEC. This approach has gone under the name of “distribution-based extrapolation methodology” (Aldenberg and Slob 1993; Forbes and Forbes 1993; Kooijman 1987; Okkerman et al. 1993; Smith and Cairns 1993; Van Straalen and Denneman 1989; Wagner and Lokke 1991.)

Revealing pancrustacean relationships: Phylogenetic analysis of ribosomal protein genes places Collembola (springtails) in a monophyletic Hexapoda and reinforces the discrepancy between mitochondrial and nuclear DNA markers

BackgroundIn recent years, several new hypotheses on phylogenetic relations among arthropods have been proposed on the basis of DNA sequences. One of the challenged hypotheses is the monophyly of hexapods. This discussion originated from analyses based on mitochondrial DNA datasets that, due to an unusual positioning of Collembola, suggested that the hexapod body plan evolved at least twice. Here, we re-evaluate the position of Collembola using ribosomal protein gene sequences.ResultsIn total 48 ribosomal proteins were obtained for the collembolan Folsomia candida. These 48 sequences were aligned with sequence data on 35 other ecdysozoans. Each ribosomal protein gene was available for 25% to 86% of the taxa. However, the total sequence information was unequally distributed over the taxa and ranged between 4% and 100%. A concatenated dataset was constructed (5034 inferred amino acids in length), of which ~66% of the positions were filled. Phylogenetic tree reconstructions, using Maximum Likelihood, Maximum Parsimony, and Bayesian methods, resulted in a topology that supports monophyly of Hexapoda.ConclusionAlthough ribosomal proteins in general may not evolve independently, they once more appear highly valuable for phylogenetic reconstruction. Our analyses clearly suggest that Hexapoda is monophyletic. This underpins the inconsistency between nuclear and mitochondrial datasets when analyzing pancrustacean relationships. Caution is needed when applying mitochondrial markers in deep phylogeny.

Population substructures in the soil invertebrate Orchesella cincta, as revealed by microsatellite and TE-AFLP markers

Microsatellite and three enzyme–amplified fragment length polymorphism (TE‐AFLP) DNA markers were used to describe the population genetic structure in the soil dwelling collembolan Orchesella cincta (L.). Two forests were sampled according to a three‐level nested hierarchical design, with fixed distances among samples within a parcel and among parcels within a forest. The largest component of variation was found at the smallest scale, within parcels (77–97%), while the smallest component of variation was found between forests. The two different methods to study population structure indicated a similar allocation of variance. Population genetic substructuring was revealed between samples on a scale of 50 m; the degree of substructuring however, varied between parcels and forests. One forest showed a high degree of structure as revealed by microsatellites, while another showed a low degree of structure. A significant deviation from random‐mating (average FIS = 0.23) over the two forests was detected. Two of 18 samples showed a difference in population genetic structure between males and females. We discuss the fact that the population genetic structure of O. cincta is significantly affected by long‐range dispersal, even though it is a small and wingless insect. This interpretation is supported by observations on tree‐climbing behaviour in this species that may facilitate air dispersal. As a consequence, the assumption that migration a priori may be neglected in demographic analysis of O. cincta is incorrect.