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Abstract

Domestication of CO2-fixation became a worldwide priority enhanced by
the will to convert this greenhouse gas into fuels and valuable
chemicals. Because of its high stability, CO2-activation/fixation
represents a true challenge for chemists. Autotrophic microbial
communities, however, perform these reactions under standard temperature
and pressure. Recent discoveries shine light on autotrophic acetogenic
bacteria and hydrogenotrophic methanogens, as these anaerobes use a
particularly efficient CO2-capture system to fulfill their carbon and
energy needs. While other autotrophs assimilate CO2 via carboxylation
followed by a reduction, acetogens and methanogens do the opposite. They
first generate formate and CO by CO2-reduction, which are subsequently
fixed to funnel the carbon toward their central metabolism. Yet their
CO2-reduction pathways, with acetate or methane as end-products,
constrain them to thrive at the "thermodynamic limits of Life". Despite
this energy restriction acetogens and methanogens are growing at
unexpected fast rates. To overcome the thermodynamic barrier of
CO2-reduction they apply different ingenious chemical tricks such as the
use of flavin-based electron-bifurcation or coupled reactions. This
mini-review summarizes the current knowledge gathered on the
CO2-fixation strategies among acetogens. While extensive biochemical
characterization of the acetogenic formate-generating machineries has
been done, there is no structural data available. Based on their shared
mechanistic similarities, we apply the structural information obtained
from hydrogenotrophic methanogens to highlight common features, as well
as the specific differences of their CO2-fixation systems. We discuss
the consequences of their CO2-reduction strategies on the evolution of
Life, their wide distribution and their impact in biotechnological
applications.