Culture-independent and culture-dependent analyses of the bacterial community in the phycosphere of cyanobloom-forming Microcystis aeruginosa

Abstract

Confocal and scanning electron microscopic observations have previously shown the strong bacterial association of Microcystis aeruginosa cells on their surfaces. DNA-based analyses of the associated bacterial communities were carried out using two M. aeruginosa strains grown in the laboratory and eight newly collected cyanobacterial bloom samples. M. aeruginosa was the most predominant species (66–100%) within the phylum Cyanobacteria. Rhizobium, Hydrogenophaga and Brevundimonas species were commonly found, and Flavobacterium species were present in all the cyanobacterial bloom samples. In total, 396 colonies from various samples were screened, revealing that most culturable bacteria belonged to the class Alphaproteobacteria (19%) including Rhizobium, Brevundimonas, and Porphyrobacter species. The genetic variation among the M. aeruginosa strains and different habitat conditions may have led to the presence of distinct bacterial populations among the tested samples. Among all the tested seven culturable isolates, Rhizobium sp. MK23 showed the best growth-promotion effect on the axenic M. aeruginosa strains. H2O2 was observed to be produced during the growth of M. aeruginosa PCC7806 under light conditions, this strain was more resistant to H2O2 when associated with Rhizobium sp. MK23. Our data suggested that Rhizobium species along with other associated bacteria might help the growth of M. aeruginosa by decomposing H2O2 under the aerobic growing conditions.

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

“…1), it is quite di cult to eliminate the heterotrophs from xenic Microcystis 905 culture by antibiotics treatment or lysozyme treatment methods. Researches indicate the bloom forming cyanobacteria in freshwater or seawater are more often occurred in nutrient-rich environments, and the cyanobacteria are surrounded by diverse communities of heterotrophic bacteria [32][33][34]. The di culty in obtaining the axenic Microcystis 905 is probably due to the lack knowledge of heterotrophs in xenic culture.…”

“…Heterotrophs can colonize within the enclosed region or directly adhere to the surface of a cyanobacterium colony [34]. By transferring and culturing xenic culture of Arthrospira platensis in fresh sterile medium, the axenic A. platensis is obtained by the technique of single-trichome manipulation performed with a microtrowel [35].…”

“…Heterotrophs can enhance or suppress the growth of cyanobacteria, or even kill them [32,34]. To better understand the general interaction between heterotroph and cyanobacterium, the effect of the strain B905-1 on the cyanobacterium M. aeruginosa FACHB-905A is studied.…”

“…In the same way, there may be some ROS produced in BG 11 solid medium and the ROS is likely a contributing factor to the growth inhibition of Microcystis 905A. It is speculated that the heterotrophic bacterium B905-1 closely associated with cyanobacterium likely consume nutrients that released by Microcystis 905, and may also produce vitamins and other beneficial metabolites useful for cyanobacterial growth [33,34]. Nevertheless, the presence of strain B905-1 for the cyanobacterial colony formation mechanism needs to be further studied.…”

“…Up to now, most studies on the interaction between heterotrophs and cyanobacteria are performed in pure cultures [33,34,41], and the growth of the axenic cyanobacteria is almost promoted by the heterotrophs [8,33,34]. However, the interaction can be profoundly different in nature, as most microbes are not axenic but grow together in communities.…”