Scale dependency of community assembly differs between coastal marine bacteria and fungi

It is often assumed that community assembly processes, i.e. deterministic or stochastic factors determining the structure of communities, vary with spatial scale. However, evidence showing such scale-dependency is challenging to gather, especially in microbial communities. Here we studied the relative importance of potential community assembly processes of bacterial and fungal communities across the coastline of China. We used null models based on phylogenetic and compositional turnover to quantify the relative importance of the processes at three spatial scales (local, regional and subcontinental). We used a generalized multinomial logistic regression to estimate the probability of observing the different assembly processes along environmental and spatial gradients. We found that, overall, deterministic processes are stronger in driving the assembly of bacterial communities compared to fungal communities, especially at the local scale. Moreover, the assembly processes in bacterial communities exhibit clear scale-dependency, with homogeneous selection being the dominant process at the local scale, and dispersal limitation dominating at regional and subcontinental scales. Conversely, drift is consistently the dominant driver of fungal community assembly across all spatial scales. Our findings demonstrate scale-dependency in community assembly processes between two marine microbial groups, and highlight the importance of adopting a multi-scale approach to gain insights into the complex and dynamic spatial patterns of microbial communities.