Stability characteristics of bacterial and protistan communities along an estuarine continuum: Diversity, composition and co-occurrence networks

Estuarine ecosystems have been threatened by increasing anthropogenic and natural pressures, yet the integral understanding of their stability characteristics of microbial communities at taxonomic, habitat, and spatial scales remains limited. In this study, the Mulan River estuary in southeastern China was selected to compare the stability characteristics of bacterial and protistan communities in water and sediments over three hydrological periods, and to explore their spatial variations along the estuarine continuum from river to ocean. The potential driving mechanisms of stability characteristics were also explored. Results revealed that the estuarine ecosystem displayed varied stability at taxonomic, habitat, and spatial scales, separately. On the whole, protistan communities had a lower stability than bacterial communities in both water and sediments, with the higher coefficients of variation in alpha diversity (av. 0.82), the higher Bray-Curtis distances of community compositions (av. 0.98), and the lower robustness of networks (av. 0.23). Compared to the sediments, the lower stabilities of two taxa in water were also indicated via the above three indices. Along the estuarine continuum, the stability of bacterial communities in the middle reach was lowest, but the stability of protistan communities was lowest in the up reach. The null model and network analysis suggested that the assembly processes with stronger variable selection and homogeneous selection, coupled with higher positive interaction (r > 0.7) of microbial communities jointly accounted for the lower stabilities in the up and middle estuarine reaches. The partial least squares path model suggested that total nitrogen, total phosphorus, and dissolved oxygen controlled the community stability by influencing microbial assembly and interaction in the up and middle reaches. This study provides new insights on the varied stability characteristics of different microbial communities along the estuarine continuum, which are helpful for maintaining estuarine ecological function and formulating management strategies.