Functional contribution of the gut microbiota to digestive capacity during ontogeny in greater amberjack (Seriola dumerili) larvae

Introduction The intestinal microbiota is increasingly recognized as an important contributor to host digestive physiology; however, its potential contribution to digestive capacity during fish larval development remains largely unknown. Here, we investigated the coordinated development of host digestive physiology and gut microbial function throughout larval ontogeny in greater amberjack ( Seriola dumerili ) larvae sampled at 10, 19, 32, 40 and 60 days post-hatching (dph), covering the main dietary transitions from live prey feeding through co-feeding with formulated diets to post-weaning. Methods Intestinal digestive enzyme activities were quantified in parallel with the enzymatic potential of the associated bacterial community, allowing, for the first time in marine fish larvae, the estimation of the potential contribution of the microbiota to digestive capacity. These analyses were integrated with 16S rRNA gene sequencing, FAPROTAX functional prediction, indicator taxa analyses and zymography to characterize the taxonomic and functional maturation of the intestinal microbiota. Results Host digestive capacity underwent marked ontogenetic changes associated with major dietary transitions. In parallel, the bacterial community progressively acquired a broader digestive repertoire. Bacterial proteolytic and chitinolytic activities increased markedly during development, while bacterial amylase activity was detected only at 60 dph. The potential bacterial contribution was highly enzyme- and stage-dependent, reaching its highest values during co-feeding and after completed weaning on formulated diet. Functional predictions consistently revealed an enrichment of fermentation and complex carbohydrate degradation pathways, coinciding with an increase in Firmicutes, particularly Clostridiales, Lactobacillales and Bacillales. Zymographic profiles also revealed increasing complexity in the bacterial proteolytic repertoire throughout development. Significant correlations between bacterial and intestinal enzyme activities further support associations between microbial enzymatic activity and host digestive capacity. Discussion and conclusion Overall, these findings suggest that the intestinal microbiota undergoes functional maturation in concert with host development and dietary transitions, potentially complementing host digestive physiology during stages of limited digestive capacity. This study provides the first quantitative framework for assessing the potential digestive contribution of the intestinal microbiota during fish larval ontogeny.