Chromium toxicity to Euglena gracilis strains depending on the physicochemical properties of the culture medium

Abstract

Chromium can be found in several oxidation states being the most stable and common forms trivalent Cr(III) and hexavalent Cr(VI), which present different chemical properties (Bagchi et al., 2002). This metal is an essential element for some organisms in relation with some metabolic pathways (Hamilton & Wetterhahn, 1987). Discharges of this metal into surface water increase its concentration in the environment due to the application of this metal in diverse industries, such as the metallurgical and tanning industries. Hexavalent chromium is an environmental contaminant whose cytotoxic effects in animals and plants are well documented (Cervantes et al., 2001; Vajpayee et al., 2001). It is considered the most toxic form of this metal, usually associates with oxygen to form chromate (CrO42-). This molecule can easily go through cell membranes, thus being an alternative substrate for the sulfate transport system (Riedel, 1985). Cr(VI) physicochemical characteristics, pH-dependent equilibrium and redox properties, as well as the thermodynamic and kinetic stability observed in various chromium oxidation states, are key elements to understand the interaction of Cr(VI) in living systems (Cieslak-Golonka, 1996). Many authors reported the great tolerance to heavy metals showed by algae cells collected from highly contaminated rivers (Rai and Rai, 1998, Devars et al., 1998). Previous works on the phytoplankton of the Matanza River, one of the most polluted rivers of Buenos Aires, Argentina, showed that the euglenoids were one of the most important groups of this community. For decades, the quality of this River has been deteriorating due to the high discharges of different pollutants derived from untreated sewage, solid wastes and petroleum. The most important industrial discharge was produced by tanning (Conforti, 1991, Conforti et al. 1995). In a previous paper (Rocchetta et al., 2003), we demonstrated the damage produced by hexavalent chromium on two strains of Euglena gracilis; UTEX 753 (from the Culture Collection of the Texas University), and MAT (isolated from the Matanza River). Both were cultured in Buetow mineral medium with a neutral pH (Buetow, 1982). Exposure to different metal concentrations showed that MAT had higher resistance than UTEX. Based on this information, we decided to compare chromium toxicity on these strains using a mineral medium but with a lower pH (Buetow, 1982) to evaluate the possible impact from some acid industrial effluents, related with the leather treatment, discharged into the River.