1National Institute of Genetics, 2JSPS Research Fellow
Microbes living in extreme environments or those with extremely small cell sizes (ultramicrobacteria) are useful models for the study of microbial diversity and evolution. We describe representative microbes from: (1) extreme environments in the polar regions and (2) extremely small filterable bacteria. (1) The aquatic moss Leptobryum forms underwater tower-like structures called “moss pillars (Koke Bouzu)” in ultra-oligotrophic Antarctic lakes. Our objective was to understand how such a unique ecosystem survives in extreme environments. We used biochemical and molecular methods to study the microbiome of aquatic moss pillars, based on the fatty acid profile, rDNA genotyping, and metagenomic information. The bacterial communities and their functions varied in different regions in the pillar, as did the eukaryotic organisms, such as algae, fungi, nematodes, and tardigrades, that cohabit within the pillar. Thus, we hypothesize that the “pillar” is a community and habitat for phylogenetically diverse microbes. (2) Our objective was to identify the smallest size microbe. We cultured ultramicrobacteria from “filter-sterile” environmental samples filtered through 0.2-_m filters. One actinobacterial isolate was characterized by ultramicro-sized cells (less than 0.1 _m3) even under eutrophic culture conditions, and also had a small, streamlined genome (1.6 Mb) and low GC content (52%). The other proteobacterial isolate had a large filamentous morphology, although some of the cells exhibited a curved rod, spiral, or small spherical form. This cellular polymorphism may explain why it was filtered through a 0.2-_m filter. This was a new isolate belonging to the novel class Oligoflexia, a member of the phylum Proteobacteria. Our results indicate that unique microbes exist in neglected filterable fractions.