Bacterial community composition in gut content and ambient sediment of two tropical wild sea cucumbers (Holothuria atra and H. leucospilota)

Abstract

Sea cucumbers play an important role in sediment bioturbation in coral reef and rocky intertidal ecosystems, and bacteria were a commonly-reported component of holothuroid diets. Bacterial community composition in the foregut and hindgut of two common tropical sea cucumbers (Holothuria atra and H. leucospilota) and the ambient surface sediment were investigated using high throughput sequencing based on 16S rRNA gene analysis. A total of 5 584 operational taxonomic units (OTUs) were obtained from 25 samples based on a 97% threshold, and more than half of the OTUs (n=3 694, 66.2%) were shared by the gut contents of two species of sea cucumbers and surrounding sediments. Bacterial richness and diversity in sediment samples were significantly higher than those in the gut content samples (P<0.05). Proteobacteria was the predominant phylum in most samples showing 45.69%±8.61%, 70.09%±4.03%, 45.88%±5.38%, and 55.19%±0.79% reads in the foregut of H. leucospilota, hindgut of H. leucospilota, hindgut of H. atra, and sediment libraries, respectively, but Bacteroidetes was the predominant phylum with the relative content of 34.98%±5.52% in the foregut of H. atra. Among the dominant genera, reads related to the genera Anderseniella, Ilumatobacter, and Ruegeria were detected in all the gut contents and sediment libraries. A comparison of gut bacteria community between the two species of sea cucumbers suggested that H. atra had stronger feeding preference than H. leucospilota, and the same types of microbes escaped digestion of the two sea cucumber species. Obvious different bacterial community composition in the foreguts of the two species of sea cucumbers and the surrounding sediments might result from the animal’s selective feeding for sediment patches. Bacterial community structure in hindgut contents of H. atra and H. leucospilota both differed clearly from adjacent sediments, which indicated feeding activity of deposit-feeding sea cucumbers could change the sedimental bacterial composition. In conclusion, from the perspective of bacteria, sea cucumber H. atra and H. leucospilota had different feeding preferences, yet they could both affect bacterial composition in sediments by feeding activity. The motivation for selective feeding and sea cucumber-sediment interaction might be explored in the future.