Biomineralization of iron-phosphates in the water column of Lake Pavin (Massif Central, France)

The availabilities of iron and phosphorus have considerably impacted biological productivity in past and present natural aquatic environments, and therefore have been key regulators of climate changes over geological time scales. Microbial organisms are known to play important roles in reactions that drive Fe and P cycling at redox interfaces in Earth's surface environments. Here we study the depth variations of Fe and P speciation in Lake Pavin (Massif Central, France), a deep (bottom depth 92 m) and permanently stratified lake with anoxic and ferruginous conditions in the water column below 60 m depth. We particularly focus on the potential roles of microbes on Fe and P transformations and traces left by these processes in the sedimenting particular matter. Bulk chemical analyses, powder X-ray diffraction and X-ray absorption spectroscopy (XAS) at the Fe K-edge were performed to characterize the mineralogy and Fe oxidation state of solid particles at different depths in the water column and in sediments deposited at the bottom of the lake. Fe is mainly hosted by Fe(III)-(oxyhydr)oxides and phyllosilicates in the shallower oxygenated water column of the lake (25 m). The amount of Fe in suspended matter increases with depth, and an additional amorphous Fe(II)-Fe(III)-phosphate phase is detected close to the chemocline (at 56 m depth), while vivianite (an Fe(II)-phosphate with a formula of Fe(II)3(PO4)2 8(H2O)) becomes dominant in the deeper anoxic water (67 m and 86 m depths). Fe-(oxyhydr)oxides are preserved down to these depths in the water column suggesting that Fe-reduction has little impact on the particulate Fe budget over the monimolimnion. These Fe-(oxyhydr)oxides undergo reductive dissolution at the surface of the sediments, where vivianite is the main Fe-bearing phase. These results are confirmed by imaging at the micrometer and nano-scales using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and synchrotron- based scanning transmission X-ray microscopy (STXM), as well as high spatial and energy resolution XANES spectra at the Fe L2,3-edges and the C K-edge. Moreover, polyphosphate-accumulating microorganisms were observed by microscopy and chemical imaging at all depths except in the deeper part of the lake (86 m), and some Fe was associated with these polyphosphate inclusions below 56 m. Finally, the Fe-phosphates encrust microbial bodies, suggesting that microbial activities might be involved in the formation of these phases. Based on comparisons with laboratory experiments, we propose that Fe-oxidation (in contrast to Fe-reduction) may play a role in the precipitation of Fe-phosphates in the water column of Lake Pavin. Polyphosphate-accumulating microorganisms could also be involved in Fe-phosphate formation in the lake, for instance by increasing dissolved phosphate concentrations in the monimolimnion.