ABSTRACT
Questions of biodiversity are so much more likely to be associated with the fate of larger plants and animals that one may wonder whether microorganisms matter at all in this debate. Nevertheless, microbial diversity has become the focus of intense research in the past decades, owing much to technical advances that now greatly facilitate the identification of microorganisms and their study. Despite being largely invisible to the naked eye, microorganisms account for a significant proportion of Earth’s biomass, species abundance, and richness. Even more importantly, it is now well-established that microorganisms drive massive biogeochemical cycles that affect the entire planet. Microbial diversity matters, and this chapter will be largely about that. Along the way, my objective will also be to point to specific philosophical questions that the study of microbial diversity raises. In the first section, I first provide a brief account of the concept of microorganisms and how the field of microbial diversity studies has recently developed in relationship to both microbiology and ecology. I then review, in the second section, a number of reasons why microorganisms are interesting to look at from a diversity point of view, in particular their quantity, ubiquity, and ecological significance. In the third section, I address the question of defining microbial diversity, and focus on the units of diversity problem in the context of microorganisms, with a special attention to taxonomic and functional perspectives. In the fourth section, I review key reasons why microbial diversity matters, including a set of applied and purely theoretical reasons. Because diversity studies venture into smaller and smaller entities, such as viruses, this raises the question of extending microbial diversity to non-cellular micro-entities, as I show in the fifth section. And in the sixth section, I review how conservation questions also arise in the context of microbial diversity.
