Effects of demand-feeding and dietary protein level on nitrogen metabolism and symbiont dinitrogen gas production of common carp (Cyprinus carpio, L.)

Abstract

Ammonia accumulation is a major challenge in intensive aquaculture, where fish are fed protein-rich diets in large rations, resulting in increased ammonia production when amino acids are metabolized as energy source. Ammonia is primarily excreted via the gills, which have been found to harbor nitrogen-cycle bacteria that convert ammonia into dinitrogen gas (N2) and therefore present a potential in situ detoxifying mechanism. Here, we determined the impact of feeding strategies (demand-feeding and batch-feeding) with two dietary protein levels on growth, nitrogen excretion, and nitrogen metabolism in common carp (Cyprinus carpio, L.) in a 3-week feeding experiment. Demand-fed fish exhibited significantly higher growth rates, though with lower feed efficiency. When corrected for feed intake, nitrogen excretion was not impacted by feeding strategy or dietary protein, but demand-fed fish had significantly more nitrogen unaccounted for in the nitrogen balance and less retained nitrogen. N2 production of individual fish was measured in all experimental groups, and production rates were in the same order of magnitude as the amount of nitrogen unaccounted for, thus potentially explaining the missing nitrogen in the balance. N2 production by carp was also observed when groups of fish were kept in metabolic chambers. Demand feeding furthermore caused a significant increase in hepatic glutamate dehydrogenase activities, indicating elevated ammonia production. However, branchial ammonia transporter expression levels in these animals were stable or decreased. Together, our results suggest that feeding strategy impacts fish growth and nitrogen metabolism, and that conversion of ammonia to N2 by nitrogen cycle bacteria in the gills may explain the unaccounted nitrogen in the balance.

Feeding strategy and feed protein level affect the gut microbiota of common carp (Cyprinus carpio)

“…This study is, to our knowledge, the first in which the effect of feeding regime on the gut microbial community is combined with individual physiological parameters. We hypothesised that demand feeding would affect gut microbiota composition to a larger extent than dietary protein content, in line with its effects on fish physiology found in the study with the same carp (Mes et al, 2023). We additionally hypothesised that microbial associations in the gut microbiota were affected by demand feeding.…”

“…In our previous study using the same carp, we found significant effects of demand feeding on specific growth rates and hepatic and intestinal GDH activities (Mes et al, 2023). Here, we determined if these individual physiological parameters could explain variations in gut microbiota composition.…”

“…However, in contrast to our study, the studies focusing on cyprinids were performed in pond culture, which may lead to different influences on the gut microbiota. Additionally, we utilised the ALDEx2 compositional differential abundance method to perform correlation analysis, which was recently found to be a more conservative test compared to the linear discriminant analysis with effect size test performed in the aforementioned studies (Nearing et al, 2022). This may have reduced the number of correlations we observed.…”

“…Carp were individually identified by passive integrated transponder tags (Nonatec™, Lutronic International, Rodange, Luxembourg) so individual growth rates and physiological parameters could be recorded. Fish were sampled at the end of the experiment, as described in detail in our earlier publication (Mes et al, 2023). Carp were divided into four groups based on the combination of feeding strategy (demand-fed vs. batch-fed) and dietary protein level (39% vs. 48% protein).…”

“…All samples were snap-frozen in liquid nitrogen and stored at À20 C until analysis. GDH activity rates in the liver and proximal intestine were measured as described and reported in Mes et al (2023) and were used here to establish potential correlations with gut microbial composition. From each of the four dietary combinations, six carp were used for analysing the gut content microbial composition.…”