Metabolite flow in the metabolic models of the endosymbionts. Metabolites with unconstrained import and export across system boundaries are represented by green arrows (8 metabolites related to usual exchange with extracellular medium) and yellow arrows (9 metabolites supposed to be directly provided by the host). Ammonia is only allowed to leave the system (blue arrow). Other external metabolites (purple arrows) can also alternatively enhance or support (depending on the strain considered) bacterial growth. Abbreviations: nac, nicotinic acid; ppbng, porphobilinogen; thm, thiamin; pan4p, pantotheine-4-phosphate; dhor-S, S-dihydroorotate.