Maintenance power requirements of anammox bacteria “ Candidatus Brocadia sinica” and “ Candidatus Scalindua sp.”

Little is known about the cell physiology of anammox bacteria growing at extremely low growth rates. Here, “ Candidatus Brocadia sinica” and “ Candidatus Scalindua sp.” were grown in continuous anaerobic membrane bioreactors (MBRs) with complete biomass retention to determine maintenance energy (i.e., power) requirements at near-zero growth rates. After prolonged retentostat cultivations, the specific growth rates ( μ ) of “ Ca . B. sinica” and “ Ca . Scalindua sp.” decreased to 0.000023 h −1 (doubling time of 1255 days) and 0.000157 h −1 (184 days), respectively. Under these near-zero growth conditions, substrate was continuously utilized to meet maintenance energy demands ( m e ) of 6.7 ± 0.7 and 4.3 ± 0.7 kJ mole of biomass-C −1 h −1 for “ Ca . B. sinica” and “ Ca . Scalindua sp.”, which accorded with the theoretically predicted values of all anaerobic microorganisms (9.7 and 4.4 kJ mole of biomass-C −1 h −1 at 37 °C and 28 °C, respectively). These m e values correspond to 13.4 × 10 −15 and 8.6 × 10 −15 watts cell −1 for “ Ca . B. sinica” and “ Ca . Scalindua sp.”, which were five orders of magnitude higher than the basal power limit for natural settings (1.9 × 10 −19 watts cells −1 ). Furthermore, the minimum substrate concentrations required for growth ( S min ) were calculated to be 3.69 ± 0.21 and 0.09 ± 0.05 μM NO 2 − for “ Ca . B. sinica” and “ Ca . Scalindua sp.”, respectively. These results match the evidence that “ Ca . Scalindua sp.” with lower maintenance power requirement and S min are better adapted to energy-limited natural environments than “ Ca . B. sinica”, suggesting the importance of these parameters on ecological niche differentiation in natural environments.