Abstract
Bacterial–mineral aggregates are the products of a tight biogeochemical coupling between microbes and geological media and play an outsized role in governing the composition of natural waters through biogeochemical cycling and mineral formation and dissolution processes. The results of combined batch column settling experiments, volumetric analyses, and microscopic investigations demonstrate that composite bacteriogenic iron oxide aggregates are sensitive to densification in response to hydrodynamic shear, a physical fluid phenomenon that introduces significant alterations to aggregate size and structure, permeability, and settling and transport behaviour. After exposing aggregate suspensions to varying degrees of shear stress, final solids volume fractions decreased by as much as 75% from initial data, while aggregate bulk density saw increases from 999 kg∙m–3 to as much as 1010 kg∙m–3. Inverse modelling of time course data yielded estimates for settling rate constants and initial settling velocities that increased with shear stress application. As well as having implications for aqueous contaminant transport and potential bacterial bioenergetic strategies, these results suggest the preservation potential of microfossils formed from bacterial–mineral aggregates may be significantly reduced with shear-induced alterations, leading to a possible underrepresentation of these microfossils in the sedimentary record and a gap in our understanding of early life on Earth.
Highlights
Owing to the ubiquity of microorganisms and associated biogenic substances in aqueous and terrestrial environments, minerals typically co-occur with various organic constituents, producing composite solids with unique physicochemical properties [1,2]
These bacterial–mineral aggregates function as focal points for interactions between the geosphere and biosphere and have been the subject of ongoing research, especially those formed in conjunction with iron oxyhydroxides; these appear in the literature variously as bacteriogenic iron oxides (BIOS) [3,4], cell-Fe(III) mineral aggregates [5], Fe cell-mineral aggregates [6,7], Fe flocs [8], and ferrihydrite–bacteria composites [2]
As BIOS aggregate mats tend to proliferate in low-energy aqueous environments, the hydrodynamic shear forces acting on them are expected to arise mainly from the shear zone above
Summary
Owing to the ubiquity of microorganisms and associated biogenic substances in aqueous and terrestrial environments, minerals typically co-occur with various organic constituents, producing composite solids with unique physicochemical properties [1,2]. Geosciences 2018, 8, x FOR PEER REVIEW This resemblance to hydrogels is relevant in the context of a natural phenomenon known asresemblance hydrodynamic shear-induced densification. Fields, In terms of fluid to poroelastic deformation shrinkage induced byand hydrodynamic stress in fluid flow resulting in mechanics, thisshrinkage behaviourand arises from the of development of localized pressure gradients around shear-induced densification the aggregates [17]. In terms of fluid mechanics, this permeable based differences in internal andaround external flow velocities, which behaviour hydrogel-like arises from thematerials development ofon localized pressure gradients permeable hydrogel-like result from more flow paths through the permeable aggregates
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