Mössbauer spectra of EGTA-washed fermenting mitochondria (Sample F12, (18)) recorded at 4.5 K and 0.05 T (A), 100 K and 0.05 T (B), and 4.2 K and 8 T (C). Solid lines simulate the contributions of NHHS FeII species (blue line in A, ~20% of total Fe), the central doublet (red line in A and blue line in C, ~ 20%), FeIII nanoparticles (red line in B, ~ 40%), and mononuclear HS FeIII species (red line in C, ~ 20%). Shown offset above B is the experimental spectrum of FeII(EGTA) (black hashmarks). The black arrows in Figures 1-3 point to the high energy absorption line of the NHHS FeII species.