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Phil. Trans. R. Soc. B (2010) 365, 3645–3653 doi:10.1098/rstb.2010.0283

Review

Biodiversity and biogeography of the atmosphere
Ann M. Womack, Brendan J. M. Bohannan and Jessica L. Green*
Center for Ecology and Evolutionary Biology, 335 Pacific Hall, 5289 University of Oregon, Eugene, OR 97403-5289, USA The variation of life has predominantly been studied on land and in water, but thisfocus is changing. There is a resurging interest in the distribution of life in the atmosphere and the processes that underlie patterns in this distribution. Here, we review our current state of knowledge about the biodiversity and biogeography of the atmosphere, with an emphasis on micro-organisms, the numerically dominant forms of aerial life. We present evidence to suggest that the atmosphere is ahabitat for micro-organisms, and not purely a conduit for terrestrial and aquatic life. Building on a rich history of research in terrestrial and aquatic systems, we explore biodiversity patterns that are likely to play an important role in the emerging field of air biogeography. We discuss the possibility of a more unified understanding of the biosphere, one that links knowledge about biodiversityand biogeography in the lithosphere, hydrosphere and atmosphere. Keywords: biogeography; biodiversity; micro-organism; air; atmosphere

Noi viviamo sommersi nel fondo d’un pelago d’aria. We live submerged at the bottom of an ocean of air. (Evangelista Torricelli 1644 quoted in Middleton 1963)

1. INTRODUCTION As humans, we have an intimate relationship with the air around us. This relationshipis by and large unconscious; we breathe in without thinking, move through the eddies and tides of air often without notice. This largely unconscious relationship has led to a delayed appreciation of the air as a biological entity. But air is as alive as soil or water. Not only does it host large macroscopic organisms such as a soaring hawk or a drifting wildflower seed, but it also hosts a widevariety of micro-organisms. Hundreds of thousands of individual microbial cells can exist in a cubic metre of air (Burrows et al. 2009b), representing perhaps hundreds of unique taxa (Brodie et al. 2007; Fierer et al. 2008; Bowers et al. 2009). The ecology of these organisms—their diversity, distribution and interactions—is poorly understood. Given our intimate relationship with air, this lack ofknowledge comes at a great cost. The life of the air, especially the microbial life, is in constant interaction with human life, both directly as a source of pathogenic and beneficial microbes (Kellogg & Griffin 2006) and indirectly through biological effects on atmospheric processes (Deguillaume et al. 2008). The atmosphere—the layers of air surrounding the Earth—has been described as ‘one of thelast frontiers
* Author for correspondence (jlgreen@uoregon.edu). One contribution of 16 to a Discussion Meeting Issue ‘Biological diversity in a changing world’.

of biological exploration on Earth’ (Rothschild & Mancinelli 2001). In this paper, we summarize our current state of knowledge of the ecology of the atmosphere, with an emphasis on the atmosphere’s biogeography. Biogeography is thestudy of patterns in the distribution of life and the processes that underlie these patterns (Lomolino et al. 2006). The air has long been recognized as an important conduit for the movement of organisms from one geographical location to another, and thus is important for the biogeography of land and water. However, it is commonly assumed that the atmosphere is not a habitat in its own right butmerely a conveyance for terrestrial and aquatic life. We review evidence that challenges this assumption and suggests the existence of metabolically active and actively reproducing organisms in the atmosphere. We argue that the atmosphere has a biogeography of its own. Our discussion will focus on micro-organisms, the numerically dominant forms of life in the atmosphere.

2. A BRIEF HISTORY OF...