Biofilms of different colours that colonize surfaces within karst caves represent a source of nutrients. They occur commonly and abundantly at sites with sediments, and close to seepages or underground rivers. Golden-yellow subaerial biofilms are particularly well observed because of their contrast with their surroundings, the characteristics of the pigment and recently, even more, due to the characteristics of light-emitting diode (LED) illumination. Yellow microbial biofilms were sampled from three caves in southwestern Slovenia, Dimnice, Križna jama and Sveta jama. The highest concentration of cultivable microbes (2.33×108 CFU/g) and the biggest number of identified bacteria (66.0%) were retrieved from a sample from Sveta jama. Using MALDI-TOF (Matrix-Assisted Laser Desorption/Ionization Time-Of-Flight) for bacterial identification showed that different species of Pseudomonas prevailed in all samples. Yellow biofilms showed an absorption peak around 400 nm, and two emission peaks, a major in the blue (~460 nm) and a minor in the orange (~600 nm) parts of the spectrum when excited at 405 nm. Microbial mats that colonize surfaces are probably frequently overlooked in caves because they are difficult to observe when they have no pigmentation and the contrast with their surroundings is low. Additional studies are needed to aid the understanding of the role of pigmented biofilms and their interactions with underlying substrata in respect of the evolution of substrate micromorphology.



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