Cave phototrophs and their relation to the rock substratum

Abstract

Aerophytic phototrophs (cyanobacteria and algae) represent special group of microorganisms capable to inhabit various substrates exposed to air, as long as the conditions for their settlement (substrate properties) and development (ecological parameters) are favourable. Rock surfaces are often susceptible to colonization of microorganisms, whether they are manmade or natural (exposed or hypogean – caves). Phototrophic biofilms that develop on rock substrates in caves (walls, ceilings, stalactites, stalagmites and other formations) are very common at naturally or artificially illuminated sites. Even though their proliferation is triggered by light and ecological/microclimatic parameters, interactions with the substratum still remain largely unknown. It is evident that substrate properties play an important role in settlement of these microorganisms and initiation of biofilm development, but further, it is uncertain to what extent contribute to their growth. Generally rocks are composed of many minerals that can be directly absorbed by cyanobacteria and algae when dissolved in water. For the purpose of this study, phototrophic microorganisms (cyanobacteria and algae) from biofilms developed on rock substrates were explored and petrographic analysis was performed in 15 Serbian caves. Considering phototrophs, four divisions (Cyanobacteria, Bacillariophyta, Chlorophyta and Xanthophyta) were recorded, and considering petrographic analysis, examined samples showed the domination of limestone with five different varieties recognized. The relationship between phototrophs and limestone varieties (divided due to different petrographical characteristics) was represented using PCA. Recrystallized, organogenic, microsparitic, micritic and clastic limestone were all separated along first PCA axis. This trend may be the consequence of different physical characteristics of these limestone types, such as porosity. However, certain phototrophc groups were connected to certain limestone types, i.e. Bacillariophyta showed positive correlation with clastic limestone, less with microsparite and micritic, while Cyanobacteria were correlated to the other two limestone types. Chlorophyta and Xanthophyta did not have clear separation according to substrate.