Bacterial phylotypes associated with the digestive tract of the sea urchin Paracentrotus lividus and the ascidian <Emphasis Type="Italic">Microcosmus</Emphasis> sp.

We used sequencing and phylogenetic analysis of PCR-amplified 16S rRNA genes from bacteria that are associated with the esophagus/pharynx, stomach and intestine of two marine sympatric invertebrates but with different feeding mechanisms, namely the sea urchin Paracentrotus lividus (grazer) and the ascidian Microcomus sp. (suspension feeder). Amplifiable DNA was retrieved from all sections except the pharynx of the ascidian. Based on the inferred phylogeny of the retrieved sequences, the sea urchin’s esophagus is mainly characterized mostly by bacteria belonging to α-, γ-Proteobacteria and Bacteriodetes, most probably originating from the surrounding environment. The stomach revealed phylotypes that belonged to γ- and δ-Proteobacteria, Verrucomicrobia and Fusobacteria. Since the majority of their closest relatives are anaerobic species and they could be putative symbionts of the P. lividus stomach, in which anaerobic conditions also prevail. Seven out of eight phylotypes found in the sea urchin’s intestine belonged to sulfate reducing δ-Proteobacteria, and one to γ-Proteobacteria, with possible nutritional activities, i.e. degradation of complex organic compounds which is beneficial for the animal. The bacterial phylotypes of the ascidian digestive tract belonged only to the phyla of Actinobacteria and Proteobacteria. The stomach phylotypes of the ascidian were related to pathogenic bacteria possibly originating from the water column, while the intestine seemed to harbour putative symbiotic bacteria that are involved in the degradation of nitrogenous and other organic compounds, thus assisting ascidian nutrition. The text was submitted by the authors in English.