This book is about the uses of diatoms (Class Bacillariophyceae), a group of microscopic algae abundant in almost all aquatic habitats. There is no accurate estimate of the number of diatom species. Estimates on the order of 104 are often given (Guillard & Kilham, 1977), and Mann and Droop (1996) point out that this estimate would be raised to at least 105 by application of modern species concepts. Diatoms are characterized by a number of features, but are most easily recognized by their siliceous (opaline) cell walls, composed of two valves, that together form a frustule (Fig. 1.1). The size, shape, and sculpturing of diatom cell walls are taxonomically diagnostic. Moreover, because of their siliceous composition, they are often very well preserved in fossil deposits, and have a number of other industrial uses.

This book is not about the biology and taxonomy of diatoms. Other volumes, for example, Round et al. (1990) and the review articles and books cited in the following chapters, provide introductions to the biology, ecology, and taxonomy of diatoms. Instead, we focus on the applications and uses of diatoms, with a further focus on environmental and earth sciences. Although this book contains chapters on direct applications, such as uses of fossilized diatom remains in industry, oil exploration, and forensic applications, most of the book deals with using these indicators to decipher the effects of long-term ecological perturbations, such as climatic change, lake acidification, and eutrophication.