Researchers isolate archaea strains closely related to eukaryotes

Researchers have reported the isolation of two archaeal strains, designated HC1 and SC1, which belong to the Heimdallarchaeia lineage, the group that branches closest to eukaryotes.

Cultivation and Characteristics

The two strains were isolated from a biofilm in a gas production well in the Boso Peninsula, Chiba Prefecture, Japan. The biofilm is located in methane-bearing saline groundwater and is intermittently exposed to air due to periodic eruptions of brine.

According to the study published in Nature, the strains were isolated using the following methods:

• Strain HC1: Isolated in a pure co-culture with the methanogen Methanocalculus sp. strain MC2.
• Strain SC1: Isolated in a pure tri-culture with Methanocalculus sp. strain MC3 and Methanolobus sp. strain MLB.

Both strains are described as obligately anaerobic, syntrophic, peptide-degrading, and mesophilic archaea. While HC1 possesses genes associated with aerobic lifestyles, such as cytochrome c oxidase (Cox), the co-cultures cannot grow under (micro)oxic conditions. The study suggests that in HC1, Cox may support oxygen detoxification rather than respiration.

Cellular Structure and Genomics

Microscopic analysis of the cells revealed several physiological features:

• Morphology: The cells are coccoid and produce protrusions and vesicles.
• Internal Structure: The cells lack persistent internal membrane structures, such as an endomembrane system.
• Cytoskeleton: Cryo-EM showed that the cytoplasm and protrusions contain actin-filament-like structures.
• Division: The cells divide through binary fission and exhibit a ring-like structure during the process.

Genomic sequencing showed that HC1 has a genome of 7.91 Mb, while SC1 has a genome of 6.75 Mb. Both strains possess eukaryotic signature proteins (ESPs), with HC1 and SC1 having 27 and 29 types of ESPs, respectively.

Implications for Eukaryogenesis

The findings provide data for the study of eukaryogenesis, the process by which eukaryotic cells emerged. The study suggests that the archaeal ancestor of eukaryotes may have shared certain features with these isolates, including anaerobiosis, the absence of persistent intracellular membranes, and the presence of membrane protrusions.

According to the researchers, the transition from archaea to eukaryotes likely involved a steep transition in cell structure and a physiological shift from anaerobiosis to (facultative) aerobiosis. They suggest that early interactions between the archaeal host and the mitochondrial ancestor may have occurred under low-oxygen conditions and were at least partially mediated by oxygen detoxification.

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Sources

• nature.com: Eukaryotes’ closest relatives are internally simple syntrophic peptidotrophic archaea | Nature

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