Reinhard Dallinger

Contaminated food and uptake of heavy metals by fish: a review and a proposal for further research

Summary1. The uptake of heavy metals via the alimentary tract can be an important factor for the metal budget of fish. 2. Concepts such as biomagnification, bioaccumulation, biotransference, or concentration factors, convey little information about the real threat originating from heavy metals in an aquatic food chain. 3. In polluted aquatic ecosystems the transfer of metals through food chains can be high enough to bring about harmful concentrations in the tissues of fish. This relationship is called the food chain effect. 4. Two kinds of ecological factors influence the food chain effect: firstly, high levels of contamination of the food, and, secondly, the reduction of species diversity. When susceptible species are eliminated, metal-tolerant food organisms may become dominant. Their tolerance may be based either on their ability to accumulate excessive amounts of metals or to exclude heavy metals from the tissues. These two strategies represent feedback mechanisms which may enhance or weaken the food chain effect. 5. It is concluded that future investigations on transference of heavy metals to fish must take into more careful consideration the specific ecological situation of a given environment.

Metallothionein in snail Cd and Cu metabolism.

Terrestrial snails tolerate elevated concentrations of cadmium and copper, accumulating both metals in their soft tissues. The snails are able to inactivate the toxic cadmium while meeting their metabolic requirement for copper. Here we report evidence for the metabolic discrimination between the two metals based on the existence of distinct metallothionein isoforms, one dedicated to cadmium detoxification and another to copper regulation.

The importance of contaminated food for the uptake of heavy metals by rainbow trout (Salmo gairdneri): a field study

Summary1.The concentrations of Zn, Cu, Cd, Pb, Cr, Ni, and Mn have been measured in water, sediments, and organisms from two contaminated rivers situated in an industrial area and near a motorway on the southern city boundary of Bolzano, Italy.2.The low metal contents of water are in contrast to the elevated concentrations of sediments indicating a chronic rather than an acute contamination of the rivers.3.The metal concentrations of the dominant moss Fontinalis antipyretrica are higher than those of sediments, the lower branches of the plant being more concentrated than its upper parts.4.Among benthic invertebrates the dominant animals are the isopod Asellus aquaticus and the snail Lymnaea truncatula. Both species feed on contaminated moss or detritus and accumulate the heavy metals to an even greater extent than the water plants.5.The dominant predator is the rainbow trout (Salmo gairdneri) which feeds mainly on metal enriched isopods and snails.6.As a consequence, there is an absorption of heavy metals from lumen of fish into the gut tissue and a flux to other organs such as liver, kidney, and muscle.7.Since the metal concentrations of water are low it is suggested that the absorption through the gills of fish may be of secondary importance compared with the excessive supply through the food.

Patterns of accumulation, distribution and liberation of zn, cu, cd and pb in different organs of the land snail Helix pomatia L

The snail Helix pomatia was fed lettuce enriched with zinc, cadmium, lead and copper for 32 days, followed by a feeding period of 40-50 days with uncontaminated lettuce. The time-dependent distribution of the four metals during loading and unloading was determined in all major organs. Four general types of response to metal loading have been distinguished. Another general feature is the rhythmic nature of accumulation and de-accumulation of metals in several organs. On the other hand, each metal also elicits specific responses which can be described in terms of acute or delayed reaction and long-term accumulation. Lead is accumulated by most organs and redistributed after loading has stopped, but a single organ, the midgut gland, accumulates about 90% of this metal. Zinc accumulates in most organs during the first part of the loading period and is then redistributed to the midgut gland which may hold up to 70% of the metal at the end of the experiment. Cadmium is not taken up at all by a number of organs but the midgut gland and anterior gut keep accumulating it. After loading has been terminated the metal appears to move from the midgut gland to the gut. Copper is distributed more evenly than the other metals in the organs of the snail, the midgut gland not playing a dominant role in the storage of this metal.

