Marine ecosystems are complex networks in which organisms form numerous associations with microorganisms, creating symbiotic relationships that are crucial for their survival and evolution. This thesis presents the intricate interactions between marine organisms and their microsymbionts, emphasizing the diversity of these relationships, ranging from mutualism to parasitism. The concept of the 'holobiont' emphasizes the complex interaction between host and microorganisms and proposes a new approach to understanding coevolution and collective functions in marine life. From invertebrates to vertebrates, marine organisms live in symbiosis with bacteria, viruses, protists, archaea, and fungi. The complexity of these associations is reflected in the responses of marine ecosystems to stressors such as climate change, ocean acidification, and anthropogenic impacts, which can lead to an alteration between states of symbiosis and dysbiosis. Stressors can enhance the resilience of some species while making others vulnerable. Microsymbionts provide essential benefits such as nutrient cycling, protection from pathogen and support for host reproduction and growth. By focusing on the adaptive mechanisms of the microbiome, this thesis discusses the adaptive strategies of hosts that strengthen symbiotic partnerships. Additionally, the thesis highlights the importance of a holistic approach to the study of marine biology that recognizes the inseparability of organisms and their symbionts in the marine environment.