Cyanobacteria: The Great Liberators - Oxygen: A Four Billion Year History - page 42

of electrons. Unlike the sulfide, Fe 2+ , and H 2 used by anoxygenic photo-
trophic organisms, water is almost everywhere on the planet surface.
This means that biological production on Earth was no longer limited
by the electron source (water in this case), but rather by nutrients and
other trace constituents making up the cells. In the end, the use of water
in photosynthesis resulted in an increase in rates of primary production
on Earth by probably somewhere between a factor of ten to a thousand,
as outlined in chapter 2. 1 For the first time, life on Earth became truly
plentiful. With the evolution of cyanobacteria, Earth was on its way to
becoming a green planet. 2
Cyanobacteria would have inhabited all manner of lakes, ponds,
streams, and puddles on land where water persisted for at least short
periods of time, as well as the whole upper sunlit layer of the oceans
(typically called the photic zone). If we imagine ancient explorers of
the solar system probing early Earth, they would have needed a micro-
scope to find much evidence for life before the evolution of cyano-
bacteria. 3 After this, however, our ancient explorers could have found
abundant life by imaging Earth from the comfort of their spacecraft,
much as we do with satellites today. here organic matter was scarce
before cyanobacteria, it would have become relatively plentiful after
they evolved. The degradation of this organic matter drives ecosystems.
More organic matter drives more active ecosystems and probably more
complex ones as well. The increase in ecosystem complexity would have
also resulted from the new availability of oxygen and the subsequent
evolution of organisms using it. Overall, there would have been an in-
crease in biological diversity as both abundant organic matter, and oxy-
gen, become available in environments ranging from the land to the sea.
All in all, the evolution of cyanobacteria was a (more likely “the”) big
deal in the history of life on Earth.
My introduction to cyanobacteria came just after I finished my PhD
at Yale with Bob Berner. Bob was infinitely patient and did his best
to turn a chemist into something resembling a geoscientist. Though the
focus of my PhD was on how organic matter was cycled by microbes in
modern marine sediments, through Bob's training and encouragement,
I became fascinated with Earth history problems and in particular with
the history of atmospheric oxygen. I realized already while doing my
PhD that if I wished to understand something of this history, I needed