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 with FA levels of 0.00 + 1% succinylsulfathiazole (SST), 0.00, 1.37, 2.43, 4.26, and 9.84 mg/kg. A total of 180 crabs (initial body weight: 15.21 ± 0.10 g) were individually reared for 8 weeks. Results showed that 4.26 mg/kg FA improved final weight, percent weight gain, specific growth rate, survival, and feed efficiency. Dietary 4.26 mg/kg FA enhanced eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) contents in hepatopancreas and muscle, whereas the 0.00 + 1% SST treatment reduced EPA + DHA contents. Crabs fed 4.26 mg/kg FA exhibited decreased hemolymph AST and ALT activities, reduced malondialdehyde (MDA) concentrations in hemolymph and hepatopancreas, and increased total superoxide dismutase (T-SOD) activity in hepatopancreas. Additionally, 4.26 mg/kg FA elevated S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) concentrations and upregulated expressions of pcft, dhfr, shmt, mthfr, and mtr in hepatopancreas. 16S rRNA analysis revealed that 4.26 mg/kg FA increased the relative abundance of Gram-negative and aerobic bacteria while reducing oxidative stress-tolerant, anaerobic, and pathogenic bacteria in the intestine. These findings indicate that juvenile mud crab have limited capacity for FA synthesis, and dietary supplementation with 4.26 mg/kg FA is necessary to support optimal growth and physiological functions, the two-slope broken-line regression of PWG against dietary FA levels indicated that the estimated optimal dietary FA level was 3.86 mg/kg for juvenile mud crab.