**2. Material and methods**

Cyanobacteria samples were collected from walls and ceiling of limestone caves where evidence of biological colonization was present. Algal crusts were scraped from the stone substrata (**Figure 1**) using nondestructive adhesive tape method also by removing the biofilm with sterilized scalpel, stored in labeled sterile plastic bags and used directly for observation under a light microscope (LM) or as inoculate for cultures on agar plates. Cyanobacterial material was cultured on agar plates or liquid medium in lab, and in consequence, filamentous and coccoid species were obtained. The samples were studied using a light microscope at 100× objective with oil immersion. Photomicrograph was taken using photomicroscope equipped with a digital camera.

For transmission electron microscopy (TEM), samples fixed in glutaraldehyde were postfixed in an osmium tetroxide solution, dehydrated in a graded ethanol series, and embedded in epoxy resin. Thin sections were collected on copper grids and stained with uranyl acetate and lead citrate. For scanning electron microscopy (SEM), cleaned sample was dried onto a coverslip, attached to an aluminum stub, and then gold coated, whereas environmental variables (temperature, light, and humidity) were measured using digital thermohygrometer and photoactinometer. For each parameter, the mean value with standard error was calculated.

**Figure 1.** Dark-green—colored cyanobacteria patches on walls in Sąspowska cave (Ojców National Park, Poland).
