Induction of cadmium tolerance in two clones of Daphnia magna straus

Analysis of Cadmium-Stress-Induced microRNAs and Their Targets Reveals bra-miR172b-3p as a Potential Cd2+-Specific Resistance Factor in Brassica juncea

The contamination of soil with high levels of cadmium (Cd) is of increasing concern, as Cd is a heavy metal element that seriously limits crop productivity and quality, thus affecting human health. (1) Background: Some miRNAs play key regulatory roles in response to Cd stress, but few have been explored in the highly Cd-enriched coefficient oilseed crop, Brassica juncea. (2) Methods: The genome-wide identification and characterization of miRNAs and their targets in leaves and roots of Brassica juncea exposed to Cd stress was undertaken using strand specific transcript sequencing and miRNA sequencing. (3) Results: In total, 11 known and novel miRNAs, as well as 56 target transcripts, were identified as Cd-responsive miRNAs and transcripts. Additionally, four corresponding target transcripts of six miRNAs, including FLA9 (Fasciclin-Like Arabinogalactan-protein 9), ATCAT3 (catalase 3), DOX1 (dioxygenases) and ATCCS (copper chaperone for superoxide dismutase), were found to be involved in the plant’s biotic stress pathway. We further validated the expression of three miRNA and six target genes in response to Cd, hydrargyrum (Hg), manganese (Mn), plumbum (Pb) or natrium (Na) stress and Mucor infection by qRT-PCR, and show that ATCCS and FLA9 were significantly and differentially regulated in the Cd-treated leaves. In addition, our results showed that DOX1 was obviously induced by Pb stress. Among the respective target miRNAs, bra-miR172b-3p (target for ATCCS) and ra-miR398-3p (target for FLA9) were down-regulated in Cd-treated leaves. (4) Conclusions: We identified bra-miR172b-3p as a potential Cd-specific resistant inhibitor, which may be negatively regulated in ATCCS in response to Cd stress. These findings could provide further insight into the regulatory networks of Cd-responsive miRNA in Brassica juncea.

Alternative splicing is highly variable among Daphnia pulex lineages in response to acute copper exposure

BACKGROUND

Despite being one of the primary mechanisms of gene expression regulation in eukaryotes, alternative splicing is often overlooked in ecotoxicogenomic studies. The process of alternative splicing facilitates the production of multiple mRNA isoforms from a single gene thereby greatly increasing the diversity of the transcriptome and proteome. This process can be important in enabling the organism to cope with stressful conditions. Accurate identification of splice sites using RNA sequencing requires alignment to independent exonic positions within the genome, presenting bioinformatic challenges, particularly when using short read data. Although technological advances allow for the detection of splicing patterns on a genome-wide scale, very little is known about the extent of intraspecies variation in splicing patterns, particularly in response to environmental stressors. In this study, we used RNA-sequencing to study the molecular responses to acute copper exposure in three lineages of Daphnia pulex by focusing on the contribution of alternative splicing in addition to gene expression responses.

RESULTS

By comparing the overall gene expression and splicing patterns among all 15 copper-exposed samples and 6 controls, we identified 588 differentially expressed (DE) genes and 16 differentially spliced (DS) genes. Most of the DS genes (13) were not found to be DE, suggesting unique transcriptional regulation in response to copper that went unnoticed with conventional DE analysis. To understand the influence of genetic background on gene expression and alternative splicing responses to Cu, each of the three lineages was analyzed separately. In contrast to the overall analysis, each lineage had a higher proportion of unique DS genes than DE genes suggesting that genetic background has a larger influence on DS than on DE. Gene Ontology analysis revealed that some pathways involved in stress response were jointly regulated by DS and DE genes while others were regulated by only transcription or only splicing.

CONCLUSIONS

Our findings suggest an important role for alternative splicing in shaping transcriptome diversity in response to metal exposure in Daphnia, highlighting the importance of integrating splicing analyses with gene expression surveys to characterize molecular pathways in evolutionary and environmental studies.

