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demise. At this point all of the oxygen
produced by photosynthesis was now able
to build up gradually in the atmosphere,
eventually reaching the present day level
of 20% (with great significance for the
later evolution of multicellular forms;
Chapter 2).
This elegant theory was put forward by
Preston Cloud in 1965 and it is still
generally accepted, although other factors
are also now known to be involved in
delaying the eventual build-up of oxygen
in the atmosphere (see Knoll, 2003 for
discussion). Nonetheless, the Precambrian
BIFs tell us that Earth's early atmosphere
was low in oxygen while its oceans were
rich in dissolved iron - in other words
the Gunflint seaway was unusual and
not at all similar to any habitat in any
modern ocean. (These early oceans
may also have been much warmer, but
the maximum temperature tolerated
by photosynthesizing organisms is
about 74°C.)
These unusual marine conditions are
reflected in the biota; the anaerobic
Gunflint ecosystems may have consisted
of nothing but minute bacteria, but
there was a wonderful variety of these
microorganisms. While eukaryotes
(protists, fungi, plants, and animals) use
either respiration, photosynthesis, or
fermentation to metabolize energy, the
prokaryotic bacteria use all of these
methods plus chemosynthesis, where
energy is harvested from chemical
reactions. Oxygen or nitrate or even
sulfate, iron oxide, or manganese oxide,
is combined with hydrogen, methane, or
the reduced forms of iron, sulfur, and
nitrogen and the cells capture the energy
released by the reaction. Moreover, some
respiring bacteria use nitrate, sulfate, iron
oxide, or manganese oxide instead of
oxygen.
It is still uncertain whether the Gunflint
stromatolites represent true microbial mat
structures built by oxygen-generating
cyanobacteria (Lanier, 1989) or whether
they are abiogenic sinters (Knoll, 2003). If
they are true stromatolites, then we can
presume that the filamentous forms were
autochthonous, growing in situ and
contributing to the mat-building process,
while the spheroidal, star-shaped and
umbrella-like types were planktonic and
allochthonous.
Whichever scenario is correct, we should
assume that the Gunflint biota included
some photosynthesizing cyanobacteria
(e.g. the larger filaments and enveloped
coccoids), but undoubtedly many do
appear to have used metabolic strategies
suited to the peculiar chemical conditions
of the Gunflint sea. For example, the iron-
coated tubes of Gunflintia minuta, the most
common fossils, resemble the sheaths of
iron-loving bacteria living in iron-rich
waters today, while the branching tubes of
Archaeorestis resemble the iron-oxidizing
bacteria Gallionella ferruginea , which
presently inhabits hydrothermal vents of
the Arctic Ocean. Similarly, the tiny,
spherical Huroniospora are similar to
coccoidal iron-loving bacteria, and the
'little dawn stars' ( Eoastrion ) can be
compared with living bacteria that use both
iron and manganese in their metabolism
(Knoll, 2003). Finally, Kakabekia persists
today in microhabitats where oxygen is
reduced and ammoniacal decomposition
takes place. These nonphotosynthesizing
microbes probably included both iron
oxidizing chemosynthesizers (which are
autotrophic) and iron respirers (which are
heterotrophic), and compare to a rare set
of modern counterparts that are not much
evident in today's iron-starved oceans.
The holistic view of Proterozoic life
concluded by Knoll (2003) is that true
microbial mat stromatolites were
abundant in carbonate-rich environments
throughout all of the Proterozoic and
during much of the preceding Archean
Eon, and that the oxygen-generating
cyanobacteria which built them were
the primary producers in early oceans.
Occasionally, however, an upwelling
of anoxic deep ocean waters containing
dissolved iron mixed with oxygenated
surface waters on the shelf-slope,
and
Gunflint-type iron-loving bacteria
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