PHYLOGENETIC ANALYSIS OF IRON OXIDASE CYC2 SUGGESTS A SHALLOW ORIGIN FOR MODERN CLADES OF IRON OXIDIZERS

Cytochrome-porin Cyc2 has recently been identified as the initial electron acceptor in iron oxidation pathways used by chemolithotrophic Gallionellales and Zetaproteobacteria as well as photoferrotrophic Chlorobi. Our phylogenetic analysis identified copies of Cyc2 in gammaproteobacterial endosymbionts of lucinid bivalves. The phylogenetic relationships between Cyc2 copies in endosymbionts mirror the relationships between their hosts species, suggesting the endosymbionts co-evolved with their bivalve hosts. Thus, we were able to use the well-dated fossil record of lucinid bivalves to time-calibrate the Cyc2 phylogeny. The resulting molecular clock shows that iron oxidation with Cyc2 in modern Gallionellales, Zetaproteobacteria, and photoferrotrophic Chlorobi originated in the Phanerozoic – and most likely in the Mesozoic, if we add more tentative constraints based on coevolutionary relationships between Cyc2-retaining plant pathogens of genus Xanthomonas and their flowering plant hosts.

Even though fossils morphologically similar to modern chemolithotrophic iron oxidizers date back to the Proterozoic, the shallow origin of the modern clades suggests that these similarities do not permit phylogenetic assignment of the fossils. While chemolithotrophic iron oxidation and photoferrotrophy as metabolisms are very ancient, the phylogenetic identity of responsible organisms has changed profoundly through time – perhaps reflecting that niches with sufficient reduced iron do not persist over long stretches of Earth history.