Mucilage formation, which started to be effective in the Sea of Marmara in the autumn period of 2020 and spread over very large areas in 2021, has been a large-scale environmental problem in our coastal waters in recent years. The microalgae-bacteria microbiome is one of the main components of the marine ecosystem and an important determinant of seawater quality. Exopolysaccharide (EPS) inputs, which increase in the environment with excessive phytoplankton bloom, make it difficult to utilise dissolved carbohydrate in water by bacteria and slow down the dissolution of organic matter, leading to the accumulation of biopolymers forming mucilage mass. The resulting mucilage mass harbours microbial pathogens as well as high carbohydrate content. In this study, the role of microalgae-pathogenic bacteria microbiome among the factors affecting mucilage formation was investigated. Morphological identification of HAB species were carried out using binocular microscope and scanning electron microscope images. As a result of morphological analyses, one isolate of coccolithophore Ochrosphaera (strain I) and one isolate of dinoflagellate Amphidinium (strain II) were purified. Isolation and cultivation of HAB species in mucilage samples were carried out under microscope by single cell culture. The isolated microalgae strains were cultured in f/2 medium at 20°C, 12 hours light/dark period, 150 Em-2 s - 1 light intensity. To obtain axenic microalgae cultures, phytoplankton stock culture was serially diluted in sterile f/2 culture medium containing a series of antibiotics and incubated for 24-48 hours under optimum growth conditions. In the last step, the samples were transferred to sterile culture medium without antibiotics. Sterile monoalgal cell cultures (control group) and sterile monoalgal cell cultures (experimental group) incubated with a mixture of 5 different bacterial species from the Sea of Marmara according to morphological preliminary identification were prepared. The inoculation time of microalgae was considered as the beginning of the experiment. To analyze microalgae growth during the experiments, the specific growth rate of microalgae was calculated based on chlorophyll-a (Kl-a) concentration and cell abundances. In the strain I experiment, a high positive correlation was found between microalgae, bacteria abundances, and Kl-a concentrations. There was also a positive correlation between microalgae abundance and Kl-a in both experimental groups, while no significant correlation (p>0.05) was detected between pathogenic bacteria abundance and Kl-a in the Strain II experiment. As a result of the study, a significant correlation was found between pathogenic bacteria abundance and specific growth rate of microalgae (strain I) based on Kl-a concentrations, but no significant (p>0.05) correlation was found between pathogenic bacteria abundance and Kl-a in the strain II experiment. The findings of the study emphasize the complexity of microalgae-pathogenic bacteria interactions and highlight the need for more detailed investigations to understand the role of the microbiome in mucilage formation.