Effects of biosecure macroalgae in formulated feeds on the gut bacterial microbiome of abalone Haliotis midae

Abstract Dietary macroalgae are known to contribute directly and indirectly to the gut microflora of abalone. However, certain farms require macroalgae to be made biosecure to eliminate microbiota and reduce their risk of acting as a vector for potential disease before being included as a dietary ingredient in aquafeeds. The aim of this study was to assess the effect of biosecure macroalgae on the gut bacterial microbiome of cultured abalone (shell length of 55–65 mm). Fresh macroalgae: brown seaweed Ecklonia maxima , green seaweed Ulva lacinulata , and red seaweed Gracilaria gracilis were subjected to a combination of three biosecurity processing treatments: disinfection with povo‐iodine solution, oven drying, and UVC treatment before being incorporated into feed and fed to abalone Haliotis midae for 150 days. Control diets that included no macroalgae or non‐biosecure macroalgae were also included in the trial. A 16S rDNA next‐generation sequencing (NGS) of the hypervariable V4 region demonstrated that the within‐sample bacterial diversity did not differ significantly across all the tested diets ( p > 0.05). No significant differences in the overall microbiome composition were detected, indicating a high degree of similarity of the bacterial communities across all the biosecure and non‐biosecure macroalgae diets. A similar core microbiome (SAR324 clade [Marine group B], Mycoplasma , and Psychrilyobacter ) was also observed in the digestive tracts of abalone fed with the biosecure and non‐biosecure diets, with fine‐scale differences between dietary treatments. It was concluded that the biosecure process previously shown to significantly reduce the abundance of bacteria, fungi/oomycetes, and viruses associated with each seaweed prior to incorporation into a formulated feed did not adversely affect other components of the seaweeds that may positively benefit the growth, health, and gut microbiome of the cultured abalone fed these feeds.