Gut microbiota and metabolic functions in herbivorous fish from Xisha coral reefs, China

Herbivorous fishes are the major consumers of macroalgae on coral reefs. Macroalgae are then degraded by fish gut microbiota. Despite the importance of this process, there remains a lack of information about the gut bacterial community of herbivorous fishes in coral reefs. This study examined the composition and potential metabolic functions of gut microbiota through full-length 16S rRNA amplicon sequencing for coral reef fish species from the Xisha islands of China, including five herbivorous species (Naso unicornis, N. lituratus, N. brevirostris, Ctenochaetus striatus, and Siganus argenteus) and one carnivorous species (Myripristis kuntee). The composition of the gut bacterial community and potential functional genes were observed and significant differences were identified between herbivorous and carnivorous fish species. The bacterial diversity (Shannon and Simpson indexes) in herbivorous fish was significantly higher than in carnivorous fish. Clostridium was significantly more abundant in carnivorous fish than herbivorous fish (p < 0.05). Conversely, Niameybacter, Alistipes, Desulfovibrio, and Akkermansia were more abundant in herbivorous fish (p < 0.05). These bacterial genera, which are shared among herbivorous fish, are believed to play crucial roles in macroalgae metabolism. A principal coordinate analysis further revealed that the gut bacterial composition of the five herbivorous fish species could be divided into two distinct clusters, which was influenced by selective feeding behaviors on different macroalgae types. Functional analysis of gut bacteria in herbivorous fishes showed a high abundance of carbohydrate metabolism pathways. Through comparison with the carbohydrate-active enzyme (CAZy) database, eight glycoside hydrolases (GHs) and one glycosyltransferase (GT) were identified in herbivorous fishes, providing further insights into the roles of gut microbiota in macroalgae metabolism. This study provides valuable insights into the roles of the gut microbiota in herbivorous fishes with regard to metabolizing various types of macroalgae.