Outbreaks of Ulva prolifera green tides reduce the network complexity and stability of cooccurring planktonic microbial communities.

Ulva prolifera green tides are becoming a worldwide environmental problem, especially in the Yellow Sea, China. However, the effects of the occurrence of U. prolifera green tides on the community organization and stability of surrounding microbiomes have still not been determined. Here, the prokaryotic microbial community network stability and assembly characteristics were systematically analyzed and compared between the green tide and non-green tide periods. U. prolifera blooms weaken the community complexity and robustness of surrounding microbiomes, increasing fragmentation and decreasing diversity. Bacteria and archaea exhibited distinct community distributions and assembly patterns under the influence of green tides, and bacterial communities were more sensitive to outbreaks of green tides. The bacterial communities exhibited a greater niche breadth and a lower phylogenetic distance during the occurrence of U. prolifera green tides compared to those during the non-green tide period while archaeal communities remained unchanged, suggesting that the bacterial communities underwent stronger homogeneous selection and more sensitive to green tide blooms than the archaeal communities. Piecewise structural equation model analysis revealed that the different responses of major prokaryotic microbial groups, such as Cyanobacteria, to environmental variables during green tides, were influenced by the variations in pH and nitrate during green tides and correlated with the salinity gradient during the non-green tide period. This study elucidates the response of the adaptability, associations, and stability of surrounding microbiomes to outbreaks of U. prolifera green tides.