Adaptation to life on land at 21% O2via transition from ferredoxin- to NADH-dependent redox balance

Pyruvate:ferredoxin oxidoreductase (PFO) and iron only hydrogenase ([Fe]-HYD) are common enzymes among eukaryotic microbes that inhabit anaerobic niches. Their function is to maintain redox balance by donating electrons from food oxidation via ferredoxin (Fd) to protons, generating H 2 as a waste product. Operating in series, they constitute a soluble electron transport chain of one-electron transfers between FeS clusters. They fulfill the same function — redox balance — served by two electron-transfers in the NADH- and O 2 -dependent respiratory chains of mitochondria. Although they possess O 2 -sensitive FeS clusters, PFO, Fd and [Fe]-HYD are also present among numerous algae that produce O 2 . The evolutionary persistence of these enzymes among eukaryotic aerobes is traditionally explained as enabling facultative anaerobic growth. Here we show that algae express enzymes of anaerobic energy metabolism at ambient O 2 levels (21% v/v), Chlamydomonas reinhardtii expresses them with diurnal regulation. High O 2 environments arose on Earth only some ∼450 million years ago. Gene presence absence and gene expression data indicate that during the transition to high O 2 environments and terrestrialization, diverse algal lineages retained enzymes of Fd-dependent one-electron based redox balance, while the land plant and land animal lineages underwent irreversible specialization to redox balance involving the O 2 -insensitive two-electron carrier NADH. Highlights - Algae express enzymes of anaerobic metabolism in 21% [v/v] O 2 atmosphere, independent of anaerobiosis - Retention of a plastid-encoded NADH dehydrogenase-like (NDH) was likely a prerequisite for the transition to life on land - Terrestrialization and adaption to high O 2 is accompanied by a shift to redox balance at higher midpoint potentials - Eukaryotes adapted to high O 2 life on land via specialization to two-electron based redox balance