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Redox metabolism is integral to the functioning of photosynthetic organisms. The highly oxidative reactive oxygen species (ROS) are products of cellular redox processes and can cause major cellular damage. ROS are also produced in the face of environmental and cellular stress, including metal-induced oxidative stress. The degree of ROS production and the subsequent anti-oxidative response will vary according to the tolerance level of the species and the degree and type of stress it experiences. The extremophilic alga Cyanidium caldarium and neutrophilic alga Chlamydomonas reinhardtii have contrasting Zn tolerances, with the EC50 of the extremophile 128 times higher than that of the neutrophile. We examined the oxidative stress responses of these algae to zinc stress, using ROS production to establish the metal-induced oxidative state of the organisms. The anti-oxidative responses measured included total free-sulphydryl groups and total protein content as well as a heat shock protein 70 (HSP70) response assay to explore the role the stress protein HSP70 might play in the contrasting metal tolerance mechanisms in these two species. Results showed that the oxidative response in Cy. caldarium was dose-dependent, while in Ch. reinhardtii the oxidative response was more dependent on the time of exposure to Zn. Cyanidium caldarium managed to regain its base level of protein (i.e. that seen in naïve cells) upon acclimation (long-term exposure) to sub-toxic Zn levels. On the other hand, Ch. reinhardtii showed reduced total protein content upon acclimation. This raises questions about the different roles that ROS signalling might play in metal stress tolerance. [ABSTRACT FROM AUTHOR]