A Silicibacter sp. strain useful for genetic transformation of marine algae and production of antibiotic agents is described. Silicibacter sp. TM1040 is a genetically tractable member of the marine Roseobacter clade of bacteria that forms an intimate and obligate interaction with algae, and is useful as a probiotic for producing antibacterial agents that are cidally effective against pathogenic bacteria such as Mycobacterium marinum, Vibrio anguillarum, V. coralliilyticus, and V. shiloi bacteria.