A hormonal network regulates growth processes and stress responses in vascular plants. There is evidence for a similar hormonal network in microalgae. This study investigated the effect of exogenous gibberellins (GAs) on Chlorella minutissima Fott et Nováková growth and biochemical composition. Two bioactive GAs i.e. GA3 and GA4 were applied at 10⁻⁸–10⁻⁵ M. Growth was monitored until cultures were harvested on day 7 when in an exponential growth phase. Primary metabolites (protein, chlorophyll and carotenoids) were quantified and endogenous GAs and phenolic acids were identified and quantified. GA3 had little beneficial effect on growth in C. minutissima while GA4 was inhibitory. GA application had little effect on the protein, chlorophyll and total carotenoid content. Analysis of the GA content suggested that GA3 was not readily taken up by the cells while GA4 was absorbed but not further metabolised. This high accumulation of GA4 could account for its inhibitory effect. Three phenolics acids were detected in C. minutissima i.e. p-hydroxybenzoic acid > salicylic acid > protocatechuic acid. Their concentrations were not affected by GA treatments or GA-type. The physiological role of GAs in microalgae is still unclear and further studies are required to gain clearer insight into uptake rates, metabolism and function.

Brassinosteroids are part of the hormonal network that regulates growth processes and stress responses in plants. There is evidence for a similar hormonal network in microalgae. In the present study, six microalgae (Chlorococcum ellipsoideum, Gyoerffyana humicola, Nautococcus mamillatus, Acutodesmus acuminatus, Protococcus viridis and Chlorella vulgaris) were subjected to salt and low temperature stress with the addition of 36 g l^–1 NaCl and transfer from 25°C to 15°C. There was a rapid response to salt stress with the brassinosteroid content (mainly castasterone with lower amounts of brassinolide, homocastasterone and typhasterol) increasing within 30 min of the salt treatment and remaining at these elevated levels after 7 h. The decrease in temperature had little effect on the brassinosteroid content. This was the first study to show that endogenous brassinosteroids increase in response to abiotic stress in a number of microalgae species.