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要旨:
The genomes of the Asgard superphylum of Archaea hold clues pertaining
to the nature of the host cell that acquired the mitochondrion at the
origin of eukaryotes1-4. Representatives of the Asgard candidate phylum
Candidatus Lokiarchaeota (Lokiarchaeon) have the capacity for
acetogenesis and fermentation5-7, but how their metabolic activity
responds to environmental conditions is poorly understood. Here, we show
that in anoxic Namibian shelf sediments, Lokiarchaeon gene expression
levels are higher than those of bacterial phyla and increase with depth
below the seafloor. Lokiarchaeon gene expression was significantly
different across a hypoxic-sulfidic redox gradient, whereby genes
involved in growth, fermentation and H2-dependent carbon fixation had
the highest expression under the most reducing (sulfidic) conditions.
Quantitative stable isotope probing revealed that anaerobic utilization
of CO2 and diatomaceous extracellular polymeric substances by
Lokiarchaeon was higher than the bacterial average, consistent with
higher expression of Lokiarchaeon genes, including those involved in
transport and fermentation of sugars and amino acids. The quantitative
stable isotope probing and gene expression data demonstrate
homoacetogenic activity of Candidatus Lokiarchaeota, whereby
fermentative H2 production from organic substrates is coupled with the
Wood-Ljungdahl carbon fixation pathway8. The high energetic efficiency
provided by homoacetogenesis8 helps to explain the elevated metabolic
activity of Lokiarchaeon in this anoxic, energy-limited setting.
