Similarly, when the structure of the bacterial community associated with the abalone digestive gland was investigated at the OTU level (Fig. 1), diet had only a minor impact on the bacterial community (ANOSIM, R = 0.05, P < 0.01, Additional file 3: Table S2). However, there was a clear grouping of samples according to season and date (R = 0.36 and R = 0.43, respectively, P < 0.01). When considering each monospecific diet individually, bacterial communities associated with P. palmata digestion were more different between sampling dates than between the cage triplicates (ANOSIM: R = 0.41–1). Bacterial communities associated with the U. lactuca diet showed noticeable differences between the beginning of the experiment (February–April 2012) and January 2013 (R = 0.83). Seasonal effects were stronger for the L. digitata diet than for the other diets, with samples from colder months separated from those from warmer months. For the microbial communities associated with the S. latissima diet, bacterial communities sampled in January 2013 were significantly different from other dates (R > 0.8, P < 0.001). Overall, diversity patterns of the abalone digestive microbiota showed seasonal patterns specific to a given monospecific diet.Fig. 1Community structure of the digestive microbiota of the abalone over 1 year. Samples were grouped a posteriori according to one of the four monospecific algal diets (a) or a sampling date (b). The Bray-Curtis dissimilarity index was calculated after Hellinger transformation of the metabarcoding data set at the OTU level. The low stress value of 18% validates the goodness-of-fit of the two-dimensional representation compared with the original matrix. Significance between groupings of samples according to algal diet, sampling date, or season was tested using ANOSIM (Additional file 3: Table S2)