BackgroundHolobionts have a digestive microbiota with catabolic abilities allowing the degradation of complex dietary compounds for the host. In terrestrial herbivores, the digestive microbiota is known to degrade complex polysaccharides from land plants while in marine herbivores, the digestive microbiota is poorly characterized. Most of the latter are generalists and consume red, green, and brown macroalgae, three distinct lineages characterized by a specific composition in complex polysaccharides, which represent half of their biomass. Subsequently, each macroalga features a specific epiphytic microbiota, and the digestive microbiota of marine herbivores is expected to vary with a monospecific algal diet. We investigated the effect of four monospecific diets (Palmaria palmata, Ulva lactuca, Saccharina latissima, Laminaria digitata) on the composition and specificity of the digestive microbiota of a generalist marine herbivore, the abalone, farmed in a temperate coastal area over a year. The microbiota from the abalone digestive gland was sampled every 2 months and explored using metabarcoding.ResultsDiversity and multivariate analyses showed that patterns of the microbiota were significantly linked to seasonal variations of contextual parameters but not directly to a specific algal diet. Three core genera: Psychrilyobacter, Mycoplasma, and Vibrio constantly dominated the microbiota in the abalone digestive gland. Additionally, a less abundant and diet-specific core microbiota featured genera representing aerobic primary degraders of algal polysaccharides.ConclusionsThis study highlights the establishment of a persistent core microbiota in the digestive gland of the abalone since its juvenile state and the presence of a less abundant and diet-specific core community. While composed of different microbial taxa compared to terrestrial herbivores, the digestive gland constitutes a particular niche in the abalone holobiont, where bacteria (i) may cooperate to degrade algal polysaccharides to products assimilable by the host or (ii) may have acquired these functions through gene transfer from the aerobic algal microbiota.Electronic supplementary materialThe online version of this article (10.1186/s40168-018-0430-7) contains supplementary material, which is available to authorized users.

The commercial culture of Haliotis tuberculata has recently started in Europe. As abalone is herbivorous, the use of local collected algae as feed may appear advantageous. The nutritional value of eight monospecific seaweed diets was studied using Palmaria palmata (Rhodophyta), filamentous algae, mainly Gracilaria sp. (Rhodophyta), Enteromorpha sp. and Ulva lactuca (Chlorophyta), together with Saccharina latissima, Saccorhiza polyschides, Laminaria digitata and Laminaria hyperborea (Ochrophyta, Phaeophyceae) and a mixed macroalgal diet. An integrative approach consisted in monitoring the seasonal composition changes of these algae in terms of protein, lipid, soluble carbohydrate, fatty acid and amino-acid contents, and to relate it to seasonal growth and reproduction investment during a largescale experiment. Abalone and algae were studied for one year in commercial sea-cage structures. Abalone fed with monospecific diet using either P. palmata or S. latissima, and with mixed diet presented the best growth rate, muscle ratio and gonad development. Seasonal daily weight gain was mainly associated with n-3/n-6 ratio, soluble carbohydrate content and total protein content. In term of aminoacid contents, the daily weight gain was associated with free phenylalanine as well as isoleucine levels. Moreover, 90% of 2-years old abalone started gonad development but less than a quarter featured a fully matured gonad. The gonad development of H. tuberculata was mostly associated to total valine, methionine, leucine, arginine and isoleucine levels. The age of initial sexual maturity in H. tuberculata turned to be a highly plastic trait in response to different growth rates and algal diets. Even if P. palmata is the best option for growth performance, mixed diets should probably be preferred to a monospecific diet in order to avoid too high pressure on a single algal resource. Highlights ► Mixed diets conferred higher fitness than the average of single-species diets, but not for the best single species ► Abalone are likely to invest more energy in muscle and gonad developments when fed on Palmaria palmata. ► At 2-year-old, an average of 90% of H. tuberculata started gonad development but <23% of the cohort had a full development ► Soluble carbohydrate, n-3/n-6 and total protein are the major algal components for reproduction and growth performance