Summary
A bioRxiv preprint uses single-cell transcriptomics to reconstruct an unusual anaerobic mitochondrion in the rhizarian flagellate PCE SSF. The proposed metabolism includes hydrogen production and a reduced, branched pathway for processing carbon.
A bioRxiv preprint reports that the rhizarian flagellate PCE SSF has an anaerobic mitochondrion whose reconstructed metabolism can produce hydrogen. The authors used single-cell transcriptomics to identify the organelle’s metabolic machinery; their findings describe a distinctive route to energy metabolism in an independently anaerobic branch of eukaryotes.
PCE SSF is a benthic eukaryotrophic flagellate and, according to the authors, the first cultivated representative of Novel Clade 12 (NC12) within Rhizaria. The study’s transcript data were used to reconstruct its mitochondrion-related organelle (MRO), a mitochondrion modified for the conditions in which an organism lives.
An anaerobic route to energy
The authors infer that PCE SSF’s mitochondrion can produce hydrogen and carry out hydrogenosome-type substrate-level phosphorylation. In substrate-level phosphorylation, ATP is formed directly through metabolic reactions, rather than being generated by the usual oxygen-dependent process of oxidative phosphorylation. Hydrogenosomes are specialised mitochondrion-related organelles associated with energy metabolism in anaerobic eukaryotes.
The study also identifies a nearly complete but probably branched tricarboxylic acid (TCA) pathway, a set of reactions that normally helps cells extract energy and carbon-building intermediates from nutrients. The reconstructed pathway lacks citrate synthase and malate dehydrogenase. The authors suggest that ATP citrate lyase, usually found in the cell’s cytosol, may perform citrate synthase’s role in this organism. They note that this possibility had previously been reported in this context only in the anaerobic cercozoan Brevimastigomonas motovehiculus.
A reduced mitochondrial toolkit
PCE SSF differs from B. motovehiculus in other parts of its reconstructed energy machinery. The preprint reports that it retains Complex II and the NuoE and NuoF subunits of the electron transport chain, while lacking a mitochondrial genome. Together, these features point to a reduced and atypical mitochondrial metabolism rather than a standard oxygen-using respiratory system.
The findings add another example of how eukaryotes have evolved distinct cellular strategies for low-oxygen environments. Because the reconstruction is based on single-cell transcriptomics, it identifies the metabolic capacity suggested by expressed genes; the proposed pathway’s activity and the precise role of ATP citrate lyase remain matters for further study. The report is a preprint.