Seasonal gut microbiota and functional dynamics in brittle star (Ophiothrix exigua) from the Yellow Sea, China

The gut microbiota forms a complex symbiotic community that performs essential functions for the host, including metabolism, nutrient absorption, and environmental adaptation, while being shaped by both environmental and intrinsic host factors. This study represents the first comprehensive investigation of seasonal gut microbiota diversity in brittle stars, examining Ophiothrix exigua from the Yellow Sea using full-length 16S rRNA gene metabarcoding. A total of 565 amplicon sequence variants were identified from gut samples, distributed across 20 phyla, 135 genera, and 46 species. The dominant phyla included Proteobacteria, and Spirochaetota, with Salinispira identified as the core genus. Seasonal variations in microbiota diversity were evident, with Caulobacter predominating in summer, and Kistimonas and Trichococcus driving winter community shift. Corresponding seasonal changes in gut microbiota functions and functional pathways were observed. Fatty acid biosynthesis pathways were enriched in winter, while aromatic compound degradation pathways showed elevated activity in summer. Although seawater microbiota exerted relatively minor influence on gut microbial diversity, correlations with abiotic factors such as pH were observed. This study highlights the intricate relationship between gut microbiota, environmental microbiota, and abiotic factors in shaping the seasonal gut microbiota diversity of O. exigua, contributing to a better understanding of the host-microbiome ecology of invertebrates.