Abstract
Aerobic biofilms were found to have a complex structure consisting of microbial cell clusters (discrete aggregates of densely packed cells) and interstitial voids. We used the Confocal Scanning Laser Microscope (CSLM) in conjunction with dissolved oxygen microelectrodes to examine the structural and chemical heterogeneity of fully hydrated, living biofilms in real time under flow conditions. The oxygen distribution within the biofilm was strongly correlated with these structures. The voids facilitated oxygen transport from the bulk liquid through the biofilm. Water could freely move through the channels within the biofilm inducing convective mass transfer of dissolved and particulate substrates.
Nuclear Magnetic Resonance Imaging (NMRI), in which the phase of the nuclear spin depends on the spin velocity, was used to show how fluid velocity varied in a conduit colonized with biofilm. Spin-lattice relaxation time was used at the same time to obtain images of biofilm density. The combined profiles revealed that the fluid velocity does not reach zero at the biofilm surface. This implied the existence of convective mixing inside the biofilm - a process which may have profound consequences for mass transport in biofilm systems. Particle tracking confirmed this hypothesis and provided detailed images of flow within the interstitial voids of the biofilm.
Structural and chemical heterogeneity may contribute to initiation of corrosion (induce microbially influenced corrosion (MIC)) on metal surfaces where biofilms have accumulated. The Scanning Vibrating Electrode (SVE) has been used to spatially and temporally map ion currents in solution above MIC anodic and cathodic sites. Chemically active surfaces often create surpluses of charged species which locally disturb the electroneutrality of the surrounding solution. As these ions diffuse into the bulk water they create concentration gradients which can be measured using the SVE. We have used the SVE to map the course of corrosion under calcium alginate, a biopolymer, which we deposit under controlled conditions to simulate the presence of a biofilm.
