Abstract: Metabolic interactions between microbiomes and algal hosts within the phycosphere of marine macroalgae are drawing increasing attention due to their roles in food webs, global nutrient cycles, industries, and their potential as food resources. However, these relations remain poorly understood. In this study, 43 marine macroalgae, including red, brown, and green algae, were collected from the coastal areas of Korea. We identified the bacterial communities within the loosely and tightly attached environments (LAEs and TAEs, respectively) of the phycosphere, along with those in the surrounding seawater, using 16S rRNA gene sequencing. β-Diversity analysis revealed significant differences between the bacterial communities among the three, with minimal variation related to sampling location or algal color. Indicator value analysis identified Pseudoalteromonas (in the LAE and TAE), Psychromonas (in the LAE), Marinomonas (in the LAE), and Litorimonas (in the TAE) as the dominant taxa in the phycosphere, in contrast to seawater. Network analysis suggested positive correlations among taxa within the same environments and negative correlations between those in the LAE and TAE, highlighting their distinct ecological conditions. Analysis using the Phylogenetic Investigation of Communities by Reconstruction of Unobserved States and Kyoto Encyclopedia of Genes and Genomes pathways revealed functional variations between the phycosphere- and seawater-residing microbes. The microbial taxa–function relationships were assessed through Spearman's rank-order correlation. Additionally, bacterial species belonging to the core taxa were isolated and their genomes sequenced. Their metabolic traits were analyzed via bioinformatics, recognizing key metabolic features essential for symbiotic interactions with algal hosts and survival within the phycosphere. The findings of this study advance our understanding of the marine algal phycosphere microbiome by detailing the metabolic characteristics of potential keystone species.