Cyanobacterial blooms specifically alter the dispersal-mediated taxonomic and functional vertical similarity of microbial communities in a subtropical reservoir

Abstract

Harmful cyanobacterial blooms, including Raphidiopsis raciborskii (basionym Cylindrospermopsis raciborskii), are an increasing environmental concern in freshwater ecosystems globally. However, the ecological consequences of cyanobacterial blooms for the vertical similarity of microbial community structure have yet to be thoroughly investigated, especially in deep waters. Here, we explored the taxonomic and functional similarity of microbial communities at different depths in a subtropical reservoir over a 7-year period following multiple R. raciborskii blooms. Our results showed that vertical microbial dispersal, rather than ecological niche, is the main process determining vertical similarity. Both particle-attached (PA) and free-living (FL) bacteria from the surface water were able to reach the deep water, particle size being a contributing factor to their vertical dispersal. Cyanobacterial blooms enhanced the vertical microbial transport of PA, impacting the composition and biogeochemical processes of deep microbial communities. During the mixing period, microbial taxonomic and functional similarities between the different water layers were high whereas they were minimal across the oxycline during the stratification period, suggesting a bottleneck in microbial vertical dispersal. In the deep water layers, the abundances of specific taxa, such as those of Burkholderiales and Desulfomonilales in PA and FL fractions respectively in stratification periods, increased during blooms. Additionally, cyanobacterial blooms enhanced sulfur compound respiration in both PA and FL fractions and suppressed nitrification in PA bacteria and denitrification in FL bacteria, simultaneously reducing light-utilization capacity in PA bacteria and altering organic matter degradation. Several mechanisms are proposed to drive variations in microbial vertical connectivity by cyanobacteria, including ecological niche shifts and alterations of physicochemical properties and nutrient dynamics. Overall, our results reveal complex effects of cyanobacterial blooms on microbial taxonomic and functional vertical similarity and highlight the contribution of surface communities to the biodiversity and biogeography of deep communities.