Polyhydroxyalkanoates (PHA), polyesters accumulated by numerous prokaryotes in the form of intracellular granules, have been for decades considered being predominantly storage molecules. However, numerous recent discoveries revealed and emphasized their complex biological role for microbial cells. Most of all, it was repeatedly reported and confirmed that the presence of PHA granules in prokaryotic cells enhances stress resistance and robustness of microbes against various environmental stress factors such as high or low temperature, freezing, oxidative, and osmotic pressure. It seems that protective mechanisms of PHA granules are associated with their extraordinary architecture and biophysical properties as well as with the complex and deeply interconnected nature of PHA metabolism. Therefore, this review aims at describing novel and unexpected properties of PHA granules with respect to their contribution to stress tolerance of various prokaryotes including common mesophilic heterotrophic bacteria, but also extremophiles or photo-autotrophic cyanobacteria. KEY POINTS: • PHA granules present in bacterial cells reveal unique properties and functions. • PHA enhances stress robustness of bacterial cells.

ARENA Arbeitsgemeinschaft für Ressourcenschonende and Nachhaltige Technologien Inffeldgasse 21b 8010 Graz Austria

Faculty of Chemistry Brno University of Technology Purkynova 118 612 00 Brno Czech Republic

Institute of Chemistry NAWI Graz University of Graz Heinrichstrasse 28 6 8010 Graz Austria

University for Natural Resources and Life Sciences IFA Tulln Konrad Lorenz Str 20 3430 Tulln Austria

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