Dietary folic acid coordinates growth and lipid metabolism through gut microbial and one-carbon networks in juvenile mud crab (Scylla paramamosain).

This study investigated the effects of dietary folic acid (FA) on growth performance, lipid metabolism, one-carbon metabolism, and gut microbiota of juvenile mud crab (Scylla paramamosain). Six isonitrogenous and isolipidic diets were formulated: one antibiotic basal diet control with 1% succinylsulfathiazole (0.00 + 1% SST), one basal diet control (0.00 mg/kg FA), and four diets with graded FA levels (1.37, 2.43, 4.26, and 9.84 mg/kg). A total of 180 juvenile crabs (initial body weight: 15.21 ± 0.10 g) were individually reared for 8 weeks. Compared with the 0.00 mg/kg FA and 0.00 + 1% SST treatments, dietary supplementation with 4.26 mg/kg FA improved final weight (FW), percent weight gain (PWG), specific growth rate (SGR), survival, and feed efficiency (FE) (P < 0.05). The 4.26 mg/kg FA also enhanced eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) contents in hepatopancreas and muscle, whereas the 0.00 + 1% SST treatment significantly reduced total EPA + DHA contents (P < 0.05). Moreover, crabs fed 4.26 mg/kg FA exhibited decreased hemolymph aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities, significantly reduced malondialdehyde (MDA) concentrations in hemolymph and hepatopancreas, and significantly increased total superoxide dismutase (T-SOD) activity in the hepatopancreas (P < 0.05). Additionally, 4.26 mg/kg FA elevated S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) concentrations and upregulated the expression of pcft, dhfr, shmt, mthfr, and mtr in the hepatopancreas (P < 0.05). The 16S rRNA gene sequencing further revealed that 4.26 mg/kg FA increased the relative abundance of gram-negative and aerobic bacteria (P < 0.05), while reducing oxidative stress-tolerant, anaerobic, and potentially pathogenic bacteria in the intestine. These findings indicate that juvenile mud crab have a limited capacity for FA synthesis and require dietary supplementation. A dietary FA level of 4.26 mg/kg supported optimal growth and physiological functions, while two-slope broken-line regression of PWG against dietary FA levels estimated an optimal dietary requirement of 3.86 mg/kg for juvenile mud crab.