一種當“甩手掌櫃”的蠕蟲（Symbiosis: doing without）

Symbiosis insights through metagenomic analysis of a microbial consortium

Olavius algarvensis是一種海洋寡毛綱蠕蟲，存在於地中海厄爾巴島沿海淺水沙子中。它們的解剖特征是引人注目的，因為它們不但沒有嘴、胃和腸，而且沒

有腎管（nephridia），即腎髒一樣的器官。消化係統的省略出現在其他動物中，但這種動物是惟一省略了自己的排泄係統以適應共生方式的寄主。現在，生

活在這些蠕蟲皮膚下麵的共生細菌群落的基因組成已被確定。多源基因組分析揭示了如此多的重要寄主任務何以被外包給了共生體。（Letter p. 950; News

and Views）

Keywords: metagenomic analysis, inability to cultivate most host-associated microbes, metabolic pathway reconstruction

Abstract: Symbioses between bacteria and eukaryotes are ubiquitous, yet our understanding of the interactions driving these associations is hampered by our inability to cultivate most host-associated microbes. Here we use a metagenomic approach to describe four co-occurring symbionts from the marine oligochaete Olavius algarvensis, a worm lacking a mouth, gut and nephridia. Shotgun equencing and metabolic pathway reconstruction revealed that the symbionts are sulphur-oxidizing and sulphate-reducing bacteria, all of which are capable of carbon fixation, thus providing the host with multiple reources of nutrition. Molecular evidence for the uptake and recycling of wormwaste products by the symbionts suggests how the wormcould eliminate its excretory system, an adaptation unique among annelid worms. We propose a model that describes how the versatile metabolism within this symbiotic consortium provides the host with an optimal energy supply as it shuttles between the upper oxic and lower anoxic coastal sediments that it inhabits.