“…1, Arrow E), such that electrons from the organic substrate are fully allocated between biomass, H 2 and CO 2 . Under these conditions, Nase is important both for nutrient acquisition and as an electron sink (Muller, 1933;Hillmer and Gest, 1977;Harwood, 2010a, 2011;Farmer et al, 2014;Gordon and McKinlay, 2014;McCully and McKinlay, 2016). To determine how simultaneous nitrogen acquisition and electron balancing constraints influence growth based on each Nase isoform, we cultured wild type R. palustris and mutant strains known to utilize only a single isoform (hereafter Mo-Nase, V-Nase and Fe-Nase strains, Oda et al, 2005) in diazotrophic photoheterotrophic media with organic substrates of varying oxidation states and assimilation pathways (Table 1) and measured growth, N 2 and CO 2 fixation, H 2 production, biomass composition and global protein abundance patterns.…”