Dietary cobalt can promote gastrointestinal bacterial production of vitamin B12 in sufficient amounts to supply growth requirements of grouper, Epinephelus malabaricus

Two growth trials were conducted to elucidate the dietary essentiality of cobalt (Co) and vitamin B12 for grouper, Epinephelus malabaricus. In Exp. 1, a vitamin B12-free basal diet was supplemented with 0.05, 6.5, 10.7, 20.7, 41.6, 82.7 and 153.1 mg Co/kg; in Exp. 2, a vitamin B12-free basal diet with 10 mg Co/kg was supplemented with 0, 8.3, 18.1, 54.0, 87.1 and 193.2 μg B12/kg diet, a vitamin B12-free basal diet with 10 mg Co/kg and 1% succinylsulfathiazole (0 + antibiotic) was also included in Exp. 2. Each diet was fed to triplicate groups of juvenile grouper (initial body wt: 8.04 ± 0.06 g in Exp. 1; 7.50 ± 0.06 g in Exp. 2) in a recirculated rearing system for 8 weeks. In Exp. 1, weight gain (WG) and feed efficiency (FE) were higher (P < 0.05) in fish fed diets with 10.7 and 20.7 mg Co/kg than fish fed diets with ≤ 6.5 mg Co/kg. Hepatic vitamin B12 concentration was highest in fish fed diets with ≥ 20.7 mg Co/kg, followed by fish fed the diet with 10.7 mg Co/kg, and lowest in fish fed diets with ≤ 6.5 mg Co/kg. Fish fed diets with ≥ 20.7 mg Co/kg had the highest plasma vitamin B12 concentration, followed by fish fed the diet with 6.5 mg Co/kg, and lowest concentration was in fish fed the diet with 0.05 mg Co/kg. Analysis by broken-line regression indicated that 10 and 20 mg Co/kg diet are required for optimal growth and maximum hepatic and plasma vitamin B12 concentrations in grouper, respectively. In Exp. 2, WG, FE and survival in fish were not affected (P > 0.05) by the dietary treatments. Hepatic and plasma vitamin B12 concentrations were lower in fish fed the 0 + antibiotic diet than those fed the diet with 193 μg B12/kg. These results revealed that grouper require 10 mg Co/kg diet for optimal growth; no vitamin B12 is needed in the diet containing adequate Co.