Dynamics of Cadmium Acclimation in Daphnia pulex: Linking Fitness Costs, Cross-Tolerance, and Hyper-Induction of Metallothionein

Acclimation increases tolerance to stress in individuals but is assumed to contribute fitness costs when the stressor is absent, though data supporting this widely held claim are sparse. Therefore, using clonal (i.e., genetically identical) cultures of Daphnia pulex, we isolated the contributions of acclimation to the regulation of the metal response gene, metallothionein 1 (MT1), and defined the reproductive benefits and costs of cadmium (Cd)-acclimation. Daphnia pulex were exposed for 50 parthenogenetic generations to environmentally realistic levels (1 μg Cd/L), and tolerance to Cd and other metals assessed during this period via standard toxicity tests. These tests revealed (1) increased tolerance to Cd compared to genetically identical nonacclimated cultures, (2) fitness costs in Cd-acclimated Daphnia when Cd was removed, and (3) cross-tolerance of Cd-acclimated Daphnia to zinc and silver, but not arsenic, thereby defining a functional role for metallothionein. Indeed, Cd-acclimated clones had significantly higher expression of MT1 mRNA than nonacclimated clones, when Cd exposed. Both the enhanced induction of MT1 and tolerant phenotype were rapidly lost when Cd was removed (1-2 generations), which is further evidence of acclimation costs. These findings provide evidence for the widely held view that acclimation is costly and are important for investigating evolutionary principles of genetic assimilation and the survival mechanisms of natural populations that face changing environments.

Inducible pesticide tolerance in Daphnia pulex influenced by resource availability

Pesticides are a ubiquitous contaminant in aquatic ecosystems. Despite the relative sensitivity of aquatic species to pesticides, growing evidence suggests that populations can respond to pesticides by evolving higher baseline tolerance or inducing a higher tolerance via phenotypic plasticity. While both mechanisms can allow organisms to persist when faced with pesticides, resource allocation theory suggests that tolerance may be related to resource acquisition by the organism. Using Daphnia pulex, we investigated how algal resource availability influenced the baseline and inducible tolerance of D. pulex to a carbamate insecticide, carbaryl. Individuals reared in high resource environments had a higher baseline carbaryl tolerance compared to those reared in low resource environments. However, D. pulex from low resource treatments exposed to sublethal concentrations of carbaryl early in development induced increased tolerance to a lethal concentration of carbaryl later in life. Only individuals reared in the low resource environment induced carbaryl tolerance. Collectively, this highlights the importance of considering resource availability in our understanding of pesticide tolerance.

MicroRNAs are involved in cadmium tolerance in Daphnia pulex

Daphnia can develop tolerance to cadmium (Cd) after multi-generational exposures. Until now, Cd tolerance in this crustacean was thought to be mainly due to its sequestration via induction of metallothioneins (MTs). Our research supports other studies showing microRNAs (miRNAs) also play a role in this enhanced tolerance. We induced Cd tolerance in Daphnia pulex after exposing them for 25 generations and examined the maintenance of enhanced Cd tolerance under a Cd-free environment for an additional three generations. Acute Cd tolerance as well as long-term effects on population dynamics were measured in selected generations via 48h LC50 tests and 21 d reproductive tests, respectively. Cd tolerance was associated with differential expression of 10 miRNAs (miR-2, miR-33, miR-92, miR-96, miR-153, miR-252, miR-279, miR-283, miR-305 and miR-615). Pathway analysis revealed these miRNAs might increase Cd tolerance by suppressing cellular growth and proliferation by GTPase and cuticle protein pathways, which switch cellular energy allocation to detoxification processes. Moreover, we found increased Cd tolerance is related with induction of MT3 and MT4 and a subsequent downregulation of MT1 and MT3 expression when animals are moved to a Cd-free environment. This is the first study linking aquatic invertebrate miRNAs with induced tolerance to environmental stressors.

