Bacterial Diversity Associated with <i>Cinachyra cavernosa</i> and <i>Haliclona pigmentifera</i>, Cohabiting Sponges in the Coral Reef Ecosystem of Gulf of Mannar, Southeast Coast of India

Sponges are abundant, diverse and functionally important organisms of coral reef ecosystems. Sponge-associated microorganisms have been receiving greater attention because of their significant contribution to sponge biomass, biogeochemical cycles and biotechnological potentials. However, our understanding of the sponge microbiome is limited to a few species of sponges from restricted geographical locations. Here, we report for the first time the bacterial diversity of two cohabiting sponges, viz. Cinachyra cavernosa and Haliclona pigmentifera, as well as that in the ambient water from the coral reef ecosystems of the Gulf of Mannar, located along the southeast coast of India. Two hundred and fifty two clones in the 16S rRNA gene library of these sponges were grouped into eight distinct phyla, of which four belonged to the core group that are associated only with sponges. Phylogenetic analysis of the core bacteria showed close affinity to other sponge-associated bacteria from different geographical locations. γ-Proteobacteria, Chloroflexi, Planctomycetes and Deferribacter were the core groups in C. cavernosa while β and δ-Proteobacteria performed this role in H. pigmentifera. We observed greater OTU diversity for C. cavernosa (Hǀ 2.07) compared to H. pigmentifera (Hǀ 1.97). UniFrac analysis confirmed the difference in bacterial diversity of the two sponge species and also between the sponges and the reef water (p<0.001). The results of our study restate the existence of a host driven force in shaping the sponge microbiome.海绵是珊瑚礁生态系统中数量丰富、类群多样且功能至关重要的生物类群。海绵共生微生物因其对海绵生物量、生物地球化学循环以及生物技术应用潜力的重要贡献，日益受到学界关注。然而，目前学界对海绵微生物组（sponge microbiome）的认知仅局限于少数来自受限地理区域的海绵物种。本研究首次报道了印度东南沿海马纳尔湾珊瑚礁生态系统中两种共栖海绵——*Cinachyra cavernosa*与*Haliclona pigmentifera*——及其周边水体的细菌多样性。本研究从两种海绵的16S rRNA基因文库中获取的252个克隆子，可划分为8个不同的细菌门，其中4个属于仅与海绵共生的核心类群。对核心细菌的系统发育分析显示，其与来自不同地理区域的其他海绵共生细菌具有较近亲缘关系。*Cinachyra cavernosa*的核心类群包括γ-变形菌纲（γ-Proteobacteria）、绿弯菌门（Chloroflexi）、浮霉菌门（Planctomycetes）与脱铁杆菌门（Deferribacter）；而*Haliclona pigmentifera*的核心类群则为β-变形菌纲（β-Proteobacteria）与δ-变形菌纲（δ-Proteobacteria）。相较于*Haliclona pigmentifera*（H'=1.97），*Cinachyra cavernosa*（H'=2.07）的操作分类单元（Operational Taxonomic Unit，OTU）多样性更高。UniFrac分析证实，两种海绵的细菌多样性存在显著差异，且海绵与礁体周边水体的细菌多样性亦存在显著差异（p<0.001）。本研究结果进一步证实，宿主驱动因素是塑造海绵微生物组的核心调控力量。