Effect of dietary phytase on water and fecal prokaryotic and eukaryotic microbiomes in a hybrid tilapia (Oreochromis aureus x O. niloticus) mixotrophic biofloc production system

Authors: Ray, Candis; Abernathy, Jason; Green, Bartholomew - Bart; Rivers, Adam; Schrader, Kevin; Rawles, Steven - Steve; McEntire, Matthew - Matt; Lange, Miles; Webster, Carl

Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/27/2023
Publication Date: 1/15/2024
Citation: Ray, C.L., Abernathy, J.W., Green, B.W., Rivers, A.R., Schrader, K., Rawles, S.D., McEntire, M.E., Lange, M.D., Webster, C.D. 2024. Effect of dietary phytase on water and fecal prokaryotic and eukaryotic microbiomes in a hybrid tilapia (Oreochromis aureus x O. niloticus) mixotrophic biofloc production system. Aquaculture. 581. Article 740433. https://doi.org/10.1016/j.aquaculture.2023.740433.
DOI: https://doi.org/10.1016/j.aquaculture.2023.740433

Interpretive Summary: Fish feed typically is supplemented with inorganic phosphorus to ensure adequate dietary phosphorus availability, especially in diets that contain high percentages of plant feedstuffs, which is the case for diets formulated for tilapia. Phosphorus is present in plant feedstuffs as phytate, which is not bioavailable to fish because their digestive system lacks sufficient phytase enzyme. Thus, phytate is excreted unmetabolized and accumulates to high concentrations in the environment. Including phytase in the feed formulation increases phosphorus bioavailability but affects the microbial communities in the culture water and tilapia gastrointestinal tract. Optimal utilization of supplemental dietary phytase requires an understanding of its influence on microbial community dynamics in biofloc water and the tilapia gastrointestinal tract during growth to market size. Hybrid tilapia were grown for five months to market size in outdoor biofloc production system tanks and fed either a phosphorus-free practical diet treated with phytase or a positive and negative control practical diets. The bacterial and eukaryotic populations in the biofloc tank water and hybrid tilapia gastrointestinal tract were sampled and characterized at four time points throughout the experiment. Overall abundance of Fusobacteria (namely Cetobacterium) and the protist Capsaspora predominated in the tilapia gut, regardless of test diet, indicating that the main driver of the gut microbiome is the biofloc rather than the manufactured diet. Compared to gut content samples, planktonic bacterial and eukaryotic communities in biofloc water exhibited greater diversity. This study demonstrated that dietary sources with variation in bioavailable P can significantly impact prokaryotic and eukaryotic communities in tilapia biofloc aquaculture and increases our understanding of synergistic fish-microbial interactions in response to dietary phytase supplementation.

Technical Abstract: In this study we evaluated feed and temporal effects on prokaryotic (16S) and eukaryotic (18S) microbiome dynamics in an outdoor mixotrophic biofloc system used to grow hybrid tilapia (Oreochromis aureus x O. niloticus) to market size. The dicalcium phosphate -free practical feed treated with 6-phytase (3000 FTU/kg; P-Free+Phytase) and positive (Control) and negative (P-Free) control practical feeds used were formulated to contain 27.7% digestible protein and 4% lipid. Sex reversed, all-male hybrid tilapia (28.9 g/fish) were stocked (29 fish/ m3) in 9 outdoor rectangular tanks (16.6 m3) and grown for 154 days. The Control feed was fed to all fish during the 14-day acclimation period after which each test feed was fed to fish in three tanks/feed. Biofloc water and fecal samples were collected from all tanks at time 0 (T0, study day 15) and T7, T70, and T140; prokaryotic and eukaryotic abundance and diversity were assessed using high-throughput amplicon sequencing of the 16S and 18S rRNA genes, respectively. Fecal 16S microbiome in all treatments was dominated by Fusobacteria (namely Cetobacterium) with minority representation by Proteobacteria, Firmicutes, Bacteroidetes, and Planctomycetes. Opisthokonts (namely Capsaspora) were the dominant eukaryote detected overall in all fecal samples. Fecal 16S and 18S relative abundances varied significantly with time. Relative abundance of fecal 16S differed significantly between Control and P-Free+Phytase feed treatments only at T140 (harvest). Fecal 18S relative abundance differences among feed treatments were detected only at T0 and T7 and only at the supergroup taxonomic level. Prokaryotic communities in biofloc water among feed treatments and across times were highly diverse and dominated by Proteobacteria, Bacteroidetes, Fusobacteria, Chloroflexi, Planctomycetes, and Actinobacteria. High relative abundances of Capsaspora across time characterized eukaryotic communities. Relative abundances of 16S and 18S taxa in water varied significantly over time. Control and P-Free feed 16S relative abundances in water differed significantly at T0 and T7. Time was the factor that explained most of the variation observed for 16S and 18S beta diversity. Overall, results of the present experiment suggest minimal influence of phytase-treated or dicalcium phosphate-free feeds on hybrid tilapia 16S and 18S water and fecal microbiomes throughout the experiment, particularly because the base feed formulation was the same for all treatments. During this experiment and at harvest tilapia in all treatments appeared healthy and robust, and no known pathogenic bacteria or eukaryotes were detected in any sample, suggesting the observed microbiomes may represent system-specific “healthy” core microbiomes.