Metallothionein and Hsp70 trade-off against one another in Daphnia magna cross-tolerance to cadmium and heat stress

The association between the insensitivity of adapted ecotypes of invertebrates to environmental stress, such as heavy metal pollution, and overall low Hsp levels characterizing these organisms has been attracting attention in various studies. The present study seeks to induce and examine this phenomenon in Daphnia magna by multigenerational acclimation to cadmium in a controlled laboratory setting. In this experiment, interclonal variation was examined: two clones of D. magna that have previously been characterized to diverge regarding their cadmium resistance and levels of the stress protein Hsp70, were continuously exposed to a sublethal concentration of Cd over four generations to study the effects of acclimation on Hsp70, metallothionein (MT), reproduction and cross-tolerance to heat stress. The two clones differed in all the measured parameters in a characteristic way, clone T displaying Cd and heat resistance, lower Hsp70 levels and offspring numbers on the one hand and higher MT expression on the other hand, clone S the opposite for all these parameters. We observed only slight acclimation-induced changes in constitutive Hsp70 levels and reproductive output. The differences in MT expression between clones as well as between acclimated organisms and controls give evidence for MT accounting for the higher Cd tolerance of clone T. Overall high Hsp70 levels of clone S did not confer cross tolerance to heat stress, contrary to common expectations. Our results suggest a trade-off between the efforts to limit the proteotoxic symptoms of Cd toxicity by Hsp70 induction and those to sequester and detoxify Cd by means of MT.

Multigenerational contaminant exposures produce non-monotonic, transgenerational responses in Daphnia magna

Generally, ecotoxicologists rely on short-term tests that assume populations to be static. Conversely, natural populations may be exposed to the same stressors for many generations, which can alter tolerance to the same (or other) stressors. The objective of this study was to improve our understanding of how multigenerational stressors alter life history traits and stressor tolerance. After continuously exposing Daphnia magna to cadmium for 120 days, we assessed life history traits and conducted a challenge at higher temperature and cadmium concentrations. Predictably, individuals exposed to cadmium showed an overall decrease in reproductive output compared to controls. Interestingly, control D. magna were the most cadmium tolerant to novel cadmium, followed by those exposed to high cadmium. Our data suggest that long-term exposure to cadmium alter tolerance traits in a non-monotonic way. Because we observed effects after one-generation removal from cadmium, transgenerational effects may be possible as a result of multigenerational exposure.

Complex interactions between climate change and toxicants: evidence that temperature variability increases sensitivity to cadmium

The Intergovernmental Panel on Climate Change projects that global climate change will have significant impacts on environmental conditions including potential effects on sensitivity of organisms to environmental contaminants. The objective of this study was to test the climate-induced toxicant sensitivity (CITS) hypothesis in which acclimation to altered climate parameters increases toxicant sensitivity. Adult Physa pomilia snails were acclimated to a near optimal 22 °C or a high-normal 28 °C for 28 days. After 28 days, snails from each temperature group were challenged with either low (150 μg/L) or high (300 μg/L) cadmium at each temperature (28 or 22 °C). In contrast to the CITS hypothesis, we found that acclimation temperature did not have a strong influence on cadmium sensitivity except at the high cadmium test concentration where snails acclimated to 28 °C were more cadmium tolerant. However, snails that experienced a switch in temperature for the cadmium challenge, regardless of the switch direction, were the most sensitive to cadmium. Within the snails that were switched between temperatures, snails acclimated at 28 °C and then exposed to high cadmium at 22 °C exhibited significantly greater mortality than those snails acclimated to 22 °C and then exposed to cadmium at 28 °C. Our results point to the importance of temperature variability in increasing toxicant sensitivity but also suggest a potentially complex cost of temperature acclimation. Broadly, the type of temporal stressor exposures we simulated may reduce overall plasticity in responses to stress ultimately rendering populations more vulnerable to adverse effects.

