The "stress' concept in microalgal biologyhomeostasis, acclimation and adaptation

The term stress' is widely used in the algal literature, usually in the context of the response of algae to changed abiotic and biotic factors. Stress' is seen as the cause of changes in algal metabolism and composition and often as a factor inducing the overproduction of particular desirable secondary metabolites. However, stress' is used differently by different authors and is often ill-defined, with no clear separation of cause and effect. This lack of a defined stress concept leads to poor experimental design, miscommunication of results and potentially erroneous conclusions. This paper reviews the stress concept as it applies to algae, especially microalgae. Here, stress is defined as the disruption of homeostasis due to a stressor and the stress response represents the changes in cell metabolism during acclimation and the restoration of homeostasis. Once homeostasis is restored the cell is no longer stressed. The stages of the stress response, i.e. alarm, regulation, acclimation and adaptation, are described. The well-studied responses of the green halophilic alga Dunaliella to changes in salinity are used as an example to illustrate the stress response and acclimation to the changed salinity.