Microbial Communities in Sunken Wood Are Structured by Wood-Boring Bivalves and Location in a Submarine Canyon

The cornerstones of sunken wood ecosystems are microorganisms involved in cellulose degradation. These can either be free-living microorganisms in the wood matrix or symbiotic bacteria associated with wood-boring bivalves such as emblematic species of Xylophaga, the most common deep-sea woodborer. Here we use experimentally submerged pine wood, placed in and outside the Mediterranean submarine Blanes Canyon, to compare the microbial communities on the wood, in fecal pellets of Xylophaga spp. and associated with the gills of these animals. Analyses based on tag pyrosequencing of the 16S rRNA bacterial gene showed that sunken wood contained three distinct microbial communities. Wood and pellet communities were different from each other suggesting that Xylophaga spp. create new microbial niches by excreting fecal pellets into their burrows. In turn, gills of Xylophaga spp. contain potential bacterial symbionts, as illustrated by the presence of sequences closely related to symbiotic bacteria found in other wood eating marine invertebrates. Finally, we found that sunken wood communities inside the canyon were different and more diverse than the ones outside the canyon. This finding extends to the microbial world the view that submarine canyons are sites of diverse marine life.

沉木生态系统的核心基石是参与纤维素降解的微生物。此类微生物既包括栖息于木材基质中的自由生活微生物，也包括与蛀食木材的双壳类动物共生的细菌——其中最具代表性的类群便是最为常见的深海蛀木生物蛀木蛤属（Xylophaga）物种。本研究采用实验沉水松木样本，分别部署于地中海布拉内斯海底峡谷的内部与外部区域，以此对比木材表面、蛀木蛤属（Xylophaga）物种粪粒以及该类动物鳃部附着的微生物群落。基于16S rRNA细菌基因标签焦磷酸测序的分析结果显示，沉木样本中存在三类显著分化的微生物群落。木材与粪粒的微生物群落结构存在明显差异，表明蛀木蛤属（Xylophaga）物种可通过向自身洞穴排泄粪粒的方式，构建全新的微生物生态位。此外，蛀木蛤属（Xylophaga）物种的鳃部携带有潜在的细菌共生体，这一结论可由其序列与其他植食性海洋无脊椎动物体内发现的共生细菌高度相似得到佐证。最后，本研究发现峡谷内部的沉木微生物群落与外部群落存在显著差异，且其多样性更为丰富。该发现将‘海底峡谷是多样海洋生物栖息地’的观点拓展至微生物研究领域。