Size Matters: Ultra-small and Filterable Microorganisms in the Environment

Area of Science:

• Microbiology
• Environmental Science
• Molecular Biology

Background:

• Ultra-small microorganisms, defined as having cell volumes <0.1 μm³, are numerically dominant in aquatic environments.
• Cultivated ultramicrobacteria, like SAR11 clade, possess streamlined genomes and unique traits.
• Many ultramicrobacteria can pass through 0.2-μm filters, challenging conventional sterilization methods.

Purpose of the Study:

• To review the diversity of filterable microorganisms often overlooked in environmental samples.
• To highlight the complexity of microbial communities passing through 0.2-μm filters.
• To discuss the implications of these findings for various scientific fields.

Main Methods:

• Review of cultivation-based studies on filterable microorganisms.
• Analysis of culture-independent "omics" approaches applied to filtered samples.
• Examination of microbial diversity in <0.2-μm filtered environmental samples.

Main Results:

• Filtered fractions contain ultramicrocells, ultramicrobacteria, filamentous bacteria (including Oligoflexia), Candidate Phyla Radiation (CPR) bacteria, and ultra-small DPANN archaea.
• CPR and DPANN groups exhibit minimal biosynthetic capacities and extremely small genomes.
• Filtered samples reveal a greater microbial diversity and complexity than previously assumed.

Conclusions:

• Standard 0.2-μm filtration does not yield sterile samples, as a diverse range of microorganisms can pass through.
• Overlooked filterable microbial agents, including CPR and DPANN, significantly expand our understanding of microbial diversity.
• Further research into these filterable microorganisms is crucial for comprehending their ecological roles and potential applications.