Use of an aluminium-26 tracer to study the deposition of aluminium species on fish gills following mixing of limed and acidic waters

Assessment of Heavy Metals in Water, Fish and Sediments of the Baleh River, Sarawak, Malaysia

This paper reports the heavy metals content in water, sediments and in fish of the Baleh River, in order to evaluate the contamination status of metals. Water and sediment samples were obtained from seven stations located upstream of the Baleh River. Fish species were collected using netting and three species were recorded. Samples were digested and subjected to metal analysis. The findings indicate that there is low risk of metal contamination in water and sediments. Nonetheless some stations were characterized by higher levels of Fe, Al and Mn in water, likely due to accelerated sediment runoff. The acid extractable metal content reported in this study represents the dissolved and weakly-sorbed metals on particulates. For fish, accumulation of heavy metals was found to be more pronounced in the gills, compared to dorsal and ventral muscles. The concentration of Fe, Mn and Al were particularly high in the gills, as these elements were abundant in water. Hg on the other hand was consistently higher in dorsal and ventral muscles. Generally, concentrations of heavy metals in fishes of the Baleh were well within the permissible limit of Food and Agriculture Organization.Keywords: contamination status, metal analysis, Fe, Mn, Al

The high tolerance to aluminium in crucian carp (Carassius carassius) is associated with its ability to avoid hypoxia

It is well known that aluminium is the principle toxicant killing fish in acidified freshwater systems, and it has been shown that crucian carp (Carassius carassius) can survive exposures to aqueous aluminium levels toxic to most other freshwater fish species. The crucian carp has a remarkable ability to survive anoxic conditions, and the aim of the present study was to reveal if the tolerance to aluminium can be associated with the ability to survive prolonged anoxia. Crucian carps were exposed to either acidic Al-rich water (pH 5.8; 960 μg Al/l), acidic Al-poor water (pH 5.8; 50 μg Al/l) or untreated control water (pH 6.5; 50 μg Al/l). Blood, muscle and gill samples were collected from exposed fish, and closed respirometry was performed to measure critical O₂-tension an normoxic O₂-consumption. The results show an increased gill surface area in Al-exposed fish, while the critical O₂-tension did not change. The normoxic O₂-consumption was lower in Al-exposed fish and might be due to a reduced metabolic rate. The results suggest that crucian carp exposed to aluminium do not become hypoxic, since haematocrit, plasma lactate and blood ethanol did not differ from that of control fish after 14 days of exposure. We also observed an initial loss of plasma chloride and sodium, followed by a stabilisation of these ions at a lower level than in control fish. The decrease in plasma ions caused a transient increase in haematocrit and water content in muscle tissue, returning to control levels when the ion concentrations stabilised, suggesting that the water balance was restored. We conclude that the high tolerance to aluminium in crucian carp is associated with its ability to avoid hypoxia as well as an ability to counteract a continuous loss of plasma ions.

Mobilization of river transported colloidal aluminium upon mixing with seawater and subsequent deposition in fish gills

During flooding events, increased concentrations of gill-reactive aluminium (Al) have been identified in estuarine water causing high Al accumulation in fish gills. By in situ filtering and ultrafiltering river water (pH 5.5-6.4, 3-5 mg L(-1) DOC) and continually mixing the size fractioned river water with seawater (30 in salinity), Al mobilization was studied in flow-through tank systems (6 in salinity) during a six-week field experiment in Western Norway. Atlantic salmon (Salmo salar L.) kept in tanks were continuously exposed to the mixtures. Based on in situ Al fractionation of the experimental waters and sampling of gills from exposed fish, results showed that Al associated with river transported colloids, probably organic material (<or=0.45 microm and >8 kDa) was mobilized to low molecular mass cationic Al-species (LMM Al(i)) upon contact with seawater. Mobilized Al(i)-species deposited immediately on fish gills. During high flow conditions with increased amounts of colloidal material, the concentration of mobilized LMM Al(i) and the Al accumulation in gills of fish exposed to river water-seawater mixtures increased by a factor of 5 and 10, respectively, compared to the input river water. The concentration of mobilized LMM Al(i) and the Al accumulation in fish gills decreased with time after mixing (from 1 to 30 min) and as high runoff subsided.

