Analytical-modeling analysis of how pore-water gradients of toxic metals confer community resistance

We develop a mechanistic explanation of how microbial communities are able to protect themselves from toxicity from inflowing metal concentrations much higher than the metal-toxicity thresholds of individual microorganisms. We propose a general bio-protection mechanism, of widespread applicability to microbial communities, by which some bacteria induce pore-water metal gradients by producing ligands that bind the metal toxicant, reducing the toxicant’s concentration to non-inhibitory levels for much of the community. Sulfate-reducing communities are a good example of community-based bio-protection. In particular, we develop analytical solutions to derive metal-resistance criteria for two distinctly different systems displaying gradient-based resistance: permeable reactive barriers (PRBs), which are advection dominated, and sediments, which are diffusion dominated. In advection-dominated systems, the most significant variables influencing the development of static gradients are groundwater velocity and the rate of ligand production. By transporting the toxicant into the PRB and by preventing ligand from moving upgradient, a fast groundwater velocity can overwhelm the chemical gradient bio-protection mechanism. Likewise, the stability of a chemical gradient bio-protection scheme increases in proportion to the rate of ligand generation. In diffusion-dominated systems, resistance depends on the rate of ligand generation and the diffusion length for movement of metal into the sediment. For both cases, we derive quantitative stability criteria that include the phenomena described here. These criteria demonstrate that diffusion-dominated systems offer greater potential for gradient-based metal resistance than do advection-dominated systems. When diffusion controls transport, metal movement into the reactive zone can be slowed down, and a greater fraction of the ligand is available for reaction with the metal, since it is not swept away by advection.