15 INTERFACIAL MICROBIAL MATS AND BIOFILMS

Biofilms and microbial mats can be defined as surface-associated layers of microbial cells embedded in extracellular polymeric substance (EPS; Characklis and Marshall, 1989; Stal and Caumette, 1994). Biofilms cover solid surfaces, while mats cover sediments. Biofilms range from a few cell layers to a maximum of a few mmillimeters in thickness, and microbial mats range from <1 mm to several centimeters in thick-ness. Furthermore, most microbial mats are characterized by high population densi-ties of photoautotrophic microorganisms that act as primary producers in the top millimeters and build up the mat matrix, while many biofilms are heterotrophic and rely on substrate supply from the surface or the surrounding water. Despite these fundamental differences, biofilms and microbial mats share many characteristics, and both represent communities with complex strategies for microbial life at surfaces where steep gradients of physicochemical variables are found (chapters 8 and 14). Mats cover sediments of shallow waters with calm hydrodynamics and little graz-ing by animals. Typically, these habitats exhibit extreme environmental conditions, and prolific mats develop, for example, in hot springs and hypersaline waters (Karste