Abstract
Adherent epilithic microorganisms recovered from rocks submerged 10 to 20 cm in two different rivers were examined by electron microscopy and enumerated after dispersion in M-9 salts by viable plate counts. Bacterial cells concentrated in microcolonies were often observed attached to the surface of algae, cyanobacteria, and organic detritus. This structured communal mode of growth was common among epilithic microbial communities of different rock types. However, counts of heterotrophic bacteria from limestone (106 to 107 cfu/cm2) were 10- to 100-fold greater than corresponding values from granite, gabbro, rhyolite, basalt, and quartz. Cyanobacteria and algae were an order of magnitude less abundant compared with their bacterial counterparts. These variations in population densities of epilithic microorganisms present on different rocks were inversely related to mineral substrate hardness.Key words: epilithic microorganisms, mineral hardness.
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