Evidence for an active rare biosphere within freshwater protists community

Studies on the active rare biosphere at the RNA level are mainly focused on Bacteria and Archaea and fail to include the protists, which are involved in the main biogeochemical cycles of the earth. In this study, the richness, composition and activity of the rare protistan biosphere were determined from a temporal survey of two lakes by pyrosequencing. In these ecosystems, the always rare OTUs represented 77.2% of the total OTUs and 76.6% of the phylogenetic diversity. From the various phylogenetic indices computed, the phylogenetic units (PUs) constituted exclusively by always rare OTUs were discriminated from the other PUs. Therefore, the rare biosphere included mainly taxa that are distant from the reference databases compared to the dominant ones. In addition, the rarest OTUs represented 59.8% of the active biosphere depicted by rRNA and the activity (rRNA:rDNA ratio) increased with the rarity. The high rRNA:rDNA ratio determined in the rare fraction highlights that some protists we...

目前针对RNA水平的活性稀有生物圈（rare biosphere）的研究，主要聚焦于细菌（Bacteria）与古菌（Archaea），却未纳入参与地球主要生物地球化学循环的原生生物（protists）。本研究通过对两座湖泊的时序调查，采用焦磷酸测序（pyrosequencing）技术，解析了稀有原生生物生物圈的丰度、组成与活性。在这些生态系统中，始终稀有操作分类单元（Operational Taxonomic Units，OTUs）占总OTUs的77.2%，占系统发育多样性的76.6%。通过计算得到的多项系统发育指数，可将仅由始终稀有OTUs构成的系统发育单元（phylogenetic units，PUs）与其余PUs区分开来。由此可见，相较于优势类群，稀有生物圈主要包含与参考数据库亲缘关系较远的类群。此外，最稀有的OTUs占以核糖体RNA（ribosomal RNA，rRNA）表征的活性生物圈的59.8%，且活性（即rRNA与核糖体DNA（ribosomal DNA，rDNA）的比值）随稀有程度升高而增强。本研究在稀有组分中检测到的高rRNA:rDNA比值表明，部分原生生物我们...