Comparative toxicity assessment of nanosilver on three Daphnia species in acute, chronic and multi-generation experiments

The antibacterial properties of nanosilver have led to a versatile application spectrum including medical purposes and personal care products. However, the increasing use of nanosilver has raised concerns about its environmental impacts. Long-term exposure studies with aquatic invertebrates are essential to assess possible adverse effects on aquatic ecosystems. In the present study, acute (48 h), chronic (21 d) and long-term effects of nanosilver (primary size 15 nm) on five successive generations of three Daphnia species (D. magna, D. pulex, and D. galeata) were investigated. Acute EC₅₀ values of nanosilver were 121 µg Ag L⁻¹ for D. magna being the least sensitive species and 8.95 and 13.9 µg Ag L⁻¹ for D. pulex and D. galeata, respectively. Chronic exposure provided EC₁₀ values of 0.92 µg Ag L⁻¹ for D. magna showing the most sensitive chronic reaction and 2.25 and 3.45 µg Ag L⁻¹ for D. pulex and D. galeata, respectively. Comparative exposure to AgNO₃ revealed a generally higher toxicity of the soluble form of silver. The multi-generation experiments resulted in effects on the population level for all tested species. Exposure of D. magna indicated an increased toxicity of nanosilver in the fifth generation of animals exposed to 10 µg Ag L⁻¹. Neonates from pre-exposed parental daphnids did not completely recover when transferred into clean water. Exposure of D. pulex and D. galeata revealed not only increasing toxicity in some generations, but also greater tolerance to nanosilver. This study contributes to the assessment of the risk potential of nanosilver on aquatic ecosystems. It shows that effects of nanosilver vary within one genus and change with exposure duration. Therefore, long-term studies considering different aquatic species are needed to better understand the possible effects of nanosilver on aquatic ecosystems.

Review on methods for determination of metallothioneins in aquatic organisms

One aspect of environmental degradation in coastal areas is pollution from toxic metals, which are persistent and are bioaccumulated by marine organisms, with serious public health implications. A conventional monitoring system of environmental metal pollution includes measuring the level of selected metals in the whole organism or in respective organs. However, measuring only the metal content in particular organs does not give information about its effect at the subcellular level. Therefore, the evaluation of biochemical biomarker metallothionein may be useful in assessing metal exposure and the prediction of potential detrimental effects induced by metal contamination. There are some methods for the determination of metallothioneins including spectrophotometric method, electrochemical methods, chromatography, saturation-based methods, immunological methods, electrophoresis, and RT-PCR. In this paper, different methods are discussed briefly and the comparison between them will be presented.

Acclimatable cardiac and ventilatory responses to copper in the freshwater crayfish Procambarus clarkii

Mortality and physiological tests following exposure to waterborne copper were performed in the red swamp crayfish Procambarus clarkii from a central Italian population. Mortality tests gave an estimated 96 h LC50 value (with 95% confidence limits) of 162 (132-211) mg L(-1) waterborne copper II. Variations in cardiac and ventilatory rates were simultaneously monitored using a non-invasive plethysmographic technique. In experiments with different sub-lethal copper concentrations (control, 0.5, 1 and 10 mg L(-1)) performed at different times (3, 6, 96 h), copper exposure elicited a reduction in both heart and scaphognatite rates. Following exposure to 10 mg L(-1) copper for 96 h, the heart and scaphognatite rates decreased to about 35% of the initial values. The reduction was fully reversible, since crayfish exposed to 0.5, 1 and 10 mg Cu L(-1) for 96 h resumed control rates after a 3-h residence in clean water. In crayfish pre-exposed (96 h) to sub-lethal copper concentrations (0.1 and 1 mg L(-1)) and then held in control water (3 h), the reduction of heart and scaphognatite rates after exposure to 10 mg Cu L(-1) were significantly lower than in specimens pre-exposed to control water. Therefore, copper induces a concentration and time dependent reduction of both cardiac and ventilatory activity in P. clarkii; these responses can be reduced or fully abolished by pre-exposure to sub-lethal levels of the metal.

Inter- and intra-species variation in acute zinc tolerance of field-collected cladoceran populations

