Overview of biopolymer-induced mineralization: what goes on in biofilms?

Bacteria are associated with mineralization and dissolution processes, some of which may enhance or compromise the physical stability of engineered structures. Examples include stabilization of sediment dikes, bioplugging, biogrouting, and self-healing of concrete and limestone structures. In contrast to ‘biologically controlled’ precipitation (e.g. shells) of eukaryote organisms, microbial precipitation primarily results from two major processes: (1) ‘biologically induced’ precipitation, where microbial activities generate biogeochemical conditions that facilitate precipitation; and (2) ‘biologically influenced’ precipitation, where passive interactions of extracellular biopolymers and the geochemical environment drive precipitation. A common location for such biopolymers is the microbial ‘biofilm’ (i.e. cells surrounded within a matrix of extracellular polymeric substances (EPS). EPS biofilms occur commonly in both natural environments and many engineered surfaces. Emerging evidence now suggests that EPS inhibit, alter or enhance precipitation of calcium carbonate. Functional groups on EPS serve as initial nucleation sites, while other moieties function to control extent and types (e.g. crystals vs. amorphous organominerals) of precipitation. Understanding how to control, or even manipulate, precipitation/dissolution processes within the confines of EPS matrices will influence long-term structural integrities of materials. The present overview explores properties of EPS, and their potentially destructive (dissolution) and constructive (precipitation) effects on precipitation. Initial insight is offered for understanding how biopolymers might be controlled for applied purposes.