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Marine sponges host complex communities of microorganisms that contribute to their health and survival, yet the mechanisms behind microbiome assembly are largely unknown. The authors of this article present the global marine sponge–microbiome network, containing over 150 sponge hosts, and almost 2 million bacterial associates, and revealed a modular organisation in both community structure and function. They show that modules are linked by a few sponge species that share microbes with other species all around the world.

This is somehow expected for tight associations such as symbioses: a strong microbial community differentiation across host species. Hosts within a given module have a set of characteristic functions given by their symbionts, that differ across modules.

The authors then tried to answer the following question: What are the drivers of such modularity? As expected, host type (i.e. modules contain sponges with very different lifestyles) and sponge phylogeny (i.e. hosts are more phylogenetically related within a module than across modules) were key predictors for modularity. What challenges the current paradigm is that abiotic conditions of the environment played a negligible role. For example, sponges inhabiting a given marine ecoregion do not share more symbionts among them than with hosts from other ecoregions.

These findings suggest that both ecological and evolutionary processes are at play in host-microbe network assembly. On the basis of this specific example, authors predict that mechanisms behind microbiome assembly should consistent across multicellular hosts throughout the tree of life.