Metallothionein research in terrestrial invertebrates: Synopsis and perspectives☆☆☆

While most of metallothionein research during the past years has been carried out on mammals or vertebrates, only relatively few studies have been directed towards invertebrates. Even fewer investigations have focussed on terrestrial invertebrates. The best studied metallothioneins and/or metallothionein genes among terrestrial invertebrates are those from an insect species (Drosophila melanogaster), a nematode (Caenorhabditis elegans) and some terrestrial gastropods (Helix pomatia, Arianta arbustorum). From these few examples it already appears that terrestrial invertebrate metallothioneins provide intriguing models to better understand the multiplicity of functions of these proteins and their evolution within the animal kingdom. Like in mammals, metallothioneins in terrestrial invertebrates seem to perform different functions simultaneously. This is exemplified by terrestrial gastropods, which are able to accumulate different metals in different tissues, in which metal-specific metallothionein isoforms or conformation forms are expressed, allowing these organisms to detoxify more efficiently nonessential trace elements such as cadmium, and at the same time to maintain the homeostasis of essential trace elements such as copper. A major proportion of metallothionein research in terrestrial invertebrates addresses the ecophysiological and ecotoxicological significance of these proteins with regard to the increasing risk due to chemical pollution. One promising aspect in this concern is the potential utilization of metallothioneins as biomarkers for risk assessment in terrestrial environments.

Invertebrate organisms as biological indicators of heavy metal pollution

Some species of invertebrate animals are known to be efficient accumulators of trace elements. Generally, metal accumulation by such organisms is based on efficient detoxification mechanisms, such as intracellular compartmentalization, or metal inactivation by binding to metallothioneins. Metal accumulators have often been used as accumulation indicators of environmental metal pollution. This means that, ideally, metal concentrations in the animal’s body reflect quantitatively or semiquantitatively environmental pollution levels. In reality, however, many factors, such as the animal’s weight and age, can disturb such quantitative relationships. These factors have, therefore, to be considered carefully before an invertebrate is utilized as accumulation indicator for metal pollution.Apart from accumulation, many invertebrates exposed to elevated metal concentrations respond to this stress by metal-induced synthesis of metallothioneins. Additionally, metallothionein in metal-loaded organisms can be present in different isoforms that are specifically synthesized in response to different metals. These facts make metallothionein a potential biomarker for metal stress in invertebrates. One possibility may be to assess parameters of metallothionein synthesis at the molecular or biochemical level. Moreover, metallothionein isoform patterns could provide information on different isoforms synthesized in response to different metals or chemicals. In any case, however, care must be taken to consider intrinsic physiological parameters, such as nutritional or developmental factors, which could also interfere with metallothionein synthesis.

The flow of copper through a terrestrial food chain

SummaryGroups of “cooper-enriched” and “copper-deficient” isopods were allowed to choose between three batches of birch litter differing in their copper contents. The relative amounts of litter ingested by the three species were the following (ranges of average contribution of each type of litter to total amount ingested): Thus the selection of food in these animals appears to be influenced by the status of the copper reserves in their bodies.

Shaping mechanisms of metal specificity in a family of metazoan metallothioneins: evolutionary differentiation of mollusc metallothioneins

