We selected four local monospecific macroalgal diets based on abalone food preferences, nutritional quality, and on their biochemical composition: the brown algae Saccharina latissima and Laminaria digitata, the red alga Palmaria palmata, and the green alga Ulva lactuca. We determined the composition of the microbiota from the abalone digestive gland, and we tested the following hypotheses: (i) the marine herbivore gastrointestinal tract offers a specific niche for specific marine bacteria, (ii) algal diet, host characteristics, and time influence microbiota composition and fluctuations, (iii) specific microbial groups are associated with the digestion of a monospecific macroalgal diet, allowing for the abalone’ polyphagy.