Tailored bacterial co-cultures improve Tisochrysis lutea growth and nutrient profiles under xenic conditions: a new pathway to improve microalgal production

• 145 culturable bacterial isolates were obtained from industrial Tisochrysis lutea. • Seven isolates were selected for designed bacterial combinations . • 21 bacterial blends were evaluated for effects on growth and biomass composition. • Roseovarius and Haliea promoted enhanced algal biomass, PUFA and vitamin B 12 contents. • Bacterial co-cultures proved effective in non-axenic microalgal cultivation systems. The marine haptophyte Tisochrysis lutea is a valuable source of high-value compounds, including polyunsaturated fatty acids like docosahexaenoic acid, and pigments (e.g., fucoxanthin). However, high production costs and variability remain major challenges for its large-scale application in aquaculture, pharmaceuticals, and biotechnology industries. Therefore, strategies to enhance biomass production and quality are actively explored. In natural environments, T. lutea establishes mutualistic interactions with bacteria to obtain essential nutrients such as vitamin B 12 , yet the role of bacteria in industrial cultures remains poorly understood. In this study, 145 bacterial strains were isolated and taxonomically identified from industrial T. lutea cultures, with members of the class Gammaproteobacteria and Actinomycetia being the most prevalent. Forty isolates were screened individually in co-culture with T. lutea revealing strain-specific effects on growth and biochemical composition. Seven beneficial strains were used to design 21 tailored bacterial blends. Several consortia enhanced biomass production (up to 74 %) and increased key bioactive compounds, particularly methylcobalamin (up to 300 %). These findings demonstrate the potential of tailored bacterial consortia to enhance T. lutea productivity and nutritional quality under production-relevant xenic conditions, enabling strategic microbiome modulation for specific industrial goals.