Abstract
The cyanobacterium Microcystis occurs as colonies of different sizes with varying abundance of toxic genotypes versus non-toxic genotypes under natural conditions. To investigate the effects of toxic Microcystis genotypes on natural colony formation, samples collected from the mainstream of Haihe River from July to October 2015 were sieved into four colony classes with sizes of <8 μm, 8-20 μm, 20-90 μm and >90 μm. Each colony size class was analyzed for the proportion of toxic Microcystis genotypes, and microcystins (MCs) cellular production of toxic genotypes. The results showed the smallest size class of Microcystis colonies (<8 μm) showed the lowest proportion of toxic genotypes and the highest MC-RR and MC-YR cellular production. With the increasing colony sizes, the proportion of toxic Microcystis genotypes increased but the MC-RR and MC-YR cellular production decreased. A negative correlation between the MCs cellular production and the proportion of toxic genotypes was observed in all four colony size classes, suggesting that the less there were toxic Microcystis cells able to produce MCs, the more each toxic cell needed to produce that molecule. Toxic Microcystis played an important role in the colony formation in natural waters via producing MCs.
Highlights
Microcystis is a globally distributed bloom-forming genus of cyanobacteria (Haande et al ; Shen & Song )
All four colony size classes were found throughout the study period
Phytoplankton identification of water samples revealed that two Microcystis morphospecies (M. aeruginosa and M. novacekii) (Figure 2) dominated the phytoplankton biovolume from July to October in the mainstream of Haihe River (Table 1)
Summary
Microcystis is a globally distributed bloom-forming genus of cyanobacteria (Haande et al ; Shen & Song ). When Microcystis colonies isolated from the natural waters are cultured in the laboratory, their colonial This morphological variation is apparently related to the lack of some environmental factors that stimulate colony formation in laboratory cultures (Li & Dai ), and these environmental factors may be contributing to colony formation in the natural conditions. Various biotic and abiotic factors, such as zooplankton grazing pressure (Yang et al ), heterotrophic bacteria co-cultivation (Shen et al ), fluid motion (Li et al a), ultraviolet radiation (Quesada & Vincent ), high calcium levels (Wang et al ), elevated heavy metal concentrations (Bi et al ), low temperature (Trainor ) and illumination (Li et al b), have been identified to be stimulators of Microcystis colony formation in laboratory experiments
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