Data from: The dominant detritus-feeding invertebrate in arctic peat soils derives its essential amino acids from gut symbionts

Supplementation of nutrients by symbionts enables consumers to thrive on resources that might otherwise be insufficient to meet nutritional demands. Such nutritional subsidies by intracellular symbionts has been well studied; however, supplementation of de novo synthesized nutrients to hosts by extracellular gut symbionts is poorly documented, especially for generalists with relatively undifferentiated intestinal tracts. Although gut symbionts facilitate degradation of resources that would otherwise remain inaccessible to the host, such digestive actions alone cannot make up for dietary insufficiencies of macronutrients such as essential amino acids (EAA). Documenting whether gut symbionts also function as partners for symbiotic EAA supplementation is important because the question of how some detritivores are able to subsist on nutritionally insufficient diets has remained unresolved. To answer this poorly-understood nutritional aspect of symbiont-host interactions, we studied the enchytraeid worm, a bulk soil feeder that thrives in arctic peatlands. In a combined field and laboratory study, we employed stable isotope fingerprinting of amino acids to identify the biosynthetic origins of amino acids to bacteria, fungi and plants in enchytraeids. Enchytraeids collected from arctic peatlands derived more than 80% of their EAA from bacteria. In a controlled feeding study with the enchytraeid Enchytraeus crypticus, EAA derived almost exclusively from gut bacteria when the worms fed on higher fiber diets, whereas most of the enchytraeids' EAA derived from dietary sources when fed on lower fiber diets. Our gene sequencing results of gut microbiota showed that the worms harbor several taxa in their gut lumen absent from their diets and substrates. Almost all gut taxa are candidates for EAA supplementation because almost all belong to clades capable of biosynthesizing EAA. Our study provides the first evidence of extensive symbiotic supplementation of EAA by microbial gut symbionts, and demonstrate that symbiotic bacteria in the gut lumen appear to function as partners for both symbiotic EAA supplementation as well as for digestion of insoluble plant fibers.

共生体（symbionts）对营养物质的补充，可使消费者得以在原本无法满足自身营养需求的资源上存活繁衍。胞内共生体（intracellular symbionts）提供的这类营养补给已得到充分研究，但胞外肠道共生体（extracellular gut symbionts）向宿主供给从头合成营养物质的相关报道却极为匮乏，尤其是针对肠道结构相对未分化的广食性物种而言。尽管肠道共生体可协助宿主分解原本无法利用的底物，但仅靠这类消化作用，无法弥补宿主在宏量营养素如必需氨基酸（essential amino acids, EAA）方面的膳食缺口。探明肠道共生体是否同时充当共生性必需氨基酸补给的伙伴，具有重要研究价值——因为部分食腐动物如何依靠营养不足的膳食存活这一问题，至今仍未得到圆满解答。为解答这一鲜有研究的共生体-宿主互作的营养生态学问题，我们选取繁盛于北极泥炭地的线蚓科蠕虫（enchytraeid worm，一类土壤大宗取食者）作为研究对象。在结合野外调查与实验室控制的综合研究中，我们采用氨基酸稳定同位素指纹图谱法（stable isotope fingerprinting of amino acids），明确了线蚓体内氨基酸的生物合成来源，区分出其分别来自细菌、真菌与植物的占比。采集自北极泥炭地的野生线蚓，其体内超过80%的必需氨基酸均来源于细菌。在针对隐吻线蚓（Enchytraeus crypticus）的受控摄食实验中，当蠕虫以高纤维膳食为食时，其体内几乎所有必需氨基酸均来自肠道细菌；而当以低纤维膳食为食时，线蚓体内大部分必需氨基酸则直接来源于膳食本身。我们对肠道微生物群（gut microbiota）的基因测序结果显示，蠕虫的肠道腔中存在若干类群，且这些类群并未出现在其摄食的膳食与环境底物中。几乎所有肠道类群均具备提供必需氨基酸补给的潜力，因为它们均隶属于能够从头合成必需氨基酸的演化支。本研究首次证实了微生物肠道共生体可广泛为宿主提供共生性必需氨基酸补给，并证明肠道腔中的共生细菌，既可作为共生性必需氨基酸补给的伙伴，亦可参与难溶性植物纤维的消化过程。