[Main reason for concentric rings plaque formation of virus infecting cyanobacteria (A-4L) in lawns of Anabaena variabilis].

Objective: To clarify an important biological characteristic of virus infecting cyanobacteria (A-4L) and to isolate, identify new bloom-forming cyanobacteria viruses, we studied A-4L concentric rings plaque formation in Anabaena sp. PCC7120.

Methods: One step growth curve was designed to estimate the latent period and burst size of A4L. The initial titer of A-4L was about 2.8 x 10(10) PFU/mL. The appropriate titer suspension of A-4L was inoculated onto the lawns of Anabaena sp. PCC 7120 which have been cultivated at different time. Pathological change of lawns was observed and recorded daily. To investigate the effect of lighting on the concentric rings plaque formation, plates were cultivated and infected under continuous lighting (L: D = 24 h: 0 h), periodic lighting (L:D = 14 h:10 h) or 3 days continuous lighting after periodic lighting for 3 days. The ultra-morphology of purified A-4L was observed by negative staining electron microscopy.

Results: The latent period of A-4 (L) was 0.5 h-2 h and the burst size was about 247 infectious units per cell. Under periodic lighting, concentric rings plaques were observed in the plate after infection 3 days to 4 days and the distance between two rings was about 3 mm. Statistic analysis showed that there was a correlation between the number of concentric rings in plaques and infection days, which was "n -1". Compared with the periodic lighting, the plaques without concentric rings were observed under continuous lighting. However, the concentric rings formed under periodic lighting disappeared gradually after turning to continuous lighting, which demonstrated that the formation of concentric rings plaques depended on the periodic lighting. Negative staining electron microscopy showed that the A-4L particle had a spheroidal head with diameter about 50 nm and a tail with length about 10 nm which was similar to the characteristic morphology of cyanobacterial podoviruses.

Conclusion: A-4L is a virus infecting cyanobacteria which can form concentric rings plaque. And periodic lighting is the key conditions for the concentric rings plaque formation of A4L.