Acute zinc toxicity was assessed for 10 freshwater cladoceran species collected in six different ecosystems across Europe and for two standard laboratory-reared species (Daphnia magna and Ceriodaphnia dubia). The collected organisms belonged to five different genera: Daphnia (subgenus Daphnia and Ctenodaphnia), Ceriodaphnia, Simocephalus, Acroperus and Chydorus. The 48-h EC50 of the field-collected organisms tested in standard laboratory water ranged from 375+/-141 to 4314+/-1513 microg Znl(-1). The laboratory clone of D. magna was less sensitive than the majority of the field-collected species, while our laboratory Ceriodaphnia dubia was the second most sensitive. Considerable inter-species variation was found within the genus of Ceriodaphnia (factor 6) and within the genus Daphnia (factor 8). Among the different (sub)genera tested, Chydorus and Ctenodaphnia were significantly more tolerant than the others (up to a factor 3 difference). A significant positive relationship (r2=0.67, p<0.05) between the mean cladoceran 48-h EC50 and the ambient zinc concentration of the different aquatic systems was demonstrated, suggesting a role of acclimation and/or adaptation. No significant correlation between the acute zinc tolerance and the length of the organisms was found.

Accumulation and elimination of silver in Daphnia magna and the effect of natural organic matter

Body burden is often used as an indicator of the toxic impact of metals such as silver. Natural organic matter (NOM) is reported to reduce silver toxicity to the highly sensitive freshwater crustacean Daphnia magna. However, the effect of NOM on silver burden in these organisms has not been investigated, and literature reports from other aquatic animals suggest that NOM can actually promote silver accumulation. In 24 h accumulation trials NOM exhibited a general trend of reducing whole body silver accumulation. Differences in accumulation profiles between NOM samples were attributed to chloride content stimulating uptake by the formation of diffusible silver chloride complexes. Silver accumulation assayed over 1 h exhibited considerable heterogeneity. Subsequent experiments conducted with varying light conditions during exposure and utilising gut dissection, suggested that these differences were in part due to variable gut silver accumulation. In addition to a general reduction in silver accumulation, NOM also facilitated enhanced elimination of silver from the animals. Rapid elimination of silver from Daphnia, coupled with speciation-, body compartment- and time of day-dependent accumulation suggests that silver body burden may be a poor indicator of silver toxicity in natural environments.

Dietary and water-borne Zn exposures affect energy reserves and subsequent Zn tolerance of Daphnia magna

Enhanced tolerance of aquatic organisms to metal toxicity is one of the important issues of environmental monitoring programs. Determination of dominant uptake route(s) of metals may help to better predict the toxic effects posed by metals. This study aimed to investigate the importance of Zn uptake routes on tolerance and energy reserves of Daphnia magna. Neonates of D. magna were exposed to water-borne zinc, dietary zinc (algae Pseudokichneriella subcapitata loaded with Zn) or to combination of both for 4 days. LC50 (48 h) values of Zn were considerably different from different zinc pre-exposures. Four-day pre-exposure of D. magna neonates produced enhanced tolerance to Zn toxicity. The lowest LC50 values were found in controls (48.2 microM) (no Zn added to their exposure medium and food) and after water-borne Zn pre-exposures (46.2 microM). The level of tolerance increased when dietary Zn was included in pre-exposures, reaching the highest level of LC50 value (70.8 microM) in the highest pre-exposure concentration of diet and water-borne combination experiment. The energy reserves of D. magna also varied significantly under different pre-exposure routes of zinc. In all cases, control animals contained lowest levels of protein, sugar and lipid. Likewise, they represented the lowest energy reserves. Protein levels were highest in the highest dietary Zn exposure, and lowest in the water-borne exposures. Highest and lowest sugar levels were measured in the lowest and highest water-borne Zn exposures, respectively. In contrast, lipid levels were higher in the higher Zn exposure of all exposure routes, the combination exposure resulting in highest lipid levels. The highest total energy reserve was measured in animals that lived in the highest Zn exposure of diet and water-borne combination experiment, mainly due to greater lipid reserves in algae reared in Zn containing media. Results suggest that the dietary exposure route should be considered carefully in natural monitoring studies, and be considered in regulatory assessments of zinc and population dynamics of cladocerans.

