Genomic insights into polysaccharide substrate utilization and novel genera resource mining in macroalgal epiphytic bacteria

Marine macroalgae, among the fastest photosynthesizing organisms, play a crucial role in the ocean carbon cycle by converting fixed carbon dioxide into polysaccharides. Macroalgal epiphytic bacteria possessing specific polysaccharide utilization loci (PULs) have a generalized polysaccharide degradation potential that facilitates their growth and colonization. In this study, we conducted extensive research on their polysaccharide degradation potential. Through sampled and purified epiphytic bacteria and metagenomic analysis revealed a high prevalence of novel genera and species. Two novel genera, 1117T and 3-347T, were taxonomically characterized and conducted detailed functional analyses. The results demonstrated that these genera possess abundant PULs and strong capabilities for synthesizing secondary metabolites. Furthermore, their high relative abundance on macroalgal surfaces aligns with global ecological distribution patterns. These traits facilitate their colonization, growth, and environmental adaptation on macroalgal surfaces. We further performed in-depth annotation of a large number of PULs and CAZyme genes of macroalgal epiphytic bacteria. Potential polysaccharide substrates for their degradation can be predicted and focused on. Additionally, we conducted growth curve analyses by starch, xylan, {beta}-1,3-glucan, and carboxymethyl cellulose substrates to validate the genomic predictions. In summary, our findings demonstrate that macroalgal epiphytic bacteria possess significant potential for degrading algal polysaccharides. This capability may enhance their competitiveness and survival probability on macroalgal surfaces. These bacteria, originating from different sources and genera, possess similar PULs, which may result from horizontal gene transfer or evolutionary relationships. ImportanceMacroalgae are major primary producers in coastal areas and their carbon sequestration capacity per unit area far exceeds that of terrestrial forests. In this work, we extensively studied macroalgal epiphytic bacteria with polysaccharide degradation potential. We found that epiphytic bacteria from different macroalgal sources and genera share similar PULs to degrade the same polysaccharide, which may be the result of horizontal gene transfer or evolutionary relationships. Core taxa on the macroalgal surface have gradually evolved polysaccharide-degrading abilities of different marine macroalgae in order to expand their colonization and survival chances. We also identified a large number of uncultivated algal biosphere species and unreported new genera and species for expansion of macroalgal epiphytic bacteria studies. These studies have thus highlighted the important ecological and research value of macroalgal epiphytic bacteria, especially in influencing polysaccharide carbon storage and marine carbon cycling.