Dual transcriptomics reveal commensal interactions between microalgae and phycosphere bacteria

The interactions between microalgae and the bacteria living in the phycosphere are pivotal to the role they play in aquatic ecosystems. This study examines how two representatives of common phycosphere bacteria, Yoonia sp. TsM2_T14_4 (Rhodobacteraceae) and Maribacter sp. IgM3_T14_3 (Flavobacteriaceae), interact with three microalgal hosts: Isochrysis galbana , Tetraselmis suecica , and Conticribra weissflogii (formerly Thalassiosira weissflogii ) using dual transcriptomic analyses of both bacteria and microalgae. Bacterial transcriptomes differed significantly depending on microalgal host, with notable changes in carbohydrate metabolism among other COG categories. Yoonia sp. expressed genes involved in anoxygenic photosynthesis in co-culture with I. galbana , presumably due to its inability to utilize carbohydrates from this algal host, whereas Maribacter sp. expressed polysaccharide degradation genes in co-culture with C. weissflogii along with T9SS genes, which can be employed to secrete these hydrolytic enzymes. Specifically, a putative glucan endo-1,3-beta-D-glucosidase was highly expressed, an enzyme that can hydrolyze laminarin and curdlan. Maribacter sp. IgM3_T14_3 could utilize laminarin as a sole carbon source in laboratory settings, a polysaccharide commonly found in marine environments and produced by C. weissflogii . Surprisingly, microalgal transcriptomes remained largely unaltered in the presence of either of the bacteria compared to transcriptomes of axenic algal cultures. These findings highlight the adaptability of phycosphere bacteria to different microalgal hosts. Furthermore, it also indicates a commensalism between microalgae, Yoonia sp. and Maribacter sp., in which the bacteria adapt to and benefit from microalgal host exudates, whereas under the conditions employed here the microalgae are unaffected by the presence of these bacterial symbionts.