Making CO ₂ Work for You: Converting carbon dioxide (CO ₂ ) to formic acid (HCOOH) is an attractive substrate for the storage and recovery of hydrogen (H ₂ ) for use as fuel. Because CO ₂ is relatively stable, however, its hydrogenation typically requires extreme conditions. Schuchmann and Müller (p. 1382 : see the Perspective by Pereira ) isolated a hydrogen-dependent carbon dioxide reductase from Acetobacterium woodii that is nearly 2000-fold more effective than the fastest known chemical catalysts. The process, which relies on the formation of formate as a chemical intermediate, also works well in whole-cell systems of A. woodii when certain metabolic pathways are inhibited. The enzyme, whether isolated or present in host bacteria, may lead to the design of more efficient fuel cells capable of functioning under less harsh conditions than fuel cells based on chemical reactions alone.

Acetogenic bacteria are a diverse group of strictly anaerobic bacteria that utilize the Wood-Ljungdahl pathway for CO₂fixation and energy conservation. These microorganisms play an important part in the global carbon cycle and are a key component of the anaerobic food web. Their most prominent metabolic feature is autotrophic growth with molecular hydrogen and carbon dioxide as the substrates. However, most members also show an outstanding metabolic flexibility for utilizing a vast variety of different substrates. In contrast to autotrophic growth, which is hardly competitive, metabolic flexibility is seen as a key ability of acetogens to compete in ecosystems and might explain the almost-ubiquitous distribution of acetogenic bacteria in anoxic environments. This review covers the latest findings with respect to the heterotrophic metabolism of acetogenic bacteria, including utilization of carbohydrates, lactate, and different alcohols, especially in the model acetogenAcetobacterium woodii. Modularity of metabolism, a key concept of pathway design in synthetic biology, together with electron bifurcation, to overcome energetic barriers, appears to be the basis for the amazing substrate spectrum. At the same time, acetogens depend on only a relatively small number of enzymes to expand the substrate spectrum. We will discuss the energetic advantages of coupling CO₂reduction to fermentations that exploit otherwise-inaccessible substrates and the ecological advantages, as well as the biotechnological applications of the heterotrophic metabolism of acetogens.