The marine microalga Ostreococcus is considered to depend on the methionine synthase METH and its methylated cobalamin cofactor for methionine synthesis because it lacks METE, the methylcobalamin-independent methionine synthase. Here I describe minimal media lacking both biotin and cobalamin yet suitable for growth of cultured Ostreococcus. Materials and MethodsOstreococcus cell cultures were obtained from the Roscoff Culture Collection, Roscoff, France, and the Provasoli-Guillard National Center for Marine Algae and Microbiota, East Boothbay, Maine, USA. Panels of candidates to be evaluated for DNA transformation were assembled. The panel for clade A consisted of NCMA2972 (Palenik et al., 2007), RCC754, RCC755, and