Microbial Endoliths as Internal Biofilms

Microbial biofilms could be defined as living organic interfaces between inorganic physical states of matter. They act as arbitrators in interactions that occur at the interfaces between the lithosphere on one side and hydrosphere and atmosphere on the other. Some dissolve the substrate, and penetrate its interior. Microbial occupants of the lithobiontic ecological niches thus include both the exterior (epiliths) and interior (endoliths) of solid substrate surfaces. Euendoliths actively penetrate calcareous substrates and thereby create the endolithic microenvironment, whereas crypto- and chasmoendoliths take advantage of the existing internal microenvironments which they modify (Golubic et al. 1981, Schneider & Le Campion-Alsumard 1999). Microbial endoliths cope successfully with ecological conditions ranging from extremes of polar and alpine deserts to the most optimized environments in tropical reefs (Jaag 1945, Chazotte et al. 1995, Büdel 1999, Pentecost & Whitton, 2000, Vincent 2000, Wynn-Williams 2000). Their colonization involves pioneers of newly exposed substrates (Le Campion-Alsumard 1975), and successions of changing populations toward establishment of stratified microbial ecosystems. The distribution of endolithic biofilms is global, but the ecological settings under which they occur are extremely diverse and so are the assemblages of microbial endoliths that occupy them. Their constituents differ significantly from one another in appearance, complexity, taxonomic composition, in metabolic and ecological properties, as well as in the timing of their activities. Within the euphotic zone, autochthonous primary producers, mostly cyanobacteria and algae, usually dominate endolithic communities. Endolithic communities, which extend into the aphotic environment are composed of heterotrophs which depend on organic matter in the substrate they penetrate, such as skeletons and bioclasts (Golubic et al. 1975). Older literature is reviewed by Radtke et al. (1997).