Aluminium bioaccumulation in the brown trout Salmo truffa fario from acidified streams: a structural, ultrastructural and microanalytical study
The major harm caused by acidic precipitation is shown by a disappearance of fish. Other factors besides acidity such as aluminium levels are significantly harmful and many studies have shown that aluminium ions are toxic to fish. The only sensible course of action is to investigate the basic mechanisms by which each of the metal pollutants enters and attacks living systems. For this, one needs to be a combination of physicist, chemist, biochemist, physiologist and toxicologist. Investigations on metal bioaccumulation require very sensitive analytical instrumentation. Total analytical methods commonly used are inadequate : absorbed and adsorbed elements cannot be distinguished. Therefore, interesting information can be obtained by using physical methods of chemical microanalysis : two available microanalytical techniques are particularly suitable, X-ray spectrometry and secondary ion mass spectrometry. X-ray spectrometry, also called Electron Probe X-ray Microanalysis or Electron Microprobe (EMP) provides a means for studying the local chemical composition and structure of biological specimens. EMP can be used in association with a photon microscope or with a transmission electron microscope allowing the detection of elements at subcellular level. The Secondary Ion Mass Spectrometry (SIMS), also called Ion Microscopy, allows to visualize, analyse and photograph the microscopical distribution of the stable or radioactive isotopes of the elements present in a histological section. The sensitivity of the method is very high, ranging from 0.1 to 1 ppm. In association with SIMS, a processing of secondary ion images is used. Two years-old samples of Salmo trutta fario, from wild populations living in acidified waters of Eastern France (near Cornimont in the Vosges moutains) were studied for aluminium detection at cellular and subcellular levels. The acidified waters were characterized by a low pH (5,42) and high aluminium level reaching 200 μg/L-1. Control trouts living in non acidified waters (pH : 7.9 and aluminium free) of central France (near Clermont-Ferrand) were used for comparison. In order top determine the tissues, cells and organelles of a possible aluminium concentration, the following organs were investigated : kidney, liver, gill and digestive tractus, using both microanalytical techniques described above. In the kidney ion images showed aluminium emission from tubule lysosomes with a ring-shaped localization along the apical border of the epithetial cells; aluminium emissions from the tubule lumen and from the pigment granules of the melanocytes were also observed. Using the electron microprobe, X-ray emission spectra of aluminium associated with phosphorus were obtained from lysosomes and pigment granules. In the liver and in the gills, ion images showed a high aluminium emission from the same organelles and X-ray spectra of aluminium and phosphorus were also obtained. Moreover, in the pyloric caeca, large extracellular deposits of aluminium were detected : they measured about 100 μm in length and were located in places where tissues had been destroyed. The same structural, ultrastructural and microanalytical investigations were performed on the control trouts from which non aluminium detection was obtained. In conclusion, two processes appear to be involved in the aluminium accumulation in the brown trout. The first one corresponds to a well known insolubilisation of aluminium phosphate inside the lysosomes, due to an acidic phosphatase enzymatic activity; aluminium is also trapped inside pigment granules. Both of these mechanisms of storage inside membrane-limited organelles, prevent cells from any interior damage. The second one corresponds to the formation of large extracellular deposits which are likely to provoke injuries leading to the tissue destruction. Such data demonstrating basic mechanisms of aluminium accumulation in a fish, could not have been obtained using total analytical methods.