Fluorescent in situ hybridization (FISH) and 16S rDNA analysis were used to characterize the endolithic colonization of silica-rich rhyolitic glass (obsidian) in a barren terrestrial volcanic environment in Iceland. The rocks were inhabited by a diverse eubacterial assemblage. In the interior of the rock, we identified cyanobacterial and algal 16S (plastid) sequences and visualized phototrophs by FISH, which demonstrates that molecular methods can be used to characterize phototrophs at the limits of photosynthetically active radiation (PAR).
Temperatures on the surface of the dark rocks can exceed 40*C but are below freezing for much of the winter. The rocks effectively shield the organisms within from ultraviolet radiation. Although PAR sufficient for photosynthesis cannot penetrate more than 250