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Although interactions between microalgae and bacteria are observed in both natural environment and the laboratory, the modalities of coexistence of bacteria inside microalgae phycospheres in laboratory cultures are mostly unknown. Here, we focused on well-controlled cultures of the model green picoalga <i>Ostreococcus tauri</i> and the most abundant member of its phycosphere, <i>Marinobacter algicola</i>. The prevalence of <i>M. algicola</i> in <i>O. tauri</i> cultures raises questions about how this bacterium maintains itself under laboratory conditions in the microalga culture. The results showed that <i>M. algicola</i> did not promote <i>O. tauri</i> growth in the absence of vitamin B₁₂ while <i>M. algicola</i> depended on <i>O. tauri</i> to grow in synthetic medium, most likely to obtain organic carbon sources provided by the microalgae. <i>M. algicola</i> grew on a range of lipids, including triacylglycerols that are known to be produced by <i>O. tauri</i> in culture during abiotic stress. Genomic screening revealed the absence of genes of two particular modes of quorum-sensing in <i>Marinobacter</i> genomes which refutes the idea that these bacterial communication systems operate in this genus. To date, the ‘opportunistic’ behaviour of <i>M. algicola</i> in the laboratory is limited to several phytoplanktonic species including Chlorophyta such as <i>O. tauri</i>. This would indicate a preferential occurrence of <i>M. algicola</i> in association with these specific microalgae under optimum laboratory conditions.