  How photosynthesis by Precambrian cyanobacteria oxygenated Earth's biosphere
remains incompletely understood. Here it is argued that the oxic transition,
which took place between approximately 2.3 and 0.5 Gyr ago, required a great
proliferation of cyanobacteria, and this in turn depended on their ability to
fix nitrogen via the nitrogenase enzyme system. However, the ability to fix
nitrogen was not a panacea, and the rate of biospheric oxygenation may still
have been affected by nitrogen constraints on cyanobacterial expansion.
Evidence is presented for why cyanobacteria probably have a great need for
fixed nitrogen than other prokaryotes, underscoring the importance of their
ability to fix nitrogen. The connection between nitrogen fixation and the
evolution of photosynthesis is demonstrated by the similarities between
nitrogenase and enzymes critical for the biosynthesis of (bacterio)chlorophyll.
It is hypothesized that biospheric oxygenation would not have occurred if the
emergence of cyanobacteria had not been preceded by the evolution of nitrogen
fixation, and if these organisms had not also acquired the ability to fix
nitrogen at the beginning of or very early in their history. The evolution of
nitrogen fixation also appears to have been a precondition for the evolution of
(bacterio)chlorophyll-based photosynthesis. Given that some form of chlorophyll
is obligatory for true photosynthesis, and its light absorption and chemical
properties make it a "universal pigment," it may be predicted that the
evolution of nitrogen fixation and photosynthesis are also closely linked on
other Earth- like planets.
