The Role of the Substrate on the Mineralization Potential of Microbial Mats in A Modern Freshwater River (Paris Basin, France)

Abstract

The relationship between environmental conditions and the development, mineralization and preservation of modern tufa microbialites was investigated in a 1.1 km long freshwater stream in Villiers-le-Bâcle, a tributary of Mérantaise river. Detailed mapping of the tufa microbialite distribution combined with sedimentological, petrographical and mineralogical analyses were coupled with chemical measurements. Six organosedimentary structures were identified; their distribution appears heterogeneous along the stream and responds to physicochemical conditions of water and specific biological components (e.g., microorganism, exopolymeric substance). Two of the organosedimentary structures show evidence of mineralization and only one is lithified. Based on field observations and in-situ deployment of mineralization markers (bricks), three zones with increasing mineralization intensities are defined, ranging from no mineralization to thick mineralized crusts forming riverine tufa. Both biotic and abiotic processes were proposed for the tufa microbialite formation. We explained changes in mineralization intensities by the specific physicochemical conditions (e.g., calcite saturation index (SIcalc) and partial pressure of CO2 (pCO2) and a closed proximity of the cyanobacterial biofilm and carbonates precipitates. The physical and chemical composition of substrate impact development of microbial communities, mineralization potential of tufa microbialite. Even though the physicochemical and biological conditions were optimal for mineral precipitation, the potential of lithification depended on the presence of a suitable (physical and chemical) substrate.

Carbonate Precipitation in Mixed Cyanobacterial Biofilms Forming Freshwater Microbial Tufa

“…This 1.1-km long stream runs from North to South, through the Oligocene Saclay Plateau and supports development of modern microbial tufa deposits (Figure 1c,d). The microbial tufas in this stream, first erroneously identified as algal concretions [37], are described in detail by [38].…”

“…Field campaigns were carried out in October, November, and December of 2016 and in March of 2017. Physicochemical characteristics along the stream were monitored and physical substrates characterized [38]. Mineralized structures in the streambed sampled included various microbial biofilms and microbial mats.…”

“…Sediment samples were collected from the end of the stream. This part (Zone 3 in [38]) has a high mineralization potential and microbial tufa form when a suitably hard substrate is available. The sediments were used as inoculum for Winogradsky-type ( [39]) columns in order to enrich for cyanobacterial biofilms.…”

“…This step produced the necessary biomass for a second stage of biofilm preparation; (2) Culturing in semi-liquid (soft-agar) medium: small (1 cm 2 ) fragments of the biofilm grown in liquid (see 1) were transferred into a Petri dishes with sterile soft agar (0.75% w/v) containing half-strength BG-11 medium (i.e., 1:1 BG-11 medium and water) with a pH of circa 8. The soft agar was used to provide a substrate for biofilm growth [38]. (Figure 1f).…”

“…Initially, the cyanobacterial enrichments grew rapidly in the fresh medium, which was supported by evolution of O 2 bubbles during the light period. The agar provided a (semi)solid surface critical for mat development and lithification [38,59]. During this initial phase, the high rates of CO 2 fixation could increase the pH and alkalinity through the production of hydroxyl ions resulting from bicarbonate dissociation into CO 2 [66] (Table 3, Figure 12a,b).…”