An assessment of the environmental toxicity of hexavalent chromium in fish

Abstract

At present chromium is a common contaminant in surface waters in many countries. In water the metal may be present in the trivalent form (CrIII) or in the hexavalent form (CrVI), the latter of which is more toxic to aquatic organisms.The investigations presented in this thesis were aimed at a thorough understanding of the mechanism of action of hexavalent chromium in fish. The process of uptake and elimination was studied in detail, while special attention was paid to physiological and histological alterations underlying the toxic effects. Water pH was taken into consideration as a varying environmental factor, because of its known influence on the oxidizing action of Cr(VI) and on the distribution of the ionic species of Cr(VI) in water. Rainbow trout (Salmo gairdneri) was used as the test-species.Chapter 1 describes the effect of pH on uptake, tissue distribution and retention of Cr(VI) in trout. In trout exposed to Cr(VI) at pH 7.8 for 2-4 days the highest contents of chromium were found in gill, liver, kidney and digestive tract. Upon transfer of exposed fish to tap-water, chromium disappeared rapidly from blood, gill and digestive tract, whereas chromium contents in liver tended to remain high and in kidney even tended to increase. When the pH was decreased from 7.8 to 6.5, a different pattern of accumulation and elimination of chromium was observed. The major differences were found in the gills which concentrated significantly more chromium at pH 6.5 than at pH 7.8, irrespective of the exposure time and concentration. As the intensity of the electron -spin - resonance signal characteristic for trivalent or pentavalent chromium in the gills was somewhat higher at pH 6.5 than at pH 7.8, the differences must have been at least partly due to the higher oxidizing action of Cr(VI) at the lower pH.Chapter 2 deals with the effect of pH on the acute toxicity of Cr(VI) to trout. The lethal action of the metal increased with decreasing pH in the range from 7.8 to 6.5. Morphological changes that could be associated with acute Cr(VI) poisoning at pH 7.8 were found in gills, kidney and stomach, whereas those at pH 6.5 appeared to be restricted to the gills. These findings are in accordance with the results obtained in the Cr- uptake experiments. Consequently, the general assumption that Cr(VI) elicits its toxic effect in some internal organ and that the gill is not the target organ in acute Cr(VI) toxicity, is only appropriate at relatively high pH levels. At pH 6.5 the gills seems to be the primary target organ.To explain the acute toxic effects, hydrochromate (HCrO 4- and chromate (CrO 42- ) were considered as the toxic species of Cr(VI) The relative toxicities of these ionic species were calculated from empirical toxicity relationships for weak acids in fish, as described in the literature. By this calculation it was found that the relative toxicity of HCrO 4- to trout was from 3.6 to 11.9 times greater than that of CrO 42- ) depending on exposure time and fish weight.Chapter 3 reports on an in vitro study on transfer of oxygen and chromium in gills of trout. Gills were perfused according to the isolated head perfusion technique and externally exposed to Na 2 CrO 4 solutions containing 51 CrO 42- . The results show that the transfer of chromium is directly coupled with the transfer of oxygen from the external solution to the internal perfusion medium. Under similar conditions of oxygen transfer, however, chromium transfer was significantly more effective at pH 6.5 than at pH 8.1.The data suggest that chromium is taken up by the blood by passive diffusion from the external solution across the epithelium of the secondary lamellae. In addition it is indicated that the availability of the metal to the fish increases with decreasing pH.Gill preparations of trout that were structurally damaged by pre-exposure in vivo to Cr(VI) exhibited an impaired oxygen transfer.Chapter 4 describes the effect of Cr(VI) on respiration and osmoregulation in trout. Recordings were made of the ventilation frequency, coughing rate and rate of oxygen uptake in trout subjected to sublethal concentrations of Cr(VI) for 4 days at pH 7.8 and 6.5.During exposure no significant effect of chromium on oxygen uptake rate was detected. The ventilation frequency and coughing rate increased proportionally to an increase in metal concentration, with fish being more susceptible at the lower pH.Alterations in blood and plasma variables determined after exposure indicated a significant dose-dependent decrease in plasma osmolality and electrolyte concentrations, and an increase in hemoglobin, hematocrit, plasma glucose and lactate levels. The pattern of these changes was dependent on pH and exposure concentration, and seemed to be related with chromiuminduced histological alterations. The results indicate, that at pH 7.8 as well as at pH 6.5 both an osmoregulatory and respiratory dysfunction are part of the physiological mechanism of hexavalent chromium toxicity.In chapter 5 toxic effects of Cr(VI) are described in trout after prolonged exposure at different pH values for periods of up to 32 weeks. Different developmental stages were studied.In all stages tested, fish were more susceptible to Cr(VI) at pH 6.5 than at pH 7.8. Fish in the embryo-through-juvenile stage were the most susceptible to Cr(VI) For this developmental stage it can be estimated that concentrations affecting survival lie between 0.2 and 2.0 mg/I Cr(VI) at pH 7.8 and between 0.02 and 0.2 mg/l Cr(VI) at pH 6.5.Finally, the observed effects of a prolonged exposure are discussed in relation with the results of the short-term toxicity tests described in the preceeding chapters.The toxicological evaluation of chromium in surface waters in the Netherlands is hampered by a lack of information on the proportion of trivalent and hexavalent forms of chromium in the total chromium content. Until now only total chromium contents have been determined at various sampling locations. If one compares the total chromium levels in Dutch surface water with the lowest toxic concentrations of Cr(VI) in trout, it can not be excluded that chromium concentrations may locally exceed toxic levels for fish. However, a final conclusion can only be drawn when additional information on the chemical status of the metal in the aquatic environment in the Netherlands has been made available.<p/