The sea urchin, Strongylocentrotus droebachiensis, is a prominent grazer in coastal ecosystems with the ability to graze down large areas of kelp forests and turn them into marine deserts, or so called barren grounds. Kelp are usually avoided by most grazers due to low protein content, thick leaf structure and grazing deterrents. The ability to consume kelp might be caused by a bacterial symbiosis in the sea urchin gut, where the bacteria contribute to the degradation of tough structural components and provide nutrients through nitrogen fixation. To investigate this, the aim of this thesis was to study how the bacterial diversity and composition of bacteria responded to various diets, and if bacteria related to the degradation of structural components and nitrogen fixation could be identified in the sea urchin gut. In order to investigate this, a no-choice feeding experiment was conducted, and the diets were: Saccharina latissima (kelp), Fucus serratus (wrack) and Palmaria palmata (dulse, red algae), each representing a uniform diet. Starved urchins served as control, and sea urchins collected prior to the experiment served as reference for natural microbiomes. The bacterial communities were analyzed from sequenced 16S rRNA gene fragments, and 614 amplicon sequence variants (ASVs) were identified, and one ASV (related to Psychromonas marina sp. nov.) accounted for 44 % of the total sequence reads and was present in all samples, suggesting that this is an important symbiont in the sea urchins. The sea urchins given uniform diets, had a significantly lower ASV diversity compared to the control and reference samples. This indicates that uniform diets promote dominant bacterial groups and a lower richness compared to the reference samples, and thus the sea urchins in nature, were probably not restricted to only one type of food source. Several types of ASVs were found related to bacteria that can degrade structural components like alginate, and possible nitrogen fixing representatives, although the latter was likely a bit restrained due to the low C:N ratio of the kelp which indicated a high nitrogen content. These findings can provide a better understanding of how sea urchins in nature are able to survive on high-carbon food sources like kelp.