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Freshwater cyanobacterial blooms have drawn public attention because they threaten the safety of water resources and human health worldwide. Heavy cyanobacterial blooms outbreak in Lake Taihu in summer annually and vanish in other months. To find out the factors impacting the cyanobacterial blooms, the present study measured the physicochemical parameters of water and investigated the composition of microbial community using the 16S rRNA gene and internal transcribed spacer amplicon sequencing in the months with or without bloom. The most interesting finding is that two major cyanobacteria, Planktothrix and Microcystis, dramatically alternated during a cyanobacterial bloom in 2016, which is less mentioned in previous studies. When the temperature of the water began increasing in July, Planktothrix appeared first and showed as a superior competitor for M. aeruginosa in NO 3 -rich conditions. Microcystis became the dominant genus when the water temperature increased further in August. Laboratory experiments confirmed the influence of temperature and the total dissolved nitrogen (TDN) form on the growth of Planktothrix and Microcystis in a co-culture system. Besides, species interactions between cyanobacteria and non-cyanobacterial microorganisms, especially the prokaryotes, also played a key role in the alteration of Planktothrix and Microcystis. The present study exhibited the alteration of two dominant cyanobacteria in the different bloom periods caused by the temperature, TDN forms as well as the species interactions. These results helped the better understanding of cyanobacterial blooms and the factors which contribute to them.- -rich conditions. Microcystis became the dominant genus when the water temperature increased further in August. Laboratory experiments confirmed the influence of temperature and the total dissolved nitrogen (TDN) form on the growth of Planktothrix and Microcystis in a co-culture system. Besides, species interactions between cyanobacteria and non-cyanobacterial microorganisms, especially the prokaryotes, also played a key role in the alteration of Planktothrix and Microcystis. The present study exhibited the alteration of two dominant cyanobacteria in the different bloom periods caused by the temperature, TDN forms as well as the species interactions. These results helped the better understanding of cyanobacterial blooms and the factors which contribute to them., (Copyright © 2020. Published by Elsevier B.V.)

Cyanobacterial blooms frequently occur in Lake Taihu (China), but the intertwined relationships between biotic and abiotic factors modulating the frequency and duration of the blooms remain enigmatic. To better understand the relationships between the key abiotic and biotic factors and cyanobacterial blooms, we measured the abundance and diversity of prokaryotic organisms by high-throughput sequencing, the abundance of key genes involved in microcystin production and nitrogen fixation or loss as well as several physicochemical parameters at several stations in Lake Taihu during a cyanobacterial bloom of Microcystis sp.. Measurements of the copy number of denitrification-related genes and 16S rRNA analyses show that denitrification potential and denitrifying bacteria abundance increased in concert with non-diazotrophic cyanobacteria (Microcystis sp.), suggesting limited competition between cyanobacteria and heterotrophic denitrifiers for nutrients, although potential bacteria-mediated N loss may hamper Microcystis growth. The present study provides insight into the importance of different abiotic and biotic factors in controlling cyanobacteria and heterotrophic bacteria spatial variability in Lake Taihu.

Physico-chemical parameters, hydrological conditions, and microbial interactions can affect the growth and persistence of cyanobacteria, but the interacting effects among these bloom-forming factors are still poorly known. This hampers our capacity to predict the occurrence of cyanobacterial bloom accurately. Here, we studied the relationship between temperature, N and P cycles, and the microbial community abundance and diversity at 0.5 m under the surface of West Lake (China) from January 21 to November 20, 2015, in order to better understand the key factors regulating temporal changes in the cyanobacterial community. Using high throughput sequencing of the 16S rRNA gene V3-V4 region, we studied the diversity and abundance of bacteria. In parallel, we measured physico-chemical parameters and followed the abundance of key genes involved in N fixation, denitrification, and nutrient uptake. Multivariate analyses suggest that P concentration and water temperature are the key factors controlling the outbreak of summer cyanobacterial bloom. RT-qPCR analyses of the bacterial community and measurements of the copy number of denitrification-related gene (nirK, nosZ, nirS) show that denitrification potential and denitrifying bacteria relative abundance (Pseudomonas and Bacillus) increased in concert with diazotrophic cyanobacterial genera (Anabaena, Nostoc, Aphanizomenon flos-aquae) and the common bloom-forming non-diazotrophic cyanobacterium genus Microcystis. The present study brings new insights on the complex interplay between physico-chemical parameters, heterotrophic bacterial community composition, nitrogen cycle, and cyanobacteria dominance in a eutrophic lake.