Cleaning of Phototrophic Biofilms in a Show Cave: The Case of Tesoro Cave, Spain

Show caves have different grades of colonization by phototrophic biofilms. They may receive a varied number of visits, from a few thousand to hundreds of thousands of visitors annually. Among them, Tesoro Cave, Rincon de la Victoria, Spain, showed severe anthropic alterations, including artificial lighting. The most noticeable effect of the lighting was the growth of a dense phototrophic community of cyanobacteria, algae and bryophytes on the speleothems, walls and ground. The biofilms were dominated by the cyanobacterium <i>Phormidium</i> sp., the chlorophyte <i>Myrmecia israelensis</i>, and the rhodophyte <i>Cyanidium</i> sp. In many cases, the biofilms also showed an abundance of the bryophyte <i>Eucladium verticillatum</i>. Other cyanobacteria observed in different biofilms along the cave were: <i>Chroococcidiopsis</i> sp., <i>Synechocystis</i> sp. and <i>Nostoc</i> cf. <i>edaphicum</i>, the green microalgae <i>Pseudococcomyxa simplex</i>, <i>Chlorella</i> sp. and the diatom <i>Diadesmis contenta</i>. Preliminary cleaning tests on selected areas showed the effectiveness of hydrogen peroxide and sodium hypochlorite. A physicochemical treatment involving the mechanical removal of the thickest layers of biofilms was followed by chemical treatments. In total, 94% of the surface was cleaned with hydrogen peroxide, with a subsequent treatment with sodium hypochlorite in only 1% of cases. The remaining 5% was cleaned with sodium hypochlorite in areas where the biofilms were entrapped into a calcite layer and in sandy surfaces with little physical compaction. The green biofilms from the entire cave were successfully cleaned.