Environmental Engineering Reference
In-Depth Information
This cluster contains nitrogenases from strict anaerobes, and may represent the
earliest form of nitrogenase. Nitrogenase genes in Cluster III are found in
modern anoxic environments [142].
The nitrogenases of cyanobacteria form a cluster of deeply branching lin-
eages not unlike ribosomal RNA cyanobacterial phylogeny [40, 133, 139].
Heterocyst-forming cyanobacteria (heterocysts are specialized cells that do not
evolve oxygen and are the site of N
2
fixation in some filamentous species) form
tight phylogenetic clusters in both
nifH
and rRNA trees, and appear to have
evolved late in the evolution of cyanobacteria [40, 139]. Akinetes of hetero-
cystous cyanobacteria have been reported in microfossils dating to as recently
as 1.5 Ga [41]. However, the concentration of oxygen in Earth's atmosphere
between 2-0.5 Ga was only a fraction of the present atmospheric level. Thus,
the heterocyst evolved prior to severe oxygen stress in the oceans, except per-
haps in shallow embayments, where the local concentration of oxygen could
have been high due to blooms of cyanobacteria. Blooms of heterocyst-forming
cyanobacteria still occur in embayments and estuaries such as the Baltic Sea and
Lake Alexandrina in Australia. In the modern world, microenvironments play
pivotal roles in shaping ecosystem diversity and function; similar microhabitats
undoubtedly were important in the evolution and diversification of early life on
the Earth.
There would have been strong selection for carbon fixation early in the
evolution of life, since organic molecules would have quickly been consumed
by heterotrophic microorganisms. Autotrophic carbon-fixing microbes were
likely present by 3.8 Ga [108] and the evolution of CO
2
fixation may have
partially been responsible for depleting ammonia on the early Earth [104,
118]. The first phototrophic autotrophs were probably phototrophic bacteria
that used Fe for electron donors [46, 88]. Chlorophyllide reductases, used in
the synthesis of bacteriochlorophyll, have a high degree of similarity in protein
structure and gene sequence to nitrogenase, and probably evolved from the
nitrogenase genes [18, 105]. Thus, N
2
fixation may have predated anoxygenic
photosynthesis. Oxygenic photosynthesis evolved more recently, followed by
aerobic respiration. Cyanobacteria appeared between 3.8 Ga [115] and 2.7
Ga [14, 115, 125], but evolved and diversified relatively quickly [40, 74].
Oxygenic photosynthesis and the diversification of cyanobacteria must have
evolved between 3.5 Ga (if the microfossils are indeed of cyanobacterial origin,
and if cyanobacteria had evolved oxygenic photosynthesis by that time) and 2
Ga, when the atmosphere became oxygenated (based on the presence of red beds
formed by precipitation of iron). Oxygenic photosynthesis would have predated
the accumulation of oxygen in the atmosphere since oxygen produced would
have had to titer the reduced Fe and sulfide in the oceans before accumulating in
the atmosphere. Atmospheric oxygen concentrations that would have selected
