Microalgal Cell Wall Dynamics at Different Growth Stages Under Cold Stress Conditions: A Study of Ultrastructure and Chemical Composition

Microalgae have been gaining attention for biotechnological purposes in different fields, such as food, energy, cosmetics, and bioremediation. However, due to habitat variation across the planet, there is still a scarcity of information on microalgae development in extreme environments, such as those with low temperatures. The aim of this study was to investigate the effects of growth stage and cold stress (5°C) on cell wall features, including ultrastructure, and main polysaccharides in three microalgal species from Northern Sweden: Coelastrella sp. 3-4, Chlorella vulgaris sp. 13-1, and Scenedesmus sp. B2-2. All microalgal strains grown at 25°C showed increased cell wall thickness between the exponential and stationary phases by around 45%, with Scenedesmus showing an increase of up to 230%. Under cold stress, the cell wall thickness between the exponential and stationary phases had the greatest increase of 68% in Scenedesmus. However, both Chlorella vulgaris sp. 13-1 and Scenedesmus sp. B2-2 showed that cold stress stimulated the formation of a thicker cell wall during exponential growth compared to control growth conditions. The layer configuration showed a differentiation, with Chlorella presenting a bilayer cell wall and the cell walls of Coelastrella and Scenedesmus being mainly composed of several layers distinguishable by transmission electron microscopy. Cold stress altered the microalgal cell wall ultrastructure and morphology. Glycosyl-linkage analysis did not show a change in the composition of the main cell wall polysaccharides under cold stress.