219sis of these polymers (Miot et al. 2016 ). How this intracel-
lular polyphosphate/orthophosphate cycle superimposes
with Fe-phosphate biomineralization will have to be deter-
mined by future studies. Moreover, the identification of the
microbial diversity specifically involved in these processes
as well as the oxidation and reduction of Fe in Lac Pavin has
yet to be assessed precisely.
Iron also interacts with sulfur at the redox transition zone,
where sulfate is reduced into sulfide, and forms insoluble
FeS particles. The fate of these FeS particles is unclear but
they ultimately transform into pyrite (FeS 2 ), which is found
in the lake sediments. An important question to elucidate in
the future is the mechanism of pyrite formation, since this
would greatly help in understanding the geochemical signa-
ture recorded in ancient sediments deposited under ferrugi-
nous conditions. Systematic analyses of sulfur isotopes and
dedicated mineralogical study may provide strong con-
straints on pyrite formation pathways in Lac Pavin. Another
Fe-bearing mineral of interest for ancient sediment interpre-
tation and paleo-environment reconstruction is siderite, a
ferrous carbonate (FeCO 3 ). Contrasting with vivianite, which
is ubiquitous in Lac Pavin sediments, siderite has only been
identified in restricted layers of the lake bottom sediment
(Schettler et al. 2007 ). Water in the monimolimnion is
grossly supersaturated with respect to siderite (Michard et al.
1994 ) but it remains undetermined whether siderite precipi-
tates in the water column or in the sediment, and what pro-
cesses can lead to siderite formation. Despite some
uncertainties in mineral formation pathways, Lac Pavin pro-
vides a well-understood analogue for organic and iron rich
sedimentary systems on early Earth. As such Lac Pavin is an
ideal spot calibration of novel geochemical and biological
signatures.
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