Observational comparisons of intestinal microbiota characterizations, immune enzyme activities, and muscle amino acid compositions of loach in paddy fields and ponds in Sichuan Province.

A balanced intestinal microbial ecosystem is crucial for the growth and health of animals because it can influence the digestion and absorption of nutrients in the intestine. Different culture conditions may change the ecology of microbial in intestine and thus affect the overall growth performance of an animal. In this study, we compared intestinal morphologies, microbiota characterizations, immune enzyme activities, and muscle amino acid compositions of loach cultured in paddy fields and ponds. The fish were fed with the same diets from May 5 to November 5 (2015) in three paddy or ponds. Fish samples were collected for analysis in the August (summer season) and November (fall season) during the feeding trial. In both culture conditions, results based on microscopy observation showed that the intestinal perimeter, fold height, fold radical, and total absorption of the gut were significantly higher in the foregut than that found in the midgut and hindgut (P < 0.01). The average final body weight of fish was similar between the two culture conditions (P > 0.05). The percentage of carcass weight to whole loach weight for samples collected from paddy field (91.6 ± 1.1) was significantly higher than the index measured for loach from pond (87.3 ± 3.4, P < 0.05). Results based on denaturing gradient gel electrophoresis demonstrated that the Shannon-diversity index, evenness, and richness of intestinal flora were increased from summer to fall in paddy cultivation. In pond culture condition, however, the above indexes obtained from mucosa and intestinal contents decreased in fish from summer to fall. The sequencing results of bands indicated that the predominant microorganisms are Proteobacteria, Firmicutes, and Actinobacteria in the intestine of fish being cultured in both cultures. Activities of alkaline phosphatase (AKP, in two culture conditions) and superoxide dismutase (SOD, in paddy field) presented a gradual decrease trend from foregut to hindgut of fish. The activities of acid phosphatase (ACP, in midgut), AKP (in midgut and hindgut), SOD (in foregut), and lysozyme (LZM, in midgut) were significantly higher in fish cultured in paddy than those in pond (P < 0.01). In addition, the percentage of some essential amino acids (valine, methionine, and phenylalanine) based on total amino acids in muscle was significantly higher in fish cultured in paddies than in ponds. In summary, the fish cultured in paddy or pond was not significantly different in growth but the two culture conditions seems to generate different carcass yield and changed the amino acid profiles of fish muscle. The similar predominance microorganisms were identified in the intestine of fish from two conditions, and the quantification of microbial in the intestine will be determined in the future, but part activities involved in immune protection were higher for fish cultured in paddy fields.