BackgroundThe degree of metal binding specificity in metalloproteins such as metallothioneins (MTs) can be crucial for their functional accuracy. Unlike most other animal species, pulmonate molluscs possess homometallic MT isoforms loaded with Cu+ or Cd2+. They have, so far, been obtained as native metal-MT complexes from snail tissues, where they are involved in the metabolism of the metal ion species bound to the respective isoform. However, it has not as yet been discerned if their specific metal occupation is the result of a rigid control of metal availability, or isoform expression programming in the hosting tissues or of structural differences of the respective peptides determining the coordinative options for the different metal ions. In this study, the Roman snail (Helix pomatia) Cu-loaded and Cd-loaded isoforms (HpCuMT and HpCdMT) were used as model molecules in order to elucidate the biochemical and evolutionary mechanisms permitting pulmonate MTs to achieve specificity for their cognate metal ion.ResultsHpCuMT and HpCdMT were recombinantly synthesized in the presence of Cd2+, Zn2+ or Cu2+ and corresponding metal complexes analysed by electrospray mass spectrometry and circular dichroism (CD) and ultra violet-visible (UV-Vis) spectrophotometry. Both MT isoforms were only able to form unique, homometallic and stable complexes (Cd6-HpCdMT and Cu12-HpCuMT) with their cognate metal ions. Yeast complementation assays demonstrated that the two isoforms assumed metal-specific functions, in agreement with their binding preferences, in heterologous eukaryotic environments. In the snail organism, the functional metal specificity of HpCdMT and HpCuMT was contributed by metal-specific transcription programming and cell-specific expression. Sequence elucidation and phylogenetic analysis of MT isoforms from a number of snail species revealed that they possess an unspecific and two metal-specific MT isoforms, whose metal specificity was achieved exclusively by evolutionary modulation of non-cysteine amino acid positions.ConclusionThe Roman snail HpCdMT and HpCuMT isoforms can thus be regarded as prototypes of isoform families that evolved genuine metal-specificity within pulmonate molluscs. Diversification into these isoforms may have been initiated by gene duplication, followed by speciation and selection towards opposite needs for protecting copper-dominated metabolic pathways from nonessential cadmium. The mechanisms enabling these proteins to be metal-specific could also be relevant for other metalloproteins.

Localization and quantification of Cd- and Cu-specific metallothionein isoform mRNA in cells and organs of the terrestrial gastropod Helix pomatia.

A quantitative assay based on real-time detection polymerase chain reaction (rtdPCR) was applied to analyze basal and metal-induced mRNA levels of two metallothionein (MT) isoforms (Cd-MT and Cu-MT) in organs of the terrestrial gastropod Helix pomatia. The results show that specific Cd-MT mRNA levels increase with Cd tissue burden, identifying hepatopancreas and gut as the main organs of Cd accumulation and, accordingly, the predominant organs of Cd-MT mRNA expression. In situ hybridization localized this isoform in epithelial cells of hepatopancreas, gut, and kidney. In contrast to the observed Cd-dependent inducibility of the Cd-binding MT isoform, gene expression of the Cu-binding MT could not be induced by either Cd or Cu exposure. Only very low mRNA amounts of the Cu-MT isoform were found in snail hepatopancreas and kidney, whereas the mantle exhibited high basal mRNA levels of this isoform. In situ localization revealed that the Cu-MT gene expression was restricted to one cell type, the so-called rhogocytes, which are present to various extents in the different organs examined. These results suggest a metal-specific sharing of functions between the two MT isoforms. The Cd-MT isoform apparently plays a crucial role in Cd detoxification, as demonstrated by the inducibility of this isoform, as well as its specific localization in the main metabolic and Cd storing organs. The predominant presence of Cu-MT in rhogocytes of snail mantle strengthens the hypothesis that this isoform may regulate Cu availability in hemocyanin synthesis.

Terrestrial isopods : useful biological indicators of urban metal pollution

SummaryEnvironmental pollution by toxic metals is widespread in urban areas. In contrast to many industrialized sites, however, metal pollution in most urban regions occurs at low or moderately elevated levels. Reliable criteria of environmental quality have therefore to be established, with the consequence that there is an increasing need for sensitive monitoring of pollution. In this present study, the isopod species Porcellio scaber was used as a bioindicator of lead and cadmium pollution in Innsbruck, Austria. During May 1988, isopods were collected at 356 points over the whole city area; lead and cadmium in whole animals were measured using flame atomic absorption spectrophotometry. Lead and cadmium contamination of urban districts were correlated with traffic density, the only exception being higher cadmium contamination of one district in which several factories are situated. Concentrations of lead, but not of cadmium, increased along the main arterial roads with increasing traffic density and with decreasing distance from the city centre. On a large scale, concentrations of lead and cadmium increased in the direction of the prevailing wind, from south-west to north-east. On a small scale, the variability of metal concentrations can be explained by the irregular distribution of active business centres, by different degrees of contamination between exposed and non-exposed sites, and by contamination gradients depending on wind direction. Compared to highly polluted sites around industrial factories and metal smelting works, lead contamination in Innsbruck can be considered as moderately elevated, whereas cadmium contamination is low.