The meta-holobiont: rethinking marine species interactions through microbial connectivity.

Positive species interactions underpin ecological resilience, which is critical for counteracting anthropogenic pressures on ecosystems. Current research focuses on intra-holobiont interactions between hosts and their microbiomes, reflecting their recognized importance for host well-being, but overlooks interactions among holobionts. We introduce the meta-holobiont framework for the marine environment, which conceptualizes ecological communities as networks of interacting holobionts connected through microbial transmission and metabolite production. Marine connectivity can facilitate horizontal transmission of microorganisms, potentially enhancing host fitness and stress tolerance across all life stages. Conceptual and empirical findings from sessile benthic species support this view. We argue that investigating meta-holobiont interactions is crucial for advancing knowledge of ecological dynamics and may have important practical implications, particularly for marine restoration strategies.