Co-occurrence between key HAB species and particle-attached bacteria and substrate specificity of attached bacteria in the coastal ecosystem.

The ecological dynamics of particle-attached bacteria (PAB) were observed through changes in the core phytoplankton phycosphere, and were associated with the dynamics of free-living bacteria (FLB) using metabarcoding and microscopic analyses over 210 days (with weekly sampling intervals) in the Jangmok coastal ecosystem, South Korea. Cluster analysis and non-metric multidimensional scaling classified the phytoplankton community into six groups comprising core phytoplankton species, including the harmful algal species Akashiwo sanguinea (dinoflagellate) in late autumn, Teleaulax amphioxeia (cryptomonads) in early winter and spring, Skeletonema marinoi-dohrnii complex (diatom) in winter, Pseudo-nitzschia delicatissima (diatom) in early spring, and diatom complexes such as Chaetoceros curvisetus and Leptocylindrus danicus in late spring. We identified 59 and 32 indicators in PAB and FLB, respectively, which rapidly changed with the succession of the six core phytoplankton species. The characteristics of PAB were mainly divided into "Random encounters" or "Attraction of motivation by chemotaxis." When Akashiwo sanguinea bloomed, bacteria of the genera Kordiimonas and Polaribacter, which are commonly observed in PAB and FLB, indicated "Random encounter" characteristics. In addition, Sedimenticola of PAB was uniquely presented in Akashiwo sanguinea, exhibiting characteristics of "Attraction of motivation by chemotaxis." In contrast, FLB followed the strategy of "Random encounters" because it was not affected by specific habitats and energy sources. Thus, many common bacteria were PAB and FLB, thereby dictating the bacteria's strategy of "Random encounters." "Attraction of motivation by chemotaxis" has characteristics of the species-specific interactions between PAB and specific harmful algal species, and is potentially influenced by organic matter of core phytoplankton cell surface and/or EPS released from phytoplankton.