Macroalgae surfaces are considered as a potentially suitable substratum present underwater for the dynamic epibiotic microbiome. They secrete various organic substances as nutrients for the multiplication of bacteria and the formation of microbial biofilms. The microbial biofilm itself acts as a biological defense mechanism to prevent the establishment of pathogenic microbes as well as to protect the surface of the seaweed from the macrofoulers. In most cases, the presence and diversity of these organisms are host-specific. The macroalgal host and their associated microbiont are often referred to as a 'Holobiont'.The surface-associated microbiome of macroalgae plays a crucial role in the growth, morphogenesis and defense in the normal favorable environmental conditions of the macroalgae (Symbiotism). Whereas, when there is a change i.e. reversed, unfavorable conditions, the microbiome reverses its symbiotic nature and affects growth and degrades the macroalgae by causing different diseases in different species (Parasitism). In parallel, marine macroalgae employ different defense mechanisms, a physical defense such as continuous shedding of the outer layer of cells, and the mucilaginous covering and chemical defense such as the production of secondary metabolites.The following chapter focuses on the macroalgal surface-associated microbiome and its defense system with a certain number of relevant examples from related literature on this subject.

Macroalgae, being important habitat formers and primary producers in the marine ecosystem, provide potential settling substrate and protective environments for the early life and development of many invertebrates. Attached to their substrate, marine seaweeds are linked to their environment and confronted with various abiotic and biotic stresses. They are affected by wave action, light radiation, desiccation, and variations in salinity, temperature, nutrient supply, herbivore predation, epibiosis, microfouling and human activity. In the marine environment, competition for space and nutrients is intense. Each species lives in this complex environment according to its own physiological parameters, and its ability to resist changes in the state of the environment, particularly, during low tides or in reduced space. Due to the extraordinary complexity of the marine environment, seaweeds establish close associations with other species to ensure their survival and competitiveness for space.

Macroalgal surfaces represent a major physiological and highly functional interface with the environment. Such an interface is always threatened by fouling, and thus macroalgae need to develop many defense strategies to protect their surface from a vast number of conse