Microbial Role in Global Biogeochemical Cycling of Metals and Metalloids at the Interfaces in the Earth’s Critical Zone

Microorganisms are intimately involved in many processes of fundamental importance to geology, and these include biotransformations of metals and minerals, as well as related substances like metalloids, and metal radionuclides (

Gadd et al., 2005

;

Gadd, 2007

). Such processes are involved in bioweathering, mineral dissolution and formation, and soil formation and development. Integral to all mechanisms are interactions with metals, and microorganisms are intimately involved in metal biogeochemistry with a variety of processes determining mobility, and bioavailability (

Gadd, 2005

;

2008

). The balance between metal mobilization and immobilization varies depending on the organisms involved, their environment, and physico-chemical conditions. Metal mobilization can arise, e.g. from leaching mechanisms, complexation by metabolites, and methylation where this results in volatilization. Immobilization can result from sorption, transport and intracellular sequestration or precipitation as a variety of “organic” and inorganic biominerals, e.g. oxalates (fungi), carbonates, phosphates and sulfides (

Sayer et al., 1999

;

Burford et al., 2003

,

2006

).