Sodium silicate as alternative to liming-reduced aluminium toxicity for Atlantic salmon (Salmo salar L.) in unstable mixing zones

When acid aluminium (Al) rich water is limed, unstable mixing zones are formed until equilibrium is reached. In such mixing zones transient high molecular mass positively charged Al-species (HMM Al(i)) being extremely gill reactive are produced, causing toxic effects in fish. The transient HMM Al(i)-species are formed due to hydrolysis and polymerization of low molecular positively charged Al-species (LMM Al(i)), e.g. initiated by liming and the subsequent increase in pH. To counteract the toxicity of transient Al polymers in such mixing zones, sodium silicate, forming non-toxic hydroxyaluminosilicate (HAS) complexes, can be used as alternative to liming. In the present work the effect of sodium silicate on polymerization of LMM Al(i) in unstable mixing zones and subsequent gill reactivity and mortality of fish was compared to results obtained from liming. Diluted sodium silicate (<1.5 g l(-1)) and lime slurry (Ca(OH)(2)), respectively, were continually added to acidified Al-rich water in six different channel-tank systems, to obtain mixing zones with pH 5.9, 6.0 and 6.4, respectively. Utilising in situ size and charge fractionation techniques and following the exposure of Atlantic presmolt (Salmo salar L.) kept in cages at defined stations along the channel-tank systems, changes of Al-species in the mixing zones, the gill reactivity of Al-species and thus Al toxicity could be followed downstream the confluences (time of reaction after mixing: 1-100 min). By increasing the pH of the acid water to 6.0 or 6.4 by sodium silicate, the detoxification of Al was faster than using lime. Using sodium silicate, the transformation of LMM Al(i), the formation of HMM Al(i), the Al deposition in fish gills and fish mortality were lower than using lime. The formation of neutral LMM Al-species (Al(o)) was, however, higher and the formation of colloidal Al-species (Al(c)) lower in the presence of silicate compared to lime. Furthermore, the Al deposition in fish gills and fish mortality decreased by increasing concentration of sodium silicate dosed. Thus, sodium silicate is a good alternative to liming, and under certain circumstances when aging of water may represent a problem (e.g. aquaculture) sodium silicate should be the preferred agent.

Gill reactivity of aluminium-species following liming

In acidified aluminium (Al) rich freshwater positively charged Al-species (Al(i)) are the key toxic components due to the accumulation in fish gills. As a countermeasure, liming is used to increase the pH and reduce the concentration of Al(i)-species; in particular low molecular mass (LMM) Al(i)-species by hydrolyses. However, very toxic high molecular mass (HMM) Al polymers can form in the unstable mixing zone immediately after liming. In the present work gill reactivity of LMM and HMM Al-species was studied under controlled conditions in eight channel-tank mixing zone systems in the field where Atlantic salmon (Salmo salar L.) kept in cages were exposed to defined mixing zone water. Mixing zones were created by continually liming acid river water (pH 5.0-5.7) high in LMM Al(i) to pH 6.0 and 6.4, respectively. Transformation processes affecting the Al-speciation as a function of time after liming were documented by in situ hollow fibre ultrafiltration interfaced with ion chromatography, while the Al accumulation in fish gills was used as bioindicator. For fish exposed to mixing zone water immediately after liming (1 min) the Al accumulation in gills (mug Al g(-1)) was higher (factor of 2) than for fish exposed to acid water prior to liming, due to the formation of gill reactive HMM Al(i)-species. The Al accumulation in gills followed a first order kinetic expression reaching steady-state conditions after 24-h exposures. The deposition rate of Al in gills (mug Al g(-1) h(-1)) correlated with the water concentrations of HMM Al(i) (R(2)=0.80) in the mixing zones, and for LMM Al(i) in the acid water (R(2)=0.92). Due to the transient nature of HMM Al(i) the deposition rate of Al decreased from the point of liming with a factor of 10 downstream the channel-tank system (i.e. 100 min after liming). The concentration of gill accumulated Al was higher (factor of 3) immediately after high level liming (pH 6.4) than following low level liming (pH 6.0). However, high level liming was more efficient in detoxifying Al in downstream waters. Furthermore, the bioavailability of a given LMM Al(i) concentration as well as bioreactivity following liming was dependent on the TOC and silicon concentration in the acid water. Increased concentration of TOC (1.5 to 4.3 mg l(-1)) and silicon (0.3 to 1.0 mg l(-1)) reduced the gill reactivity of Al(i) by approximately 50%.

