Geoscience Reference
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weathering
Fe
2+
hv
Photic zone
Anoxygenic
photosynthesis
Fe
2+
+ CO
2
à
FeOOH
Fe
2+
Iron reduction
FeOOH + CH
2
O
à
Fe
2+
Hydrothermal Fe
2+
Figure 2.2. Possible structure of an Fe-based ecosystem in the oceans. see text for details.
redrawn from Canfield et al. (2006).
coming from below, is oxidized as it meets the fading light from above.
hile these phototrophs have evaded isolation, Sean and CarriAyne
concluded, after a variety of considerations, that they most likely oxi-
dize ferrous iron to rust in the lake. Lake Matano is a long way from
Fritz Widdel's ditch, but both environments point to the possibility that
iron-oxidizing anoxygenic phototrophic bacteria could have been im-
portant contributors to the biological productivity of early Earth.
We imagine that these iron-oxidizing organisms would have been
partners in ecosystems involving fermenting bacteria and also so-called
iron-reducing bacteria. Iron reducers are a well-known group of mi-
crobes who grow by reducing iron oxide back to ferrous iron, and they
oxidize organic matter or H
2
in the process. They would have done the
same job in the ancient oceans by recombining the products of photo-
synthesis, the iron oxides, and the cell biomass, oxidizing the cell bio-
mass back to CO
2
, and reducing the iron oxides back to dissolved fer-
rous iron (
ig. 2.2
). This ecosystem would effectively recycle the iron,
and the activity of the phototrophs would ultimately be controlled by
the availability of ferrous iron and nutrients. Minik, Christian, and I
also attempted to model the activity level of this ancient phototrophic
population. Our calculations were highly imprecise and fraught with
many uncertainties and assumptions,
13
but recognizing these issues, we
