B-vitamins are organic micronutrients that play a vital role in cellular metabolism by providing enzyme cofactors. Most animals have lost the ability to synthesise these cofactors and obtain them in their diet, whereas plants can produce all their own compounds, and indeed are the major source of them for human nutrition. The situation is more complex for microbes with both producers and requirers are found in different species. Intriguingly this is also the case for microalgae, which despite their photosynthetic lifestyle are often auxotrophic for one or more B-vitamin. In particular more than 50% of microalgal species require vitamin B12 (cobalamin), the cofactor for methionine synthase, the central enzyme of C1 metabolism, essential for both DNA synthesis and production of S-adenosyl-methionine (SAM) the universal methyl donor, which in turn is required for methylation marks in DNA and histones. Here I will describe our work on elucidating the molecular basis for B12-auxotrophy in microalgae, how this impacts metabolism and gene expression, and the possible role of vitamin exchange in initiating and maintaining microbial communities.