The substantial and impressive changes in microbial ecology can scarcely be chronicled in a meaningful fashion, and a review series such as Advances in Microbial Ecology can thus not do justice to the numerous studies that have been published in recent years. On the other hand, the mere existence of this series bears testimony to the many and diverse activities. The growing concern with microbial communities and processes in natural ecosystems is not restricted to scientists in one region and is not limited to particular groups of organisms or to individual theoretical or applied problems. The recent and successful international symposium on microbial ecology held in New Zealand-sponsored in part by the International Commission on Microbial Ecology, as is the Advances-and the general microbiology and ecology conferences and congresses have included reports from investigators from all corners of the globe and have explored both new and traditional areas, agricultural and public health problems, individual species and complex communities, and heterotrophs and autotrophs as well as ecosystem models relying on mathematical concepts and environmental processes needing sophisticated chemistry for their definition. The reviews in the present volume thus can offer only a minute sampling of the multitude of topics being actively explored at the present time. Two of the reviews focus attention on biogeochemical cycles regulated by microorganisms, in particular the way these organisms contribute to or control the levels and identities of chemical substances in the atmosphere. The chapter by Y. Dommergues, L. W. Belser, and E. L.