10.3: Spatial scales of interactions

Microbial interactions can occur over scales that are much larger than microbial cells themselves. As described in Chapter 3, cellular microorganisms exist on the scale of microns. As such, direct interactions between them may often occur on the micron scale. However, microorganisms can directly interact over longer distances through filamentous structures. For example, most fungi exist within hyphae, which are only a few microns in diameter but can extend over the scale of meters and serve multiple functions, including nutrient absorption and transport. Cable bacteria, as a second example, construct filamentous structures over centimeter distances and use them to transfer electrons across redox zones (Marzocchi et al., 2014; Nielsen et al., 2010; Pfeffer et al., 2012).

In addition to direct interactions, microorganisms can also interact indirectly by altering the chemistry of an environment (Konopka, 2009). For example, aerobic respiration in recharge areas of aquifers can create downgradient anoxic zones that host the activity of anaerobic microorganisms (Bethke et al., 2008). By lowering oxygen concentrations, the aerobes create conditions that are favorable for the anaerobes, which is arguably consistent with commensalism. Indirect interactions are not necessarily as strong as direct interactions, but they have the potential to span larger distances.

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