The following proposal is aimed to support the computational work of the ERC Starting Grant TANGO2 The vast majority of formally described eukaryotes are multicellular and fall into two phylogenetic groupings: Archaeplastida (plants and algae) and Opisthokonta (animals and fungi). Nevertheless, most of the eukaryotic biodiversity can be found in the microbial world of single-celled eukaryotes, also known as protists, a group that is well-worth studying. These organisms are the least studied collection of eukaryotes and represent a majorly unexplored frontier of eukaryotic biodiversity but, excitingly, progress is being made and the rate at which new "kingdom"-level eukaryotic lineages are uncovered shows no signs of slowing down. Protists are a group of organisms that display complex morphologies and life cycles and represent significant community members of many ecosystems on Earth, such as environments having low concentrations of oxygen. To shine new light onto this understudied environment, the following project aims to interrogate the evolutionary history and phylogenetic distribution of oxygen-dependent and independent metabolism across the eukaryotic diversity. 1/ Catalogue the complement of respiratory chain components across the tree of eukaryotes with an emphasis on anaerobic eukaryotes; 2/ Catalogue the complement of respiratory chain cofactor biosynthesis (e.g. heme, quinones) across the tree of eukaryotes with an emphasis on anaerobic eukaryotes; 3/ Compare the paralogue copy number of respiratory chain components between anaerobic animals and anaerobic protists 4/ Genomic investigations of protists and their bacterial symbionts.