Biofilms in caves: easy method for the assessment of dominant phototrophic groups/taxa in situ

Domination of certain aerophytic phototrophic group or specific taxon in biofilms is connected with biofilm features recognised in situ. Well-developed, gelatinous, olive to dark-green biofilms are composed mostly of coccoid cyanobacterial forms. The same features, characterised biofilms dominated by one coccoid taxon, except the latter were vividly coloured. Gloeobacter caused the appearance of purple, Gloeocapsa representatives yellow and Chroococcidiopsis black biofilm. The brown to the dark colour of heterocytous biofilms was mainly caused by Nostoc. Simple trichal Cyanobacteria were occasionally present in biofilm, except in one blue-coloured sample. According to the principal component analysis (PCA), well-developed and gelatinous biofilms were correlated with Cyanobacteria, while scanning electron microscopy (SEM) revealed richness of extracellular polymeric substances (EPS) in such biofilms. Biofilm with calcified cyanobacterium (Geitleria cf. calcarea) was also found. Chlorophyta-abundant biofilms (many rich in Desmococcus), thinner than cyanobacterial, were predominantly green and occasionally yellow and blue. Many were dry when observed in situ (confirmed with PCA), with few being moistened (i.e. Klebsormidium-dominant). Diatom biofilms were usually developed on sediment, mosses or near seeping water (demonstrated by PCA) and were also thinner than cyanobacterial ones. Compared to cyanobacterial biofilms, SEM showed less developed EPS in those rich in diatoms and green algae, where microorganisms are more exposed to the environment. The study demonstrates an easy method for biofilm assessment based on visual characterisation and provides encouragement for more frequent biofilm investigation in caves that can be important from an ecological, biological, biotechnological point of view and which assessment can have an irreplaceable role in potential monitoring and protection.