Environmental Engineering Reference
In-Depth Information
evolved during periods when the oceans were anoxic, reducing, and rich in
ferrous Fe (Fe
2
+
) [104]. The early ocean probably had high concentrations of
Fe
2
+
, but low concentrations of sulfate and sulfide [110]. Although the high
concentrations of Fe
2
+
may have removed some of this sulfide by precipitation,
chemical models suggest that relatively high concentrations of sulfide remained
[110]. Fe and S, required for nitrogenase, were probably present in the Archean
ocean. Extant anoxic basins have high sulfate which differs from these early
Earth scenarios [110]. However, the availability of Mo and V in the Archean
oceans may have been low, due to the precipitation of MoO
4
and VO
4
with
sulfides or FeS [7]. Once the surface oceans became oxic these metals might
have become more biologically available [7, 105], and could have selected for
the Mo and V nitrogenases.
It is difficult to accurately model the chemistry of the early oceans since we
know so little about the ionic composition at that time in Earth's history. The
early ocean may have even been much more saline [67], which would make
the modern anoxic basins good analogues for the early Earth (except for the
presence of sulfate). During the Proterozoic, the surface oceans appear to have
become oxic, while the deep ocean remained anoxic, mildly reducing, and rel-
atively Fe
2
+
-rich. Sediment records indicate that sulfate reduction became an
increasingly important process during the Proterozoic [116]. The subsequent
oxidation of sulfide to sulfate would have titered any atmospheric oxygen that
was formed by oxygenic photosynthesis. This may be part of the explanation
for the lag between the evolution of oxygenic photosynthesis and the oxygena-
tion of the atmosphere (Fig. 2). Contemporary anoxic basins where sulfate
reduction plays a central role in anaerobic metabolism are likely analogous to
the microbiology of the deep oceans during this period. Intriguingly, numerous
sulfate reducers have N
2
fixation genes, suggesting that sulfate reducers could
have been involved in early N
2
fixation.
There is a large degree of uncertainty regarding when life evolved. Earth be-
came habitable around 3.8 Ga after the frequent meteor bombardments ended,
and evidence of life in the form of microfossils has been reported from 3.5
Ga rocks [69, 115]. Considerable controversy surrounds the interpretation of
these fossils, ranging from chemical artifacts [12], to evidence for early mi-
crobes, including cyanobacteria. The first microbes probably predated these
3.5 Ga microfossils, with prokaryotic life likely arising approximately 3.8 Ga
[88].
The first microbes are thought to have been anaerobic chemoheterotrophs
[30, 88] with methanogenesis, a process catalyzed by the Archaeal methanogens,
likely the earliest metabolic process [63, 84]. Various taxa of methanogens
contain nitrogenase genes [76]. Similarly, sulfate reducers likely evolved prior
to 3.5 Gya [20, 116], and the nitrogenase of sulfate reducing Proteobacteria as
well as some Archaea, fall within the deeply branching Cluster III
nif
sequences.
