Fungi active in weathering of rock and stone monuments

Abstract

Fungal colonization of sandstone and granite from Antarctica was studied. Granite from a church, sandstones from a monument and a courthouse in Germany, glazed bricks from a German cathedral, and some other stone types were also examined. All samples contained fungi and heterotrophic bacteria, often also cyanobacteria or algae. For cell counting and enrichment of microorganisms the samples were crushed aseptically, suspended in NaCl–Tween-80, plated on oligotrophic media, and incubated at 16–25 °C dark or in dim light. Total biomass was determined as phospholipid-bound phosphate (PLP). Rock samples were also viewed by electron microscopy. Typical viable fungal cell numbers (CFU∙g−1 dry weight) were the following: sandstone 1.6 × 104, granite 6.2 × 105, and glazed bricks 1.2 × 105. Total biomass ranged from 41 (sandstone) to 137 (glaze) nmol PLP∙g−1 dry weight; antarctic sandstone had 88 nmol PLP∙g−1. Fungal genera identified were the following: Alternaria, Aspergillus, Aureobasidium, Candida, Cladosporium, Paecilomyces, Phoma, Penicillium, and Sporobolomyces. Scanning electron microscopy revealed fungal bridging of open spaces with their hyphae or close contact between fungal hyphae and coccal cells believed to be algae. In some cases fungi were found to produce secondary minerals or their hyphae were covered with partially etched mineral layers. Exopolymer formation was common among the endolithic microorganisms. Mites were found to feed on epiliths. In pure culture, fungi from sandstone produced in 24 h cultures organic acids (citrate, glutamate, pyruvate, malate, succinate, lactate, formate, fumarate, and oxalate). Many of these acids could also be extracted from rock samples. Fungal growth on glucose and mineral powder resulted in up to 60% cation extraction from the mineral. Some fungi were antibiotically active against bacteria or yeasts but also stimulated other bacterial isolates. In summary, epi- and endo-lithic communities contained fungi that probably contribute substantially to the deterioration of many stones and monuments. Key words: rock weathering, biomass, acid excretion, cation extraction.