Fungi promote cross-domain interactions even in deep anoxic mangrove sediments

Microbial communities in mangrove sediments play vital ecological roles that underpin the functioning of the overall mangrove ecosystem. Fungal communities, in particular, are known to play crucial roles across sediment systems, yet their roles in mangrove sediments, especially in deeper layers, remain poorly understood without a comprehensive inter-domain characterization. To better understand fungal roles in sediment horizons, 10 sediment cores extending down to a depth of 1 m were taken in three mangrove sites to characterise the archaeal, bacterial, and fungal communities at 10 cm depth intervals.

We demonstrate that sediment depth has distinct effects on the three microbial communities. While fungal community compositions were similar across sediment depths, bacterial and archaeal community compositions were stratified into three distinct layers, surface (10-30 cm), subsurface (40-60 cm), and deep (70-100 cm). Co-occurrence networks were then constructed to investigate the roles of fungi in these sediment layers, where fungi were consistently identified as keystone taxa in maintaining the microbial network topology, with co-domain interactions constituting more than half of all interactions. Even in the deepest layer, fungal nodes still retained high betweenness centralities, acting as network hubs to potentially augment microbial interactions vital for the functioning of the overall ecosystem.

Overall, our results emphasise the important role of fungi in mediating microbial interactions across sediment depths even in deep, anoxic sediment layers, and highlight the importance of cross-domain interactions as integral to a more holistic understanding of the mangrove microbiome.