Abstract
Nano-sized and filterable microorganisms are thought to represent the smallest living organisms on earth and are characterized by their small size (50–400 nm) and their ability to physically pass through <0.45 μm pore size filters. They appear to be ubiquitous in the biosphere and are present at high abundance across a diverse range of habitats including oceans, rivers, soils, and subterranean bedrock. Small-sized organisms are detected by culture-independent and culture-dependent approaches, with most remaining uncultured and uncharacterized at both metabolic and taxonomic levels. Consequently, their significance in ecological roles remain largely unknown. Successful isolation, however, has been achieved for some species (e.g., Nanoarchaeum equitans and “Candidatus Pelagibacter ubique”). In many instances, small-sized organisms exhibit a significant genome reduction and loss of essential metabolic pathways required for a free-living lifestyle, making their survival reliant on other microbial community members. In these cases, the nano-sized prokaryotes can only be co-cultured with their ‘hosts.’ This paper analyses the recent data on small-sized microorganisms in the context of their taxonomic diversity and potential functions in the environment.
Highlights
Recent technological advances in microbiology have helped to reveal the enormous diversity of prokaryotic life on our planet (Kuczynski et al, 2010; Caporaso et al, 2011; Thompson et al, 2017)
The second scenario conjunctures that some distinct taxa, independently of growth conditions, nutrients’ availability or age of their culture do constantly exhibit small cell sizes. One source describes these organisms in the following way: the microorganisms must be 0.1 μm3 or smaller (
Some studies showed that the results were inconclusive, meaning that there were conflicting conclusions in the literature. ∗Colonies were slow-growing taking up to a few months to become visible. ∗∗Can be co-cultured in association with the host. ∗∗∗Proposed Candidatus status. †Parasitic ultramicrobacteria discussed in Duda et al (2012) review
Summary
Recent technological advances in microbiology have helped to reveal the enormous diversity of prokaryotic life on our planet (Kuczynski et al, 2010; Caporaso et al, 2011; Thompson et al, 2017). While this has enabled us to characterize and map prokaryote populations across a diverse array of ecosystems, the functional role of most of these organisms remains unknown, due to our inability to culture, and study them in the laboratory. As the review of Duda et al (2012) discusses a number of issues related with ultramicrobacteria, the aim of present review was to highlight the latest discoveries related to (1) taxonomic diversity, (2) biogeography, (3) current experimental approaches to characterize these organisms and (iv) potential role of ultra-small Bacteria and Archaea within a contrasting range of environments
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