Biochemical and ultrastructural aspects of chromium toxicity and resistance in two contrasting strains of Euglena gracilis.

IARA ROCCHETTA

Universidad de Konstanz

Conferencia; Biochemie Aktuell; 2005

Universidad de Konstanz

Euglena gracilis is an excellent organism for toxicity studies because of its easy growth in vitro and particular characteristics. Oxidative stress happens when the antioxidant defenses are not enough to counteract the effect of a toxic compound. Hexavalent chromium is an environmental pollutant which cytotoxic effects in plants and animals are well documented by epidemiological and experimental studies. Previous works showed a higher resistance in heavy metal treatments using a strain of Euglena gracilis (strain MAT) isolated from a highly polluted river (River Matanza, Bs.As.) in compared with one from a Culture Collection (UTEX 753). Based on this information, it was decided to study in more details the mechanism of the hexavalent chromium toxicity, its relation with oxidative stress and clarify the differences found in both strains. Toxicity studies were carried out, with both strains, growth in autotrophic and heterotrophic conditions, in presence of different chromium concentrations. The effect of hexavalent chromium was evaluated on the antioxidant defenses, reactive oxygen species (ROS), lipid peroxidation and the fatty acid composition. Photosynthetic and respiration activities were measured, and ultrastructural changes were observed by electronic microscopy with X microanalysis. Despite of the major antioxidant enzyme activities, the lipid peroxidation values were too high. Besides, the respiration and the photosynthesis were damaged by a high ROS amount. It was detected an increase in the phospholipase C (PLC) activity with the treatment related with an activation of a cell signal pathway. Ultrastructural studies showed a nucleus fragmentation, an increase in the number of vacuoles with dense inclusions and a great damage in chloroplasts. These changes were correlated with the fatty acid variation, specially, those related with the photosynthetic structures (linolenic acid). We can conclude that hexavalent chromium produce oxidative stress in E. gracilis, independently of the growth condition. Metabolic changes were observed depending on the culture conditions. The most important differences between both strains, MAT and UTEX, were that one isolated from a highly polluted river showed a more efficient antioxidant defense mechanism, being more remarkable between other proteins, the superoxide dismutase (SOD) activity. X ray analysis showed some elements presence in MAT strain suggesting the use of phosphates as ion metal quencher and the metallo-proteins production as a part of its defense mechanism.