ABSTRACT
Nitrogen fixation is today envisaged as a nutritional advantage. One may say that it slowly became one, and that everything was ultimately due to the advent of photosynthesis. A debate on ''who came first'' between rubisco and nitrogenase sees the first as a favorite. Atomic evidence from fossils (1) seems to indicate that, after a rich phase of microbial development occurred between 3.9 and 3.5 billion years ago, the massive sedimentation of carbon-containing compounds could have made carbon availability a major limiting factor for life. Selective pressure would therefore have favored the onset of carbon-fixing mechanisms using C02 as a source. These would have been initially chemoautotrophic or photoautotrophic and not yet leading to oxygen development. However, given the necessity of maintaining the C/N ratio below 10, as required in bacterial living matter, the capabilities of fixing carbon in unlimited fashion would have, in turn, rendered availability of nitrogen the new limiting factor. Considering the cell requirement of reduced forms and the abundance of gaseous N2, a strong selection would have at that stage privileged the appearance of nitrogen-reducing reactions. Such potentialities were presumably already present among microbial communities, and thence they could have been recruited. It has been postulated that nitrogenase, presently the key enzyme for nitrogen autotrophy, could have been initially just a system of cyanide detoxification (2). In today's world the capability of converting molecular nitrogen, amounting to a wealthy 78% of the surface atmosphere but too inert and oxidized to be easily drawn into biochemis-
try, is the privilege of a guild of prokaryotes to whom all remaining living beings, as consumers, owe a great deal of the nitrogen that shapes their proteins and nucleic acids. What, however, is the link bridging nitrogen fixation and symbiosis? Once again photosynthesis may have imposed its early influence on the events. The picture, a distant one, has to be seen in proper perspective. In a still aquatic habitat, under a reducing oxygen-free atmosphere, the reductions required for biosyntheses did not have to face the high redox slope imposed by today's oxidizing conditions. Therefore, the first nitrogen-fixing systems did not require the elaborate devices aimed at protecting nitrogenase from oxygen, which are so evident in modern symbioses. The key to the change that has led to the present status is to be searched for around 2.5 billion years ago and consists in the onset of oxygenic photosynthesis. Initially run by cyanobacteria, it was then joined by eukaryotic algae, and finally, during the last 500 million years, by higher plants. An oxygen-rich atmosphere, besides providing a new and versatile terminal electron acceptor that promoted aerobic life, has enabled the buildup of the ozone layer, at first catalyzed by UV radiation and eventually providing a shield from the same. Such protection allowed the nonlethal exploration and colonization of a highly irradiated environment, as the emerged land is. The possibility of living under high photosynthetic intensity brought plants to the massive development that characterizes their recent history; a history full of interaction with microbes.
