Keystone microbial isozyme enhances crustacean growth by utilizing L-citrulline

Abstract

Under standardized rearing conditions, the genetically uniform “Keyong No. 1” swimming crab exhibits pronounced inter-individual variability in growth performance. While the gut microbiota is recognized contributors to crustacean growth, the specific microbes and mechanistic pathways that govern host growth are yet to be elucidated. By comparing gut bacterial communities in fast- and slow-growing swimming crabs ( Portunus trituberculatus ), we identified Roseovarius litoreus as a pivotal low-abundance keystone taxon enriched in fast growers, whose abundance positively correlated with host growth. Dietary supplementation with R. litoreus promoted growth in shrimp ( Litopenaeus vannamei ), selectively bred and wild-type crabs, restructuring gut microbial networks and increasing community metabolic capacity, particularly L-citrulline utilization. These growth benefits likely arise from the coordination of R. litoreus with resident microbes. Mechanistically, R. litoreus functions as a L-citrulline–utilizing commensal that enhances L-arginine biosynthesis through a microbial–host isozyme pair of argininosuccinate synthase (ASS) and argininosuccinate lyase. The microbiota-mediated conversion of L-citrulline to L-arginine stimulated muscle development through a MyoD-dependent pathway potentially governed by NO–sGC–PKG signaling. Inhibition of ASS suppressed growth, whereas R. litoreus supplementation restored this pathway. R. litoreus encodes a simpler yet more efficient ASS than the host enzyme, and deletion of its core ASS domain eliminated citrulline metabolism and the growth-promoting effect. Supplementation with wild-type R. litoreus rescued growth stunting by reestablishing microbial citrulline metabolism. Our findings demonstrate that ASS derived from keystone taxon R. litoreus enhances L-Cit utilization and promotes crustacean growth potentially through the L-Arg-mediated NO–sGC–PKG signaling. Targeting the gut-muscle axis via microbial keystone taxa may represent a promising probiotic strategy for growth promotion in crustaceans. Video Abstract