Geology Reference
In-Depth Information
Inexorable oxygen production by hydrogen loss might have turned Earth's surface rusty
red over a similar multibillion-year period, but it can't have had much effect on Earth's
early environment. Even with the most extreme estimates, less than one atmospheric mo-
lecule in a trillion was O
2
before the Great Oxidation Event. (Today it's one in five.)
That trivial amount of oxygen would have been snapped up, as fast as it could be gener-
ated, at Earth's surface by huge quantities of iron atoms just waiting to be oxidized in the
oceans and in the soils. Even if Earth had remained lifeless and eventually sported reddish
weathered zones in older, stable parts of the continents, such a superficial coating of rouge
would have been purely cosmetic.
Life, too, may have contributed a small inventory of oxygen prior to photosynthesis. In
fact, cells have learned at least four different ways to make oxygen from their surround-
ings. Oxygenic photosynthesis is the big one today, but other biochemical pathways may
have played small roles in ancient times.
Life scavenges energy from its environment any way it can. The easiest way to gain
energy while releasing oxygen is to start with a molecule that is already oxygen rich and
highly reactive. Thus a number of microbes have learned to exploit molecules of peroxide
(H
2
O
2
, produced by reactions high in the atmosphere) to generate O
2
plus energy. Admit-
tedly, such molecular species would have been scarce before the rise of atmospheric oxy-
gen,andsuchmicrobialmechanismscan'thaveplayedmuchofaroleinmodifyingEarth's
early environment.
A team of microbiologists in Holland recently reported a more relevant oxygen-produ-
cing scenario: they discovered remarkable microbes that gain energy by decomposing ni-
trogen oxides. Early in Earth's history, these so-called NOX chemicals were produced in
small amounts through reactions of nitrogen gas with minerals—during lightning storms,
for example. Today, owing to widespread use of nitrogen-rich fertilizers, many lakes,
rivers, and estuaries are heavily polluted with NOX compounds, which promote large mi-
crobial blooms. The newly discovered microbes are able to decompose nitrogen oxides in-
to nitrogen plus oxygen, then use the oxygen to “burn” natural gas, or methane, and thus
enjoy a jolt of energy. Such a clever chemical strategy might prove especially useful on a
nitrogen-rich, oxygen-starved world like Mars.
Fossil Evidence
Of all the oxygen-producing mechanisms, photosynthesis is the undisputed champion, but
how early did photosynthesis and the production of oxygen begin? Paleontologists, who
scrutinize the fragmentary tangible remains of ancient living worlds, see the connections
between life past and life present more vividly than any other scientists. Perhaps it's not
surprising, therefore, that they were among the first to find evidence for an oxygenated
