Ultra-small microorganisms are ubiquitous in Earth’s environments. Ultramicrobacteria, which are defined as having a cell volume of <0.1 μm3, are often numerically dominant in aqueous environments. Cultivated representatives among these bacteria, such as members of the marine SAR11 clade (e.g., “Candidatus Pelagibacter ubique”) and freshwater Actinobacteria and Betaproteobacteria, possess highly streamlined, small genomes and unique ecophysiological traits. Many ultramicrobacteria may pass through a 0.2-μm-pore-sized filter, which is commonly used for filter sterilization in various fields and processes. Cultivation efforts focusing on filterable small microorganisms revealed that filtered fractions contained not only ultramicrocells (i.e., miniaturized cells because of external factors) and ultramicrobacteria, but also slender filamentous bacteria sometimes with pleomorphic cells, including a special reference to members of Oligoflexia, the eighth class of the phylum Proteobacteria. Furthermore, the advent of culture-independent “omics” approaches to filterable microorganisms yielded the existence of candidate phyla radiation (CPR) bacteria (also referred to as “Ca. Patescibacteria”) and ultra-small members of DPANN (an acronym of the names of the first phyla included in this superphyla) archaea. Notably, certain groups in CPR and DPANN are predicted to have minimal or few biosynthetic capacities, as reflected by their extremely small genome sizes, or possess no known function. Therefore, filtered fractions contain a greater variety and complexity of microorganisms than previously expected. This review summarizes the broad diversity of overlooked filterable agents remaining in “sterile” (<0.2-μm filtered) environmental samples.

中文翻译：

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大小很重要：环境中的超小和可过滤微生物

超小型微生物在地球环境中无处不在。被定义为细胞体积 <0.1 μm3 的超微细菌通常在水环境中在数量上占主导地位。这些细菌中的培养代表，例如海洋 SAR11 进化枝（例如，“Candidatus Pelagibacter ubique”）和淡水放线菌和 Betaproteobacteria 的成员，具有高度精简的小基因组和独特的生态生理特征。许多超微细菌可能会通过孔径为 0.2μm 的过滤器，该过滤器通常用于各个领域和过程中的过滤除菌。专注于可过滤小微生物的培养工作表明，过滤后的部分不仅包含超微细胞（即由于外部因素导致的微型细胞）和超微细菌，但也有细长的丝状细菌，有时带有多形性细胞，特别提到了 Oligoflexia 的成员，这是变形菌门的第八类。此外，独立于培养物的可过滤微生物“组学”方法的出现产生了候选门辐射 (CPR) 细菌（也称为“Ca. Patescibacteria”）和 DPANN 的超小成员（名称的首字母缩写词）包括在这个超门中的第一个门）古细菌。值得注意的是，CPR 和 DPANN 中的某些群体被预测具有很小或很少的生物合成能力，这反映在它们极小的基因组大小上，或者没有已知的功能。因此，过滤后的部分含有比以前预期的更多种类和复杂性的微生物。