The facultative protist Euglena gracilis, a heavy metal hyper-accumulator, was grown under photoheterotrophic and extreme conditions (acidic pH, anaerobiosis and with Cd2+) and biochemically characterized. High biomass (8.5 x 10(6) cells mL(-1)) was reached after 10 days of culture. Under anaerobiosis, photosynthetic activity built up a microaerophilic environment of 0.7% O-2, which was sufficient to allow mitochondrial respiratory activity: glutamate and malate were fully consumed, whereas 25-33% of the added glucose was consumed. In anaerobic cells, photosynthesis but not respiration was activated by Cd2+ which induced higher oxidative stress. Malondialdehyde (MDA) levels were 20 times lower in control cells under anaerobiosis than in aerobiosis, although Cd2+ induced a higher MDA production. Cd2+ stress induced increased contents of chelating thiols (cysteine, glutathione and phytochelatins) and polyphosphate. Biosorption (90%) and intracellular accumulation (30%) were the mechanisms by which anaerobic cells removed Cd2+ from medium, which was 36% higher versus aerobic cells. The present study indicated that E. gracilis has the ability to remove Cd2+ under anaerobic conditions, which might be advantageous for metal removal in sediments from polluted water bodies or bioreactors, where the O-2 concentration is particularly low. (C) 2015 Elsevier B.V. All rights reserved.