Phycobacteria Biodiversity, Selected Isolation, and Bioactivity Elucidation of New Bacterial Species of Highly Toxic Marine Dinoflagellate Alexandrium minutum amtk4

Phycosphere niches host rich, unique microbial consortia that harbor complex algae-bacteria interactions with fundamental significance in underpinning most functions of aquatic ecological processes. Therefore, harvesting the uncultured phycobacteria is crucial for understanding the intricate mechanisms governing these dynamic interactions. Here, we characterized and compared microbial community composition of the phycosphere microbiota from six harmful algal bloom-forming marine dinoflagellates, <i>Alexandrium</i> spp., and their bacterial associations. Furthermore, based on a combinational enhanced cultivation strategy (CECS) procedure for the selected isolation for cultivable phycobacteria, a new yellow-pigmented bioactive bacterium designated ABI-6-9 was successfully recovered from cultivable phycobacteria of the highly toxic <i>A. minutum</i> strain amtk4. An additional phylogenomic analysis fully identified this new isolate as a potential novel species of the genus <i>Mameliella</i> within the family <i>Roseobacteraceae</i>. The bioactivity evaluation observed that strain ABI-6-9 can significantly promote the cell growth of its algal host and altered the gonyautoxin accumulation profiles in the co-culture circumstance. Additionally, the bacterial production of active bioflocculanting exopolysaccharides (EPSs) by strain ABI-6-9 was also measured after culture optimization. Thus, these findings revealed the potential environmental and biotechnological implications of this new microalgae growth- promoting phycobacterium.