The nitrogen cycle in the archaean: an intricate interplay of enzymatic and abiotic reactions
Résumé
On modern planet Earth, a multitude of nitrogen cycle enzymes equilibrate the atmospheric reservoir of dinitrogen with the more oxidized and more reduced nitrogen compounds essential for life. The respective enzymes are elaborate entities and the reactions performed are complicated and in cases energetically challenging. Nitrogen, however, must have been a crucial element already at life's very beginnings which raises the question how the primordial nitrogen cycle of emerging life in the Archaean - necessarily using simpler and likely fewer enzymes - may have evolved into the very complex network of present planet Earth. To address this question, we have analysed molecular phylogenies of the presently known enzymes involved in the present day nitrogen cycle. The results collected and presented in this chapter indicate that in the Archaean, the enzymatic part of this cycle was restricted to a partial segment of the modern energy conserving denitrification pathway and that abiotic redox conversions of nitrogen specific to the geoenvironment of the Archaean were the evolutionary precursors of many reactions now requiring enzyme catalysis. As found in recent years for other core metabolic processes, the biological nitrogen cycle appears to be evolutionarily rooted in inorganic chemistry.