Genomic analysis and molecular dating of core iron transporters suggests early Cyanobacteria could not take up Fe(II) in the Archean ocean

Archean Cyanobacteria oxygenated Earth’s atmosphere during the Great Oxygenation Event (GOE) through the action of oxygenic photosynthesis. The photosynthetic apparatus relies on metalloproteins, many of which contain iron. Cyanobacteria use several specific transporters to meet their high iron requirements. In the ferruginous anoxic Archean ocean, the FeoABC transporter was thought to be the primary means of Fe(II) uptake. Our goal is to investigate the distribution of inorganic iron uptake mechanisms among Cyanobacteria and to determine the emergence of core iron receptors in the Cyanobacterial lineage. Essential iron uptake transporters and regulators were identified in 125 Cyanobacteria using in silico analysis. We reconstructed the Baysean phylogeny of the Fe(II) receptor FeoB, the high affinity Fe(III) permease, FutB, and cyanobacterial FTR1. Additionally, the arrival of these iron receptors in the Cyanobacterial lineage was timed using a molecular clock. The expression of cftr1 (Pse7367_Rs12485), furA (Pse7367_Rs06445) and cyoC (Pse7367_Rs00935) was determined by quantitative RT-PCR against the reference gene, rpoC1 (Pse7367_Rs07505), in the basal clade cyanobacterium Pseudanabaena PCC7367, grown under simulated Archean conditions. Genome analysis shows an absence of the high affinity Fe(II) transporter, FeoB, in most basal Cyanobacteria. Moreover, evolutionary dating timed the arrival of FeoB, cFTR1 and FutB in the cyanobacterial lineage during the Proterozoic. Furthermore, cftr1 is constitutively expressed in Pseudanabaena PCC7367, even after the addition of Fe(II). This study highlights the need for a reappraisal of iron uptake systems in early Cyanobacteria, as Fe(II) does not appear to have been their primary source of iron in the ferruginous Archean oceans.