H2 partial pressure switches autotrophic pathways in an anaerobic bacterium

Abstract

Abstract Most of our knowledge on microbial physiology and biochemistry is based on studies performed under laboratory conditions. For growing hydrogen-oxidizing anaerobic, autotrophic prokaryotes, an H2:CO2 (80:20, v/v) gas mixture is typically used. However, hydrogen concentrations in natural environments are usually low, but may vary in a wide range. Here we show that the thermophilic anaerobic bacterium Ammonifex degensii balances its autotrophic carbon fixation between two pathways depending on the H2 partial pressure. At 80% H2, favoring ferredoxin reduction, it uses the ferredoxin-dependent Wood-Ljungdahl pathway. In contrast, during growth at 10% H2, it switches to the more ATP-demanding, ferredoxin-independent Calvin-Benson cycle. The study reveals that the H2 redox potential is an important factor influencing the usage of different autotrophic pathways. This type of metabolic adjustment may be widespread in the microbial world.