Insights into the role of external layers in cyanobacteria at varying temperatures and their regulatory mechanism

Abstract

The dominance of cyanobacteria and their adaptability to changing climates might be attributed to their morphological plasticity related to external layers. The content of exopolysaccharides in cyanobacterial external layers has been shown to increase at elevated temperatures. However, the answers to the questions of why and how cyanobacteria adapt to varying temperatures by adjusting the physiological and biophysical properties of their external layers are still not clear. We cultivated three cyanobacterial strains, Nostoc sp. FACHB‐2009, Dolichospermum sp. FACHB‐82 and Microcystis sp. FACHB‐3296, which are embedded in three typical types of external layers—sheath, capsule and slime, respectively––and each grown at three different temperatures (10°C, 25°C and 35°C). The results showed that varying the temperature significantly changed the physicochemical properties of the external layers. High temperature conditions promoted increases in colony sizes compared to the control, with the external layers becoming thicker, looser and more adhesive, whereas cell sizes did not significantly change. Additionally, the expression patterns of related genes varied with temperature; it represents several temp dependent genes related to synthesis of nucleotide sugars were upregulated at high temperature but downregulated at low temperature. As the external layers are wrapped outside cells, we assumed a “spherical shell heat conduction model”. Through that, we inferred that the presence of external layers influenced heat transfer in the cyanobacteria, thereby playing a role in stabilising cell activity at various temperatures. Transcriptome tests showed that the use of intracellularly stored nucleotide sugars for external layer synthesis was the preferred process for cyanobacteria at low temperature, whereas de novo external layer synthesis was the preferred process for cyanobacteria at high temperature.