The dynamic core microbiome: structure, stability and resistance

15 The long-term stability of microbiomes is crucial as the persistent occurrence of beneficial microbes and their associ16 ated functions ensure host health. Microbiomes are highly diverse and dynamic, but are they complex to the point of 17 being impossible to understand? We present an approach that while embracing this complexity it allows to identifying 18 meaningful patterns: the dynamic core microbiome. We study the structure, dynamics and stability of microbiomes 19 belonging to six marine sponges sampled monthly over three years. We show that microbiome temporal stability is not 20 determined by the diversity of their microbial assemblages, but by the abundance density of those microbes that con21 form their core microbiome. High-density cores confer hosts resistance against the establishment of occasional taxa 22 to which sponges are constantly exposed through their filter-feeding activities. The core microbial interaction network 23 consisted of complementary members interacting weakly with a dominance of commensal and amensal interactions 24 that have likely coevolved to maintain host functionality and fitness. 25