How cyanobacterial cells overwinter is crucial in understanding the formation of water bloom. The bloom-forming cyanobacteria, such as Microcystis sp., overwinter in the form of vegetative cells and do not form akinetes. Molecular mechanism for overwintering of cyanobacteria as this kind is one of the difficulties in the biological study of cyanobacteria.

In recent years, researchers from the Research Group of Algal Genetics and Biotechnology (Principal Investigator: Prof. XU Xudong) at Institute of Hydrobiology, Chinese Academy of Sciences (IHB) found that cyanobacteria was unable to survive the chill (5℃)-light stress without preconditioning at low temperature, yet could acquire chill-light tolerance after preconditioning at 15℃ for two or more days and reinitiate growth under suitable conditions (Journal of Bacteriology, 190: 1554-1560, 2008). Under natural conditions, exposure of cyanobacteria to suboptimal growth temperatures in late autumn and early winter may greatly enhance their ability to survive the chill-light stress in shallow areas of lake in winter. What effect does preconditioning at a low temperature have on cyanobacteria so that they acquire the chill-light tolerance? This is the core issue that needs to be clarified so as to understand the molecular mechanism for overwintering of cyanobacteria. Recently, Prof. Xu’s group revealed that a RNA binding protein, named Rbp1, was shown to be accumulated during preconditioning. This might be directly involved in the development of chill-light tolerance in cyanobacteria. The paper was published in the recent issue of Journal of Bacteriology (Journal of Bacteriology, 193: 2675-2683, 2011).  

There are different RNA binding proteins (Rbp) in cyanobacterial cells. Rbp1 was found to be gradually accumulated during preconditioning at a low temperature, and the accumulation of Rbp1 was correlated with the increase of chill-light tolerance. Rbp1 accumulated during preconditioning was maintained at a relatively high level after exposure to chill-light stress. However, without preconditioning, Rbp1 almost disappeared after a transient accumulation when exposed to chill-light stress. Prof. Xu’s group constructed an rbp1 mutant by inserting a chloramphenicol/erythromycin resistance cassette（C.CE2）into its coding sequence. They found such mutation caused the cyanobacterium to lose its ability of both living at 15℃ and acquiring the chill-light tolerance after preconditioning. On the contrary, ectopic expression of rbp1 enabled the cyanobacterium to survive the chill-light stress without preconditioning. RNA binding protein is generally thought to regulate some life process of cells by controlling mRNA abundance. Employing microarrays to analyze changes of mRNA expression profile in the wild type cyanobacterium, the rbp1mutant and the rbp1-overexpressing strain, Prof. Xu’s group found that the enhancement of chill-light tolerance by Rbp1 may not be due to its influence on mRNA abundance of certain genes. They also tested the Rbp1 level in Microcystis cells collected from Meiliang Bay of Lake Taihu, a large shallow eutrophic lake in eastern China, finding that from September, with the downshift of temperature, Rbp1 was accumulated in Microcystis. Cyanobacterial cells in the lake kept a high level of Rbp1 over the winter until April and May, when the temperature allowed them to reinitiate growth. With the increase of temperature, the abundance of Rbp1 or its homologues was slightly reduced in April and May and greatly reduced from June to August. 

Unlike cyanobacteria in the shallow areas of lake, benthic cyanobacteria on the surface of mud in the deep areas might face shortage of irradiation when recruited in spring. The previous research of Prof. Xu’s group revealed that reutilization of peptidoglycan degradation products may promote growth under light-limiting conditions. (Journal of Bacteriology, 192: 2239-2245, 2010).  

This research was supported by the National Natural Science Funds for Distinguished Young Scholar and the State Key Basic Research Development Program of China.