Microcystis is a cosmopolitan genus of cyanobacteria and has been recognized as the most dominant bloom-forming organism in eutrophicated bodies of freshwater world widely. This organism always occurs in a colony under natural conditions. It’s observed that the aggregation, enhancement and maintenance of colonial form are preconditioned in the formation, dominancy and persistence of Microcystis bloom. According to the laboratory studies, colonial Microcystis has several competitive advantages in many aspects. Recently, the Research Group of Algal Resources and Toxicology (Principal Investigator: Prof. SONG Lirong) at Institute of hydrobiology, Chinese Academy of Sciences (IHB) made new progress in the maintenance of Microcystis colonies and their competitive advantages. 

Although some studies have suggested that grazing by zooplankton, the presence of heterotrophic bacteria, and certain physical and chemical factors can induce aggregation of Microcystis, little is known on the mechanisms of maintenance of colonial form. Prof. Song’s group found that microcystins (MCs), released by Microcystis cells during their growth, can be as a signal to activate the expression of some polysaccharide biosynthesis-related genes in both toxin-producing and non-toxin-producing ones, and induce the release of exopolysaccharides, then promote the aggregation.  

More importantly, the colony size of Microcystis decreased significantly when the released extracellular MCs were persistently removed in time. This research indicated the important role of microcystins in maintaining colonial form and revealed the mechanism of the dominancy of Microcystis bloom from a new angle. The results were published online in Environmental Microbiology. GAN Nanqin and XIAO Yan contributed equally to this work. 

During the investigation upon competitive advantages of Microcystis, Prof. Song’s group found that irradiance can enhance the colony size. Two buoyant types of colonial Microcystis showed heterogeneity of buoyancy in response to light. This heterogeneity was associated with the interplay between photosynthetic activity, gas vesicles, ballast, and colony size. The increased colony size provided a higher sinking velocity for buoyant strains to escape from high irradiance. On the contrary, the increased size of negatively buoyant ones decreased their cell density so that they can float to the surface. The result illustrated the presence of heterogeneity of buoyancy may be implicated in the dominancy and persistence of Microcystis bloom in ever-changing environment. The paper was published in the recent issue of Hydrobiologia (Xiao et al., 679:297-311, 2012). 

In addition, in comparison with unicellular strains, colonial Microcystis exhibited dramatically higher inorganic carbon affinity at 25 and 35℃, but no significant differences were observed at 15℃. Moreover, carbon anhydrase (CA) activities of Microcystis increased with temperature, but no significant difference was observed between the unicellular and colonial Microcystis. The relative transcription levels of the CA genes (ecaA, ecaB, icfA) in all colonial Microcystis were significantly higher than those in unicellular Microcystis. The present results may indicate that the capacity for utilizing inorganic carbon plays a role in the persistence and succession of different Microcystis blooms. The results have been published in Hydrobiologia (Wu et al., 675:175-186, 2011). 

These works were supported by grants from “973” programme, the Key Project of Knowledge Innovation Program of Chinese Academy of Sciences, the Natural Science foundation of China-Yunnan Project and the State Key Laboratory of Freshwater Ecology and Biotechnology.  

Related Links: 

http://onlinelibrary.wiley.com/doi/10.1111/j.1462-2920.2011.02624.x/pdf 

http://www.springerlink.com/content/8530528097694045/fulltext.pdf 

http://www.springerlink.com/content/e83536631911t884/fulltext.pdf