Defence on surface: macroalgae and their surface-associated microbiome

Macroalgae surfaces are considered as potentially suitable substrata 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 defence 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 defence in the normal favourable environmental conditions of the macroalgae (Symbiotism). Whereas, when there is a change i.e. reversed, unfavourable conditions, the microbiome reverses its symbiotic nature and affects growth and degrades the macroalgae by causing different diseases in different species. In parallel, marine macroalgae employ different defence mechanisms, a physical defence such as continuous shedding of the outer layer of cells, and the mucilaginous covering and chemical defence such as the production of secondary metabolites. The following chapter focuses on the macroalgal surface-associated microbiome and its defence system with a certain number of relevant examples from related literature on this subject.