Characterization of Bacterial Communities Associated with the Tyrian Purple Producing Gland in a Marine Gastropod

Abstract

Dicathais orbita is a marine mollusc recognised for the production of anticancer compounds that are precursors to Tyrian purple. This study aimed to assess the diversity and identity of bacteria associated with the Tyrian purple producing hypobranchial gland, in comparison with foot tissue, using a high-throughput sequencing approach. Taxonomic and phylogenetic analysis of variable region V1-V3 of 16S rRNA bacterial gene amplicons in QIIME and MEGAN were carried out. This analysis revealed a highly diverse bacterial assemblage associated with the hypobranchial gland and foot tissues of D. orbita. The dominant bacterial phylum in the 16S rRNA bacterial profiling data set was Proteobacteria followed by Bacteroidetes, Tenericutes and Spirochaetes. In comparison to the foot, the hypobranchial gland had significantly lower bacterial diversity and a different community composition, based on taxonomic assignment at the genus level. A higher abundance of indole producing Vibrio spp. and the presence of bacteria with brominating capabilities in the hypobranchial gland suggest bacteria have a potential role in biosynthesis of Tyrian purple in D. orbita.

Fig 1. Alpha diversity showing the richness of bacterial community diversity within Dicathais orbita foot (F2F, F3F, M2F and M3F) and hypobranchial gland samples (F1H, F2H, M1H and M2H) (F = female; M = male).

The phylogenetic diversity metric consists of genus richness based on 3585 observed OTUs at the 97% sequence similarity level and 443 possible observed genus. Sample with reads of more than 3000 are visible.

Fig 2. Phylum-level taxonomic diversity associated with the female (F) and male (M) hypobranchial gland (F1H, F2H, M1H and M2H) and foot (F2F, F3F, M2F and M3F) of Dicathais orbita bacterial profiling.

All the minor phyla and unnamed, but previously identified bacterial phyla (such as BD1-5, CKC4, candidate division BRC1, OD1, OP8, SR1, TM7, SHA-109, and TM6) are grouped into “Bacteria Other”.

Fig 3. Phylogenetic tree of Dicathais orbita samples generated from 16S rRNA sequences by MEGAN.

A = Female hypobranchial gland (F2H); B = Male hypobranchial gland (M1H); C = Female foot (F3F); D = Male foot (M3F). All these sample types have more than 15,000 reads.

Fig 4. Mean (+s.e.) number of OTUs in the hypobranchial gland and foot tissue of Dicathais orbita, showing the mean proportion unique to individuals samples of foot and hypobranchial gland tissue.

(A) = OTUs richness, (B) = H index/diversity.

Fig 5. Principal Coordinates Ordination (PCO) of bacterial genus composition, based on a Bray Curtis similarity matrix of the relative abundance of OTUs at 97% sequence similarity level for the hypobranchial gland (purple) and foot (orange) of female (F) and male (M) Dicathais orbita.

Fig 6. Venn diagram showing shared and non-shared bacterial species between the hypobranchial gland and foot of Dicathais orbita.

The number of species that have biosynthetic capabilities relevant to Tyrian purple production are highlighted in different colours (Orange = indole producers; Blue = brominating enzymes; Purple = indole producers and brominating capabilities).