Abstract
The long-term stability of microbiomes is crucial as the persistent occurrence of beneficial microbes and their associated functions ensure host health. Microbiomes are highly diverse and dynamic, but are they complex to the point of being impossible to understand? We present an approach that while embracing this complexity it allows to identifying meaningful patterns: the dynamic core microbiome. We study the structure, dynamics and stability of microbiomes belonging to six marine sponges sampled monthly over three years. We show that microbiome temporal stability is not determined by the diversity of their microbial assemblages, but by the abundance density of those microbes that conform their core microbiome. High-density cores confer hosts resistance against the establishment of occasional taxa to which sponges are constantly exposed through their filter-feeding activities. The core microbial interaction network consisted of complementary members interacting weakly with a dominance of commensal and amensal interactions that have likely coevolved to maintain host functionality and fitness.