Importance of acclimation to environmentally relevant zinc concentrations on the sensitivity of Daphnia magna toward zinc

Daphnia magna was acclimated for six generations to an acclimation range of 0.02 to 74 microg/L of Zn2+. This range was determined by combining physicochemical water characteristics of European surface waters with total Zn concentrations in these waters in such a way that they resulted in minimal and maximal free (i.e., assumed bioavailable) Zn ion activities. No significant differences were found in acute Zn tolerance between the different acclimation concentrations: Average 48-h median effective concentration (EC50) values ranged from 608+/-94 to 713+/-249 microg/L of Zn2+. Also, no significant shifts in chronic tolerance were observed: Average 21-d EC50 (based on net reproductive rate) ranged from 91+/-20 to 124+/-22 microg/L of Zn2+. However, at test concentrations less than the 21-d EC50, acclimation significantly increased the reproductive capacity of the offspring produced. This indicates that metal acclimation is not necessarily accompanied by an increase in tolerance but also may manifest in other responses (e.g., reproduction rate). Organisms acclimated to a range from 6 to 22 microg/L of Zn2+ produced significantly more offspring than organisms acclimated to lower and higher Zn concentrations in test concentrations up to 50 microg/L of Zn2+. This range corresponds to a previously established optimal concentration range for D. magna. Bioconcentration factors indicated that Zn was actively regulated in the acclimation range tested.

Hyper-osmoregulatory capacity of the Chinese mitten crab (Eriocheir sinensis) exposed to cadmium; acclimation during chronic exposure

The aim of this study was to evaluate the effects of waterborne cadmium on hyper-osmoregulatory capacity of the Chinese mitten crab Eriocheir sinensis acclimated to freshwater. For this purpose, crabs were submitted to acute (0.5 mg Cd L(-1) for 1, 2 or 3 days), chronic (10 or 50 microg Cd L(-1) for 30 days) or chronic, immediately followed by acute, exposure. While no effect was observed after 1 or 2 days, hemolymph osmolality, Na(+) and Cl(-) concentrations were significantly reduced after 3 days of acute exposure. Under this latter condition, the respiratory anterior gill ultrastructure, Na(+)/K(+)-ATPase and cytochrome c oxidase activities were significantly impaired. In contrast, the osmoregulatory posterior gill was unaffected for all treatments. As a consequence, we suggest that the observed hyper-osmoregulatory capacity impairment is the result of increased dissipative flow of ions and/or water through anterior gills. In contrast to acute exposure, chronic exposure did not induce any observable effect. However, crabs submitted to a known deleterious acute condition (0.5 mg Cd L(-1) for 3 days) directly after chronic exposure to 50 microg Cd L(-1) for 30 days showed normal hyper-osmoregulatory capacity with no change in gill Na(+)/K(+)-ATPase activity, and only little disturbance of anterior gill ultrastructure. These results demonstrate that a chronic cadmium exposure can induce acclimation mechanisms related to osmoregulation in this euryhaline decapod crustacean.

Tissue-specific cadmium accumulation and metallothionein-like protein levels during acclimation process in the Chinese crab Eriocheir sinensis

Aquatic organisms chronically exposed to cadmium can increase their resistance to a subsequent elevated exposure. In order to investigate mechanisms involved in acclimation process in the Chinese crab Eriocheir sinensis, we compared Cd level as well as metallothionein-like protein (MTLP) content in different tissues after direct acute exposure (i.e. 500 microg Cd L(-1) for 3 days), and after acute following chronic (i.e. 10 or 50 microg Cd L(-1) for 30 days) exposure. Cadmium accumulation occurred in the following order: anterior gill>hepatopancreas>posterior gill>carapace>hemolymph>muscle. As high concentrations as 188 microg Cd g(-1) w.w. were reported in anterior gills and seem to reach a saturation level. In these gills, the highest MTLP induction was observed after a direct acute exposure, for which a correlation with Cd content occurred. However, the Cd-binding potential by MTLPs was exceeded for any exposure condition. In hepatopancreas, the highest Cd level was reported for crabs acclimated during 30 days to 50 microg Cd L(-1) before challenging with an acute exposure. Moreover, we showed that MTLPs were induced during the acclimation process. In this organ, MTLPs are theoretically sufficient to bind all Cd. These results suggest that during a chronic exposure to 50 microg Cd L(-1), Chinese crabs acquire the capacity to hold more cadmium in hepatopancreas where it can be sequestrated by MTLPs. On the contrary, MTLP induction seems to be a rapid response to acute exposure in anterior gill, but is not sufficient to sequester all Cd. Other sequestration and/or detoxification mechanisms must take place in anterior gill to cope with high Cd levels.

