The interplay between thiol-compounds against chromium (VI) in the freshwater green alga Monoraphidium convolutum: Toxicology, photosynthesis, and oxidative stress at a glance

• Published in: Aquatic toxicology 2012-08, Vol.118-119, p.80-87
• Main Authors: Takami, R., Almeida, J.V., Vardaris, C.V., Colepicolo, P., Barros, M.P.
• Format: Article
• Language: English
• Subjects: antioxidant activity; ascorbate peroxidase; Ascorbate Peroxidases - metabolism; Chlorophyceae; chlorophyll; Chlorophyll - metabolism; Chlorophyta - drug effects; Chlorophyta - metabolism; Chromium; Chromium - toxicity; electron transfer; Freshwater; Glutathione; Glutathione - metabolism; glutathione peroxidase; Glutathione Reductase - metabolism; glutathione-disulfide reductase; Green alga; lipid peroxidation; Monoraphidium; mortality; Oxidation-Reduction; Oxidative stress; Oxidative Stress - drug effects; Photosynthesis; Photosynthesis - drug effects; photosystem II; Photosystem II Protein Complex - metabolism; plant physiology; Sulfhydryl Compounds - metabolism; Thiobarbituric Acid Reactive Substances - metabolism; Thiols; toxicity; toxicology
• ISSN: 0166-445X; 1879-1514
• DOI: 10.1016/j.aquatox.2012.03.018

► Oxidizing Cr(VI) forms are highly toxic to freshwater green algal species. ► Cr(VI) toxicity is possibly related to redox metabolism unbalances in chloroplasts. ► Low oxidative challenges are mainly counterbalanced (not fully efficient) by small thiol peptides. ► Higher Cr(VI) doses, induces GSH defenses followed up by GR and APX enzymes. In this paper, the multifaceted Cr(VI) toxicity over the freshwater green alga Monoraphidium convolutum was assessed by concomitantly monitoring thiol-dependent redox balances, photosynthesis activity and growth-survival scores. Control group showed exponential growth rate at (5.78±0.29) division/day until 8th day with linear increasing chlorophyll a/protein ratios (CHLa/PROT) throughout the period. Cultures of M. convolutum were exposed for 5 days to Cr(VI) concentrations from 0 up to 100mg/L showing that CHLa/PROT ratios were sensibly affected, in agreement to the calculated LC50,48h (5.38±0.72)mg/L from the concentration-response curve of cell mortality after 48h. Regarding photosynthesis effects, Cr(VI) concentrations >1.0mg/L showed significant increases in short-term (after 2h) electron transfer rates (ETR) and quantum yields of photosystem II (ΦPSII), followed by subsequent decline of both parameters after 48 and 72h. Biochemical analyses showed that maximal GSH concentrations in algal cultures were observed upon 1mg Cr(VI)/L and higher dichromate concentrations dramatically increased the activity of antioxidant GSH-dependent enzymes ascorbate peroxidase and glutathione reductase. However, no variation was observed in the cellular GSH levels, whereas GSSG and lipid peroxidation indexes abruptly increased upon 10mg Cr(VI)/L exposure. Altogether, plant physiology, photosynthesis and biochemical data suggest that the GSH-dependent antioxidant system is capable to sustain M. convolutum viability through efficient photosynthesis activity and adequate antioxidant responses up to Cr(VI) concentrations of 1.0mg/L, when redox unbalances were first evidenced.