Marine bacteria significantly influence the development and productivity of algal communities. The green seaweed Ulva ( Chlorophyta) relies on bacteria that secrete algal growth and morphogenesis-promoting factors (AGMPF). In a reductionist model system of Ulva compressa (cultivar U. mutabilis ), the diverse microbiome can be substituted by two key bacteria, Roseovarius sp. MS2 and Maribacter sp. MS6, both of which release AGM...