Phycospheric bacteria enhance temperature tolerance of Isochrysis zhanjiangensis: Unveiling the key functional member Labrenzia algicola 520.

Under global climate change, temperature stress constrains the stable large-scale cultivation of microalgae. Here, we evaluated the role of algal-associated bacteria in the temperature tolerance of Isochrysis zhanjiangensis and identified a functional bacterium with application potential. The presence of the associated bacterial community increased the final cell density of I. zhanjiangensis, reaching 1.28-fold that of the axenic control under heat stress. Analysis of 16S rRNA gene amplicon sequencing revealed that heat stress reshaped the bacterial community and significantly enriched Labrenzia. At an optimal algae-to-bacteria ratio of 1:50, Labrenzia algicola 520 promoted algal growth under both heat and cold stress, with elevated chlorophyll content, osmolyte accumulation, and antioxidant capacity. Genomic and phenotypic analyses further revealed multiple growth-promoting traits, including indole-3-acetic acid (IAA) biosynthesis, siderophore production, antioxidant activity, and biofilm formation. Moreover, L. algicola 520 maintained its beneficial effects under xenic conditions and increased the biomass of Isochrysis galbana by 29.6% in scaled-up winter cultivation. Collectively, these findings establish L. algicola 520 as a promising probiotic candidate for improving the production stability of commercial microalgae under fluctuating temperature conditions and provide a basis for developing bacteria-based bioaugmentation strategies in microalgal cultivation.