LAUSR

Ascidians host abundant and diverse bacterial symbionts, and ascidian-associated bacteria have been shown to benefit ascidian hosts. However, these host-symbiont interactions are often overlooked in coral reef ecosystems, where ascidians frequently occur as key components. In this study, we explored the bacterial communities associated with Atriolum robustum, a common ascidian from Xisha coral reef, China, using Illumina HiSeq high-throughput sequencing of the 16S rRNA gene amplicon. A total of 1411 OTUs were identified from A. robustum, revealing a high bacterial diversity with a Shannon index of 6.26 ± 1.04. Proteobacteria (in particular Alphaproteobacteria and Gammaproteobacteria) were dominant in A. robustum, followed by Bacteroidetes and Chloroflexi. Ten OTUs comprised the core microbiome of A. robustum, matching taxa with reported capabilities for heavy metal resistance, nutrient cycling, and defense against pathogens. Predictive metabolic profiles of the bacterial community associated with A. robustum, using Tax4Fun, showed a high abundance of genes involved in carbohydrate metabolism and amino acid metabolism. A comparison of the bacterial composition and predictive functions associated with A. robustum and other reported ascidians, suggested that horizontal transmission played a key role in the formation of bacterial communities in A. robustum. It was found that A. robustum from Xisha coral reef harbored a great diversity of bacteria, and bacterial symbionts had a potential functional role in metabolism and in the immune response of their ascidian host, and so played a beneficial role in maintaining the health of the coral reef ecosystem.