Diplonemids are considered marine protists and have been reported among the most abundant and diverse eukaryotes in the world oceans. Recently we detected the presence of freshwater diplonemids in Japanese deep freshwater lakes. However, their distribution and abundances in freshwater ecosystems remain unknown. We assessed abundance and diversity of diplonemids from several geographically distant deep freshwater lakes of the world by amplicon‐sequencing, shotgun metagenomics and catalysed reporter deposition‐fluorescent in situ hybridization (CARD‐FISH). We found diplonemids in all the studied lakes, albeit with low abundances and diversity. We assembled long 18S rRNA sequences from freshwater diplonemids and showed that they form a new lineage distinct from the diverse marine clades. Freshwater diplonemids are a sister‐group to a marine clade, which are mainly isolates from coastal and bay areas, suggesting a recent habitat transition from marine to freshwater habitats. Images of CARD‐FISH targeted freshwater diplonemids suggest they feed on bacteria. Our analyses of 18S rRNA sequences retrieved from single‐cell genomes of marine diplonemids show they encode multiple rRNA copies that may be very divergent from each other, suggesting that marine diplonemid abundance and diversity both have been overestimated. These results have wider implications on assessing eukaryotic abundances in natural habitats by using amplicon‐sequencing alone.

中文翻译：

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双烯酮的淡水辐射。

Diplonemids被认为是海洋生物，据报道是世界海洋中最丰富多样的真核生物。最近，我们在日本深层淡水湖中检测到淡水二烯酮的存在。但是，它们在淡水生态系统中的分布和丰度仍然未知。我们通过扩增子测序，shot弹枪宏基因组学和催化的报告分子沉积-荧光原位分析法评估了世界上几个地理上较远的深水湖泊中双烯酮类化合物的丰度和多样性杂交（CARD-FISH）。我们在所有研究过的湖泊中都发现了双烯隆，尽管其丰度和多样性较低。我们从淡水二烯酮中组装了长的18S rRNA序列，并表明它们形成了不同于多种海洋进化枝的新谱系。淡水二甲龙类是海洋进化枝的姐妹群，主要是沿海和海湾地区的分离株，表明最近的生境从海洋生境过渡到了淡水生境。CARD-FISH靶向淡水二烯酮的图像表明它们以细菌为食。我们对从海洋二倍体动物的单细胞基因组中检索到的18S rRNA序列的分析表明，它们编码多个rRNA拷贝，这些拷贝可能彼此非常不同，这表明海洋二倍体动物的丰度和多样性都被高估了。