3.5: Biofilms

Microorganisms can live planktonically, which is to say they can live freely within solutions. However, biofilms are the dominant mode of microbial life in terms of cell abundance and metabolism in ecosystems that can supply nutrients sufficient for growth (Costerton et al., 1995; Flemming and Wuertz, 2019). A biofilm is a community of microorganisms that is encased in extracellular polymeric substances (EPS) (Fig. \(3.5\)). Biofilms are often attached to a solid surface or an interface, such as the interface between air and water. Biofilms can also be dispersed in solutions. Where that is the case, they can be referred to as flocs or granules (Davey and O’Toole, 2000) (Fig. \(3.6\)).

Life within a biofilm provides cells with a more secure existence. Microbial biofilms are diverse in terms of their composition and morphology and usually house a mixture of species (Flemming and Wingender, 2010). Cells within biofilms are physiologically distinct from free-living cells of the same species (Costerton et al., 1995). The biofilm provides these cells with shelter that helps minimize exposure to environmental stresses (Hall-Stoodley et al., 2004). In addition, biofilms help cells maintain proximity to extracellular resources (Flemming and Wingender, 2010).

As an example of the sheltering effect of biofilms, research on microbial aspects of geological carbon storage indicates that biofilms can help subsurface microorganisms endure the stress of exposure to high levels of carbon dioxide. During geological carbon storage, carbon dioxide is captured at a point source, compressed, and injected into a rock layer within the subsurface (Bickle, 2009). Over time, the carbon dioxide will react with groundwater and minerals in the bedrock, causing it to become trapped as dissolved inorganic carbon and carbonate minerals.

Microorganisms catalyze many of the reactions that occur in the subsurface, so numerous recent studies have considered how high pressure carbon dioxide will affect subsurface microbial communities and how those communities might ultimately affect the fate of carbon dioxide (Kirk et al., 2016; Mu and Moreau, 2015). Those studies have shown that microorganisms that live within biofilms, as well as those with gram-positive cell walls and the ability to form spores, can better endure the stress associated with high levels of carbon dioxide (Bertoloni et al., 2006; Furukawa et al., 2004; Mitchell et al., 2008; Watanabe et al., 2003; Zhang et al., 2006). Thus, these properties of cells may help determine how microbial communities change after carbon dioxide injection in carbon storage reservoirs.

Figure \(3.5\): Heterogenous composition of microbial biofilm. Biofilms are composed of microorganisms encased within their extracellular polymeric substances (EPS). Compositional gradients in species composition and distribution occur alongside gradients in nutrient supply and redox conditions. Numerous components are found within the matrix, including polysaccharides, proteins, and environmental DNA (eDNA). Entanglement of these components helps provide cohesion. Image modified from Boudarel et al., (2018).

Figure \(3.6\): Cultures containing strains of the bacterium Zoogloea resiniphila. The culture on the left contains floc formed by a strain of Z. resiniphila that was isolated from a sludge wastewater treatment reactor. The culture on the right contains a mutant version of the bacterium that has lost the ability to form a floc. Instead, the cells are suspended in the culture solution, causing it to become turbid. This photo was taken by Ms. Na Gao for the purpose of donation to the public domain. Public Domain, https://commons.wikimedia.org/w/index.php?curid=74138940