Isolation of axenic cyanobacterium and the promoting effect of associated bacterium on axenic cyanobacterium

Abstract

Background: Harmful algal blooms have attracted wide attention all over the world as they cause water quality deterioration and ecosystem health issues. Microcystis aeruginosa associated with a large number of bacteria is one of the most common and widespread bloom-forming cyanobacteria that secret toxins. These associated bacteria are considered to benefit from organic substrates released by the cyanobacterium. In order to avoid the influence of associated bacteria on the target cyanobacteria for physiological and molecular studies, it is urgent to obtain an axenic M. aeruginosa culture and further investigate the specific interaction between the cyanobacterium-associated bacterium and the cyanobacterium.Results: A traditional and reliable method based on solid-liquid alternate cultivation is carried out to purify the xenic cyanobacterium M. aeruginosa FACHB-905. On the basis of 16S rDNA gene sequences, two associated bacteria named strain B905-1 and strain B905-2, are identified as Pannonibacter sp. and Chryseobacterium sp. with a 99% and 97% similarity value, respectively. The axenic M. aeruginosa FACHB-905A is not able to form colonies on BG11 agar medium without the addition of strain B905-1, while it grows well in BG11 liquid medium. Although the presence of B905-1 is not indispensable for the growth of M. aeruginosa FACHB-905A, B905-1 has a positive effect on promoting the growth of M. aeruginosa FACHB-905A. Conclusions: The associated bacteria are eliminated by solid-liquid alternate cultivation method and the axenic M. aeruginosa FACHB-905A is successfully purified. The associated bacterium B905-1 has the potential to promote the growth of M. aeruginosa FACHB-905A. Moreover, the purification technique for cyanobacteria described in this study is potentially applicable to a wider range of unicellular cyanobacteria.