Sea urchin intestinal bacterial communities depend on seaweed diet and contain nitrogen-fixing symbionts.

Kelp deforestation by sea urchin grazing is a widespread phenomenon globally, with vast consequences for coastal ecosystems. The ability of sea urchins to survive on a kelp diet of poor nutritional quality is not well understood and bacterial communities in the sea urchin intestine may play an important role in digestion. A no-choice feeding experiment was conducted with the sea urchin Strongylocentrotus droebachiensis, offering three different seaweeds as diet, including the kelp Saccharina latissima. Starved sea urchins served as experimental control. Amplicons of the 16S rRNA gene were analyzed from fecal pellets. One dominant symbiont (Psychromonas marina) accounted for 44% of all sequence reads and was especially abundant in the sea urchins fed seaweed diets. The starved and field-captured sea urchins consistently displayed higher diversity than the seaweed-fed sea urchins. Cloning and sequencing of the nifH gene revealed diverse nitrogen fixers. We demonstrate that the sea urchin intestinal microbiome is dynamic, with bacterial communities that are plastic, depending on diet and have the capacity for nitrogen fixation. This reflects the dietary flexibility of these sea urchins, and their intestinal microbiota could be a key component in understanding catastrophic kelp forest grazing events.