Muriel Pacton

Going nano: A new step toward understanding the processes governing freshwater ooid formation

Marine and freshwater ooids were historically thought to form by purely physicochemical processes in turbulent environments. Recently, organomineralization has been identified as a key process for the initiation of freshwater ooid cortex formation, but the exact biochemical mechanism(s) involved and subsequent contribution to the development of the growing cortex remain unknown. Here, we show that photosynthetic microbes not only enhance early carbonate precipitation around the ooid nucleus but also control the formation of the entire cortex in freshwater ooids from Lake Geneva, Switzerland. Microbial extracellular polymeric substances are first permineralized as amorphous magnesium silicates ( am Mg-Si) before being calcified. An ∼5‰–6‰ depletion of 13 C in ooid cortices compared to both bulk values and carbonate nuclei supports this photosynthetic microbial mechanism and argues against contributions from sulfate-reducing bacteria or methanogens. These data have significant implications for paleoenvironmental studies since photosynthetic microbes now provide an alternative to turbulent hydrodynamic conditions in the formation of freshwater ooids.

Effects of Hydrostatic Pressure on Growth and Luminescence of a Moderately-Piezophilic Luminous Bacteria Photobacterium phosphoreum ANT-2200

Bacterial bioluminescence is commonly found in the deep sea and depends on environmental conditions. Photobacterium phosphoreum ANT-2200 has been isolated from the NW Mediterranean Sea at 2200-m depth (in situ temperature of 13°C) close to the ANTARES neutrino telescope. The effects of hydrostatic pressure on its growth and luminescence have been investigated under controlled laboratory conditions, using a specifically developed high-pressure bioluminescence system. The growth rate and the maximum population density of the strain were determined at different temperatures (from 4 to 37°C) and pressures (from 0.1 to 40 MPa), using the logistic model to define these two growth parameters. Indeed, using the growth rate only, no optimal temperature and pressure could be determined. However, when both growth rate and maximum population density were jointly taken into account, a cross coefficient was calculated. By this way, the optimum growth conditions for P. phosphoreum ANT-2200 were found to be 30°C and, 10 MPa defining this strain as mesophile and moderately piezophile. Moreover, the ratio of unsaturated vs. saturated cellular fatty acids was found higher at 22 MPa, in agreement with previously described piezophile strains. P. phosphoreum ANT-2200 also appeared to respond to high pressure by forming cell aggregates. Its maximum population density was 1.2 times higher, with a similar growth rate, than at 0.1 MPa. Strain ANT-2200 grown at 22 MPa produced 3 times more bioluminescence. The proposed approach, mimicking, as close as possible, the in situ conditions, could help studying deep-sea bacterial bioluminescence and validating hypotheses concerning its role into the carbon cycle in the deep ocean.

Organomineralization processes in freshwater stromatolites: a living example from eastern Patagonia

Living stromatolites have been mostly described within shallow marine and (hyper)saline lacustrine environments. Southernmost South America lacks detailed investigations of these (organo)sedimentary buildups, particularly in regions experiencing extreme and variable environmental conditions. Here, we report and describe living freshwater stromatolites in the Maquinchao region, north‐western Patagonia, Argentina. Fossil stromatolites characterized by globular and cauliflower shapes are also present in a continuous palaeoshoreline of a former lake at an altitude of 830 m, whereas their living counterparts only occur in the calm waters of sheltered or meandering sections of the Maquinchao River. The living stromatolites and their host waters have been sampled and studied using various chemical and microscopic techniques to better constrain the environmental versus biological factors controlling their development. Our results indicate that today stromatolites only proliferate in freshwater when Ca2+ levels are high. A microscopic inspection of the living stromatolite mat indicates stronger photosynthetic activity in the upper green layer associated with crypto/microcrystalline calcite (nanoglobules) compared to the lower beige‐white biofilm. This biofilm contains more low‐Mg calcite (rhombohedra) precipitates, which can form millimetre‐sized aggregates in the underlying anoxic layer. Although sulphate‐reducing bacteria are living in the entire mat, they appear more abundant and widely distributed in the lower beige‐white layer and are always associated with Mg calcite. Low salinity and low‐turbidity water along with microbial (photosynthetic and heterotrophic) activity are the most important factors promoting low‐Mg calcite precipitation in the Maquinchao Basin. These conditions are very different from those proposed for recently described lacustrine stromatolites at high altitude in the subtropical and tropical Andes as well as in Chilean Patagonia. Hence, all these observations in modern freshwater stromatolites show the importance of geomicrobiological studies in identifying proxies of the hydrological conditions prevailing during their formation.