Experimental formation of clay-coated sand grains using diatom biofilm exopolymers

Abstract

In sedimentary environments, clay and sand are segregated by hydrodynamic processes. Yet, clay coats, thin clay envelopes lining sand grains, are abundant in modern and ancient coastal sedimentary deposits. Here, we present laboratory experiments in which we produced clay-coated quartz sands similar to those observed in modern and ancient estuarine sands. These coats were produced at ambient temperature by mixing exopolymeric substances (EPS) derived from intertidal diatom biofilms with clay minerals and quartz reference materials. The imaging of sediment-EPS mixes using cryo–scanning electron microscopy and atomic force microscopy demonstrated that EPS form organic bridges between clay and quartz. The physicochemical properties of the EPS were characterized independently through wet chemical assays and Fourier transform infrared spectroscopy. The results indicated that several EPS components (e.g., proteins, polysaccharides) had a potential to complex with quartz and clay. Our findings provide novel insights in the importance of biofilms in the aggregation of clay and sand. Detrital coats from ancient estuarine sandstones show textural similarities to the experimental clay-EPS complexes and could be considered as biosignatures of biofilm development and EPS production in past environments.

Preservation of exopolymeric substances in estuarine sediments

“…EPSs extracted from the core displayed several major infrared absorption peaks typical of polysaccharides and proteins, and also of nucleic acids and lipids (Braissant et al, 2007(Braissant et al, , 2009Duteil et al, 2020; Figure 5; Supplementary Figures S6-S8): (i) the vibrational bands in the spectral range 2,800-3,000 cm −1 represent the symmetric and antisymmetric stretching of C-H from CH 2 and CH 3 groups; (ii) the peak at 1710 cm −1 could be attributed to protonated carboxylic acid groups (iii) the peak at 1630 cm −1 was likely due to amide I C=O stretching; (iv) the peak at 1535 cm −1 could be attributed to the amide II C-N stretching and NH bending; (v) the peak at 1450 cm −1 represented the CH 2 scissoring vibration; and (vi) the doublet at 1404 and 1,365 cm −1 was likely due to C-OO − stretching; (vii) the peaks at 1235 cm −1 could be attributed to stretching of P=O bond in phosphate and the C-O-C group in esters, and at 1155 cm −1 to S=O stretches vibration from sulfate; (viii) the peak at 1024 cm −1 represented the carbohydrate C-O stretching vibrations. While present in the surface biofilm EPS, protonated carboxylic acid groups disappeared at 0.06 m and 4.33 m. This group could be characterized again by a very small peak at 6.08 m (Figure 5).…”

“…The negatively charged reactive functional groups provide the EPS a cation binding capacity to sequester metal ions or metalloids ( Braissant et al, 2007 ). EPS also possess electrostatic, hydrophilic and hydrophobic properties allowing their sorption to mineral surfaces ( Decho, 2000 ; Duteil et al, 2020 ). As a consequence, EPSs promote the cohesiveness and stability of the sediment ( Tolhurst et al, 2002 ), impacting bedform morphology ( De Winder et al, 1999 ; Malarkey et al, 2015 ).…”

“…As a consequence, EPSs promote the cohesiveness and stability of the sediment ( Tolhurst et al, 2002 ), impacting bedform morphology ( De Winder et al, 1999 ; Malarkey et al, 2015 ). In estuaries, EPSs also bind clay and sand particles, resulting in the formation of thin clay envelopes, also referred to as detrital clay coats, that surround sand grains ( Wooldridge et al, 2017 ; Virolle et al, 2019b , 2021 ; Duteil et al, 2020 ).…”

Critical Shear Stress for Erosion of Sand-Mud Mixtures and Pure Mud

“…The sand particles in sand-dominated mixtures have been found usually coated by a thin layer of the mud particles (Revil et al, 2002;Duteil et al, 2020;Van Rijn, 2020;Worden et al, 2020). The phenomenon is often documented as grain coating and ascribed to the adhesive force (the attraction/bonding between two particles of different media), diagenesis, biological actions, etc.…”

“…The phenomenon is often documented as grain coating and ascribed to the adhesive force (the attraction/bonding between two particles of different media), diagenesis, biological actions, etc. (Duteil et al, 2020;Worden et al, 2020). Here, we assume that the sand particles in the sand-dominated mixtures are coated by a thin layer of the mud particles, and when a sand particle is disrupted from the bed surface, the coating layer is entrained together with the sand particle (Figure 3).…”