Brine assemblages of ultrasmall microbial cells within the ice cover of Lake Vida, Antarctica

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The anoxic and freezing brine that permeates Lake Vida's perennial ice below 16mcontains an abundance of very small (≤0.2-μm) particles mixed with a less abundant population of microbial cells ranging from>0.2 to 1.5 μmin length. Fluorescent DNA staining, electron microscopy (EM) observations, elemental analysis, and extraction of high-molecular-weight genomic DNA indicated that a significant portion of these ultrasmall particles are cells. A continuous electron-dense layer surrounding a less electron-dense region was observed by EM, indicating the presence of a biological membrane surrounding a cytoplasm. The ultrasmall cells are 0.192±0.065 μ, with morphology characteristic of coccoid and diplococcic bacterial cells, often surrounded by iron-rich capsular structures. EM observations also detected the presence of smaller unidentified nanoparticles of 0.020 to 0.140 μmamong the brine cells. A 16S rRNA gene clone library from the brine 0.1- to 0.2-μm-size fraction revealed a relatively low-diversity assemblage of Bacteria sequences distinct from the previously reported>0.2-μm-cell-size Lake Vida brine assemblage. The brine 0.1- to 0.2-μm-size fraction was dominated by the Proteobacteria-affiliated genera Herbaspirillum, Pseudoalteromonas, and Marinobacter. Cultivation efforts of the 0.1- to 0.2-μm-size fraction led to the isolation of Actinobacteria-affiliated genera Microbacterium and Kocuria. Based on phylogenetic relatedness and microscopic observations, we hypothesize that the ultrasmall cells in Lake Vida brine are ultramicrocells that are likely in a reduced size state as a result of environmental stress or life cycle-related conditions. © 2014, American Society for Microbiology.

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Applied and Environmental Microbiology

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