Biogeochemical controls on metal behaviour in freshwater environments

Abstract

The biogeochemical controls on metal behaviour in aqueous environments involve complex linkages of biological, principally bacterially driven, and geochemical processes, which occur at both microscopic and macroscopic scales. The framework of aqueous surface chemistry and aquatic geochemistry continues to provide the foundations of the emerging paradigm: (1) metal behaviour (e.g., transport, toxicity, bioaccumulation) is governed by solid-solution reactions; (2) pH, ionic strength, redox potential, the types and concentrations of solution elements, and solid surfaces all interact to determine metal behaviour in any given system; (3) metal sorption reactions show both metal ion and solid surface specificity; (4) sorption reactions are dynamic and reversible; and (5) processes are at sufficient pseudo-equilibrium or dynamic steady state that thermodynamics can be applied to describe such reactions. Reactions controlling metal behaviour are increasingly modelled, with some success, using a variety of geochemical modelling approaches all based on this framework. However, not yet considered in the majority of these thermodynamic treatments of metal dynamics is that these reactions are highly influenced by biological factors, which will affect their location, magnitude and rate. The extent of this influence will be largely driven by microbial ecology, and thus, a fundamental identification and mechanistic understanding of how these factors will drive the geochemistry of a particular system is required. The lack of substantive biogeochemical understanding stems from the fact that the field of environmental microbiology, with its crossover to environmental geochemistry, has only recently begun to receive attention. The developing evidence strongly underscores the impact of bacterial reactions for a number of highly relevant processes related to metal dynamics such as solid solution partitioning, mineral precipitation and dissolution reactions, and intense changes in system geochemical conditions. The development of new molecular level microscopic and spectroscopic techniques provides powerful tools to promote an integrated approach to understanding the mechanisms underlying metal dynamics, which encompasses both the geochemical and biological components of this dynamic and complex cycle. Particularly, when used in conjunction with new molecular biological tools, these multiple lines of evidence will provide a mechanistic model of the controls on metal behaviour that will reflect the inherently complex reality of natural systems.