Ferredoxins are a ubiquitous family of protein electron carriers that support electron transfer in a massive number of pathways across the tree of life. As central and ancient distributors of redox equivalents, they are advantageous targets for controlling electron flow and for studying the evolution of electron flux in cells. Herein, I describe my efforts to catalogue ferredoxins and their natural substitute, flavodoxins, from over 7000 organisms to understand the usage of protein electron carriers throughout the three domains. I apply my findings to study the distribution of cyanophage ferredoxins, focusing on the ferredoxin from a phage that infects the world’s most abundant phototroph. I demonstrate this viral Fd is unusually unstable yet presents a midpoint potential similar to bacterial homologs and can transfer electron to host oxidoreductases. When this viral Fd is recombined with a thermostable homolog, I am able to generate chimeric Fds with variable heat tolerances and potentials shifted by up to 50 mV that are capable of conducting electron transfer in vivo. Additionally, I will describe my efforts to reconstruct missing links in the evolution of [4Fe-4S] ferredoxins, demonstrating that symmetric ferredoxins and fragmented ferredoxins are capable of supporting electron transfer. These results expand our knowledge of natural ferredoxins and elucidate the design rules we might use to make synthetic variants.