Acclimation of Daphnia magna to environmentally realistic copper concentrations

It may be hypothesised that as the bioavailable background concentration of an essential metal increases (within natural limits), the natural tolerance (to the metal) of the acclimated/adapted organisms and communities will increase. In this study the influence of acclimation to different copper concentrations on the sensitivity of the freshwater cladoceran Daphnia magna Straus was investigated. D. magna was acclimated over three generations to environmentally relevant copper concentrations ranging from 0.5 to 100 microg Cu/l (copper activity: 7.18 x 10(-15) to 3700 x 10(-12) M Cu2+). A modified standard test medium was used as culture and test medium. Medium modifications were: reduced hardness (lowered to 180 mg CaCO3/l) and addition of Aldrich humic acid at a concentration of 5 mg DOC/l (instead of EDTA). The effects of acclimation on these organisms were monitored using acute mortality assays and long-term assays in which life table parameters, copper body concentrations and energy reserves were used as test endpoints. Our results showed a two-fold increase in acute copper tolerance with increasing acclimation concentration for second and third generation organisms. Copper acclimation concentrations up to 35 microg Cu/l (80 pM Cu2+) did not affect the net reproduction and the intrinsic growth rate. The energy reserves of the acclimated daphnids revealed an Optimal Concentration range (OCEE) and concentrations between 5 and 12 microg Cu/l (0.5-4.1 pM Cu2+) and 1 and 35 microg Cu/l (0.023-80 pM Cu2+) seemed to be optimal for first and third generation daphnids, respectively. Lower and higher copper concentrations resulted in deficiency and toxicity responses. It was also demonstrated that up to 35 microg Cu/l, third generation daphnids were able to regulate their total copper body concentration. These results clearly indicate that bioavailable background copper concentrations present in culture media have to be considered in the evaluation of toxicity test results, especially when the toxicity data are used for water quality guideline derivation and/or ecological risk assessment for metals.

Alterations of physiological energetics, growth and reproduction of Daphnia magna under toxicant stress

The study investigates the relationship between changes in physiological energetics of organisms and alterations of growth, development and reproduction of Daphnia magna. Groups of primiparous daphnids were subjected to 8-day exposures to the heavy metals cadmium and copper or to the cationic surfactant, cetyltrimethylammonium bromide (CTAB). Energetic alterations were estimated from the measurement of oxygen consumption and feeding activity which was performed during the last 3 days of the exposure period and from the calculation of simplified carbon balances. The physiological effects were compared to effects on organismal growth and reproduction as obtained from 17-day exposure experiments. Toxicant exposure reduced weight and body length of daphnids indicating an impaired growth rate, but effects on total metabolic costs measured as weight-specific oxygen consumption could not be detected. Net carbon gain of individuals decreased in a concentration-dependent way for the tested chemicals reflecting effects on biomass of daphnids. In the case of cadmium and copper, reproduction ( summation operatormx: number of offspring per female of age x born during the time interval x-1 to x, summarised over the entire exposure period) and the estimate for the intrinsic rate of natural increase, derived from the 17-day exposure-experiment, were affected at concentrations comparable to the effect levels as observed for growth. In the case of copper, the concentrations affecting growth and reproduction were close to the 17-day LC(50) value. CTAB caused a reduction in body length of primiparous daphnids whereas a decrease in the reproductive performance was not apparent. In conclusion, the chemicals did not change metabolic costs of exposed daphnids as it would be expected as a consequence of resistance or repair mechanisms, however, they induced alterations of SFG, growth, reproduction and intrinsic rate of natural increase. These alterations were chemical-specific. The fact that toxicant-related effects on growth and reproduction could not be linked to an elevated metabolic rate of daphnids may indicate that demand side effects occurred early during exposure - before the start of respirometric measurements - or that effects on growth were caused by an altered energy uptake. The results illustrate the importance of trade-off processes in regulating the distribution of energy among growth and reproduction of daphnids.