Mechanisms by which biofilms thriving on metallic materials influence near-surface electrochemical processes are subject of extensive studies. Deterioration of a metal due to biofilm presence and/or activity is referred to as biocorrosion or microbiologically-influenced corrosion (MIC). The necessity of the contact between the cell and the surface has been demonstrated as prerequisite for MIC and sampling of biofilm populations is advocated in order to diagnose biocorrosion. There is, however, a tendency to implicate MIC as the cause of corrosion failures, based solely on sampling of bulk fluids, i.e. examining planktonic bacterial populations and mitigation strategies are often designed based on planktonic data. This approach can, perhaps, be partially justified when applied to systems where, e.g. seawater is re-circulating over an extended time period (years). Although planktonic and biofilm populations in such systems are, most likely, not too dissimilar, metal surface-associated population may still have markedly different phylogenetic and/or metabolic profile than the community present in uppermost part of the biofilm or/and in the bulk phase. It is also documented that in open environments, structures of biofilm and planktonic populations vary considerably. Thus, it is conceivable, that laboratory testing of MIC using planktonic organisms can be misleading. Indeed, laboratory measurements of corrosion rates in monocultures, enrichments, or even as-received biotic bulk fluids, rarely, if ever, match the rates reported in the field.
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