'Every dogma has its day'*: a personal look at carbon metabolism in photosynthetic bacteria

Dogmas are unscientific. What is perhaps the greatest biological dogma of all time, the 'unity of biochemistry' is, in the main, still having its day. According to present knowledge, the exceptions to this dogma are mere details when seen in relation to the biosystem as a whole. Nevertheless the exceptions are scientifically interesting and the understanding of them has led to a better comprehension of photosynthesis and ecology. Until the discovery of C-14, photosynthetic CO2 fixation was like a slightly opened black box. With C-14 in hand scientists mapped out the path of carbon in green plant photosynthesis in the course of a few years. The impressive reductive pentose phosphate cycle was almost immediately assumed to be universal in autotrophs, including anoxygenic phototrophs, in spite of the odd observation to the contrary. A new dogma was born and held the field for about two decades. Events began to turn when green sulfur bacteria were found to contain ferredoxin-coupled ketoacid-oxidoreductases. This led to the formulation of a novel CO2-fixing pathway, the reductive citric acid cycle, but its general acceptance required much work by many investigators. However, the ice had now been broken and after some years a third mechanism of CO2 fixation was discovered, this time in Chloroflexus, and then a fourth in the same genus. One consequence of these discoveries is that it has become apparent that oxygen is an important factor that determines the kind of CO2-fixing mechanism an organism uses. With the prospect of the characterization of hordes of novel bacteria forecast by molecular ecologists we can expect further distinctive CO2 fixation mechanisms to turn up.