The microbial community spatially varies during a Microcystis bloom event in Lake Kinneret

Cyanobacterial blooms in lakes frequently display high spatial heterogeneity, which may drive temporal changes in microbial community structure. Here we report on the spatial (horizontal and vertical) structure of the cyanobacteria community and of the co-occurring heterotroph microbial community during a major lake-wide bloom event at Lake Kinneret (Sea of Galilee, Israel). We sampled the upper water layer (0, 1 and 5 m depths) at five locations across the lake, spatially representing north, south, east, west and centre of the lake, on two dates during winter 2016 when the Microcystis biomass was high. We aimed to characterise the spatial distribution of the bloom and the associated microbiota, as well as testing the extent to which variation in the structure of the microbial community could be associated with Microcystis biomass. During the winter 2016 bloom, Microcystis biomass was spatially heterogeneous, but the Microcystis population structure was rather homogenous, dominated by a single genotype (ITS sequence variant). By contrast, the composition of the bacterial community, assessed using 16S rRNA gene sequencing, was spatially heterogeneous, differing between depths, sampling locations and time. The largest difference in microbial community structure was between particle-associated (PA) and free-living (FL) fractions. The PA community was characterised by a relatively lower diversity, and dominated by Proteobacteria, whereas the FL community was significantly more diverse and dominated by members of Proteobacteria, Bacteroidetes and Actinobacteria. Variation in the microbial community was significantly correlated with geographical location and sampling depth. However, there was no clear relationship between the Microcystis biomass density (cells biovolume) and the structure of the microbial communities. These results suggest that the structure of the bacterial community is partly decoupled from Microcystis population density, and potentially more affected by other environmental factors such as temperature and/or physicochemical conditions. Our study highlights the importance of spatial information on bloom composition and intensity in clarifying the factors affecting the progression of cyanobacterial harmful algal blooms and how this impacts their microbial counterparts.