Microbial influence on erosion, grain transport and bedform genesis in sandy substrates under unidirectional flow

Abstract

Interpreting the physical dynamics of ancient environments requires an understanding of how current‐generated sedimentary structures, such as ripples and dunes, are created. Traditional interpretations of these structures are based on experimental flume studies of unconsolidated quartz sand, in which stepwise increases in flow velocity yield a suite of sedimentary structures analogous to those found in the rock record. Yet cyanobacteria, which were excluded from these studies, are pervasive in wet sandy environments and secrete sufficient extracellular polysaccharides to inhibit grain movement and markedly change the conditions under which sedimentary structures form. Here, the results of flume experiments using cyanobacteria‐inoculated quartz sand are reported which demonstrate that microbes strongly influence the behaviour of unconsolidated sand. In medium sand, thin (ca 0·1 to 0·5 mm thick) microbial communities growing at the sediment–water interface can nearly double the flow velocity required to produce the traditional sequence of ripple→dune→plane‐bed lamination bedforms. In some cases, these thin film‐like microbial communities can inhibit the growth of ripples or dunes entirely, and instead bed shear stresses result in flip‐over and rip‐up structures. Thicker (ca≥1 mm thick) microbial mats mediate terracing of erosional edges; they also, foster transport of multi‐grain aggregates and yield a bedform progression consisting of flip‐overs→roll‐ups→rip‐ups of bound sand.

Deformed microbial mat structures in a semiarid temperate coastal setting

“…On the other hand, deformed microbial structures are the more outstanding structure to be found in a sedimentary cross-section; in that sense, involuted structures ( Fig. 7c) may indicate that folds and roll-up structures can be later preserved as those mentioned by Hagadorn and McDowell (2012) in ancient environments.…”

“…The first ones entangle particles, and most of the motile benthic microorganisms present in the surface are embedded in highly adhesive mucilages, collectively known as EPS (extracellular polymeric substances, Decho, 1990). Consequently, the characteristics of sediments are altered, thus increasing the erosive critical threshold in aquatic environments under unidirectional currents (Hagadorn and McDowell, 2012); tidal currents (Noffke, 2010), and even wave-shear stress (Cuadrado et al, 2014).…”

Bedform migration in a mixed sand and cohesive clay intertidal environment and implications for bed material transport predictions

“…Laboratory experiments and field measurements have demonstrated that bedforms can be inhibited from forming (Hagadorn and McDowell, 2012) and stabilized once formed (Grant et al, 1986) due to biological cohesion from extracellular polymeric substances (EPS) produced by benthic organisms. Recent laboratory experiments using mixed cohesive and non-cohesive sediment, and with added bacterial polymers as a proxy for natural biogenic stabilization, have shown that the dimensions of sedimentary bedforms decrease with increasing bed clay fraction and that the development rate of the bedforms is reduced by both physical and biological cohesion (Baas et al, 2013;Malarkey et al, 2015;Schindler et al, 2015;Parsons et al, 2016).…”

Why and How Do We Study Sediment Transport? Focus on Coastal Zones and Ongoing Methods

“…Bioturbation is another subject that calls for coupled physico-chemico-biological studies. Seabed erodibility is largely affected by algae, seagrasses, polychaete worms or cyanobacteria [152,[330][331][332][333]. In addition, there are many examples of coastal ecosystem-based management (see [334]) where interdisciplinarity is requested.…”