species competition has expanded biotic colonization towards recondite niches (Sala and Knowlton, 2006;Mestre et al., 2020). Symbiotic alliances in this sense enable marine holobionts to thrive and flourish in habitats with extremely challenging conditions, which may include low temperature waters, sulfidic sediments, or oxygen minimum zones to mention some (Middelburg et al., 2015;González-Pech et al., 2024). Under these seemingly ecologically liming environments, microbial associates furnish their hosts with expanded metabolic machineries, allowing the exploitation of alternative substrates -otherwise inaccessible for metazoans-to produce energy (Dubilier et al., 2008;Ponnudurai et al., 2017). These microorganisms often include stress tolerant taxa, which aside from participating in holobiont nourishment, they also promote vital detoxification pathways, which warrant life under harsh conditions (Van Der Heide et al., 2012). Other protective mechanisms afforded through microbial associations are able to guarantee cellular stability under severe conditions, via membranal re-organizations, and preservation of enzymatic functional integrity (La Motta et al., 2024;Sudo et al., 2024). Metaorganismal symbioses established in challenging environments frequently derive in obligate partnership interactions, and represent clear examples of coevolutive events (O'Brien et al., 2019).Holobiont systems may acquire revamped versions of interorganismal serviceability by symbiotic anatomical compartmentalization. Hosts can generate spatially confined body structures (cells, organs, regions), representing specialized niches that harbour distinctive microbial communities. Dedicated microbiome compartmentalization promotes diversification of functionalities to maintain homeostasis, optimizing nutrient exchanges, while controlling symbiont cooperation. Besides playing customized roles in host physiology, these specialized symbiotic structures allow to segregate multi-partner aggregations to avoid outcompetition among beneficial strains, and prevent cooperative microbes from being attacked by the host's immune system (Chomicki et al. 2020;Mohd-Radzman and Drapek, 2023). Therefore development of anatomical compartments with characteristic operational microbiome populations constitutes an optimization in the evolution of symbiotic relationships in the sea (Chomicki et al. 2020;Hughes et al. 2022).The present topic captures examples of some of the mentioned fascinating processes by which metazoan and microbial consortia generate upgraded biological units (holobionts), able to prosper in adverse -at times inhospitable habitats, and to refine and diversify the functional repertoire of marine host Martínez et al., 2026), in this collection we had the opportunity to include a varied representation of invertebrates and also vertebrates.Different hosts living in the same environment may establish variable symbiotic associations, while same host species may acquire a variety of microbes when inhabiting different locations (Burgsdorf et al., 2014). All in all, these different microbes structured by habitat, geographic location or host species exert often similar biochemical functions, resulting in a metabolic redundancy (Lesser et al., 2022). Compartmentalization may refine symbiotic partnerships and optimize the metabolism of anatomical regions with specialized functions, while providing the ecological niche for selected microorganisms that can co-evolve with the host (Chomicki et al., 2020). Ultimately, these findings emphasize the contribution of associated and specialized microbes within integrated holobionts, where the adaptation to a lifestyle or the acclimatization to specific environmental conditions is not only a feature of the host, but a collective outcome of a microbial integrative partnership.
Núñez‐Pons et al. (2026) studied this question.