Introduction

Looking to the future, what are the greatest challenges our society (and the world) faces? Ensuring an adequate food supply, clean air, and clean water, are problems intimately linked to the environment. Given the rate of accumulation of environmental damage, it seems appropriate to ask, can science and technology solve the problems associated with pollution and global change before it is too late? Where should we invest our scientific and technological efforts, and what might these investments yield?

One of the mysteries concerning environmental processes is the role of extremely small particles that, to date, have defied detection and/or characterization. We now realize that materials with dimensions on the nanometer scale (intermediate between clusters and macroscopic crystals) are abundant and persistent in natural systems. Nanoparticles are products of, and substrates for, nucleation and growth in clouds. They are also the initial solids formed in water, soils, and sediments. They are generated in chemical weathering and biologically mediated redox reactions, during combustion of fuel, and in manufacturing. For example, nanoparticles are by-products of microbial energy generation reactions that utilize inorganic ions (e.g., Mn, Fe, S, U) as electron donors or acceptors. They are highly reactive due to their large surface areas, novel surface structures, and size-dependent ion adsorption characteristics and electronic structures (including redox potentials). It is likely that they exert a disproportionately large, but as yet incompletely defined, influence on environmental geochemistry because they provide a means for transport of insoluble ions and present abundant novel, reactive surfaces upon which reactions, including catalytic reactions, occur.

It is widely accepted that the most rapid growth in knowledge in recent years has occurred in the field of biology. In the environmental context, the biology of single-celled organisms represents a critically important focus, for several reasons. First, microbes are extraordinarily abundant. They underpin many of the biogeochemical cycles in the environment and thus directly impact the bioavailability of contaminants and nutrients in ecosystems. They are responsible for the formation of reactive mineral particles and contribute to mineral dissolution. With analysis of these connections comes the ability to use microbes to solve environmental problems. Second, microorganisms are relatively simple, hence detailed analysis of how they work represents a tractable problem. Third, microbes have invented ways to carry out chemical transformations via enzymatic pathways at low temperatures. These pathways have enormous industrial potential because they provide energetically inexpensive routes to extract, concentrate, and assemble materials needed by society. Identification of the relevant microbial enzymatic or biosynthetic pathways requires analysis of the full diversity of microbial life, with emphasis on organisms in extreme natural geologic settings where metabolisms are tested at their limits.

Where does our understanding of microbes and nanoparticles in the environment stand today? Despite the fact that microbes dominate every habitable environment on Earth, we know relatively little about how most microbial cells function. Similarly, we have only just begun to connect the novel properties and reactivity of nanoparticles documented in the laboratory to phenomena in the environment. Although our understanding of these topics is in its infancy, science is changing quickly. The center of this revolution is the combination of molecular biology, nanoscience, and geoscience.

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