Interaction of mucus with freshly neutralised aluminium in freshwater

This study examined the interaction of mollusc trail mucus, and its biochemical constituents, with environmentally relevant concentrations of freshly neutralised aluminium (Al) in freshwater. Upon neutralisation Al starts to polymerise. In the presence of mucus the metal is rapidly localised into the hydrated mucus gel resulting in a likely reduction of its overall degree of polymerisation. A simple Al binding assay identified large-M(r) glycoconjugates as major Al-complexing molecules in mucus. Subsequent isolation and purification of these mucus glycoconjugates showed the metal readily bound to the carbohydrate portion and, in particular, to acidic components such as those containing carboxyl functionality. It is suggested gel-forming extracellular glycoconjugates play a crucial role in preventing the diffusion of Al into biological systems and thus serve to maintain metabolic homeostasis.

Chemical speciation dynamics and toxicity assessment in aquatic systems

The key to risk assessment of contaminant effects in the environment (water, sediments, soil) is the ability to document cause-and-effect relationships. In ecotoxicological research, biotic responses are related to quantified contaminant concentrations, which in most cases are still expressed in terms of "total elemental concentration" and not in terms of "elemental species." However, it becomes evident that the abundance and distribution of pollutants in the environment, their bioavailability, and their toxicity to aquatic and terrestrial organisms (including humans) can often be better understood in terms of "elemental species." The persistence, mobility, chemical reactivity, sorption dynamics, and so on of contaminants in soil and water are governed by a range of changing physicochemical parameters (pH, temperature, organic matter, suspended solids, etc.), which finally dictate the effects at the organism level. Examples are given to demonstrate that knowledge of the nature and concentration of elemental species of pollutants is crucial in assessing the impact of contaminants on aquatic ecosystems. The experimental approach to evaluate chemical speciation dynamics in relation to toxic effects is illustrated in a case study on the acute toxicity of aluminum in mixing zones at the confluence of rivers with different pH values. This study under nonequilibrium ecosystem conditions has provided new insights into the mechanism of toxicity of aluminum to freshwater organisms. In conclusion, an integrative approach by environmental chemists and ecotoxicologists is recommended to evaluate environmental pollution. Studies on the assessment of the impact of changing physicochemical parameters on the transformation kinetics and chemical speciation of pollutants, which finally determine toxicity and bioconcentration in organisms, deserve more attention in environmental toxicology.

Determination of aluminium-26 using a low-level liquid scintillation spectrometer

The lack of a suitable tracer has severely limited the use of radioisotope tracer experiments in environmental and biological studies on aluminium. The only aluminium radioisotope with a half-life greater than 10 min is the long-lived positron emitter 26Al (t1/2 7.2 x 10(5) years). The exposure of this nuclide and the difficulty in attaining sufficiently low determination limits with standard radioanalytical techniques have restricted its application as a radioactive tracer. The development of a simple method for the routine determination of 26Al using Cerenkov counting and a Quantalus 1220 low-level liquid scintillation spectrometer is reported. For a 300 min counting time, determination limits of about 60 mBq in water samples and 120 mBq in biological (fish gill) samples can be attained. With these levels of determination, the use of 26Al as a tracer is a viable technique for the study of aluminium in environmental and biological systems.