Phototrophic CO2 Fixation: Recent Insights into Ancient Metabolisms

Abstract

The production of all (autotrophy) or a substantial proportion (mixotrophy) of newly synthesized biomass from carbon dioxide is a hallmark of plants and a number of bacteria and archaea. Carbon dioxide fixation enables autotrophs to form the basis of entire ecosystem foodwebs as primary producers and mixotrophs to efficiently utilize simple forms of carbon present in various environments. During fixation, CO2 must ultimately be reduced to the level of formaldehyde for assimilation into biomass and, for this reason, CO2 fixation is also utilized by numerous organisms to dispose of excess reducing power. Currently, six distinct CO2 fixation pathways are recognized amongst autotrophic prokaryotes and eukaryotes. The impact of genomics on our understanding of the distribution, function(s), and regulation of these pathways and their unique enzymes will be described as will two recently discovered pathways, one for autotrophic or mixotrophic CO2 fixation in some photosynthetic bacteria and the other for acetate assimilation by a wide range of bacteria, including phototrophs, that involves a novel carboxylation step.