Members of the Roseobacter group are ubiquitous across marine ecosystems where they can interact with algae and play important roles that can be both beneficial and at times detrimental to the host. In this talk I will firstly discuss studies showing that members of the Roseobacter group are effective colonisers of different algal surfaces and can influence subsequent microbiome development of the host. Secondly, I will present our recent work aiming to understand bacterial causes of bleaching disease in the red seaweed Delisea pulchra and the discovery of new Roseobacter isolates with seaweed probiotic activity. These findings not only demonstrate the ecological importance of these marine bacteria but also pave the way for effective microbiome manipulation strategies for conservation and/or biotechnology applications.