The marine green seaweed Ulva (Chlorophyta) coexists with a diverse microbiome. Many Ulva species proliferate and form green tides, which occur when nutrient-rich wastewater is flushed into the sea. Hereby, bacteria are required for Ulva‘s adhesion to its substrate, growth, and the development of its thallus morphology. When compared to freshly isolated algae, the microbiome gardening of Ulva reduces the operational taxonomic units in aquacultures, indicating a selection process. Under axenic conditions, however, Ulva mutabilis develops a callus-like morphotype.
While bacterial strains of the Roseobacter-clade cause blade cell division, bacteria of the genus Maribacter promote differentiation of basal cells into a rhizoid and support cell wall formation through the morphogen thallusin (EC50 = 4.8 pM). Interestingly, a reductionist tripartite community of Ulva with two essential bacteria can restore the entire morphogenesis. The tripartite Ulva community now serves as a model system to study, for example, what the associated bacteria need for the holobiont to withstand environmental stresses and develop into a multicellular organism.
Macroalgae may adapt to changing environments in two ways: (i) through intrinsic changes in algal metabolism caused by differential gene expression and metabolite production; and (ii) through extrinsic changes provided by the associated and stress-adapted microbiome caused by its continuous support with algal growth and morphogenesis promoting factors (AGMPFs). This talk will focus on recent research and discuss how U. mutabilis has evolved into a model organism in chemical ecology, taking advantage of the specific perspective that a reductionist model system allows and discussing possible applications.