Distribution of phototrophic biofilms in cavities (Garraf, Spain)

The distribution of biofilms, the diversity of phototrophic organisms, and their relation to environmental conditions were studied in three limestone cavities in the karstic Garraf massif (Barcelona, NE Spain). Sixty-two taxa: 28 Cyanobacteria (42.8% Chroococcales, 21.4% Oscillatoriales, 7.1% Nostocales and 28.6% Stigonematales), 11 Chlorophyta, 16 Bacillariophyta and 7 lichens were identified. The environmental data obtained at three cavities by measuring temperature, relative humidity and light, showed a clear gradient from the entrance to a certain depth beyond which they remained stable. In a broad sense, the existence of three different levels can be assumed: a) Entrance level. The microclimate was strongly influenced by the outdoors. Scarcely attenuated light and abiotic factors fluctuated throughout the year. On the highly illuminated dry rocks the microflora colonies were quite rich, with special abundance of mucilaginous biofilms composed of algae and cyanobacteria typical of terrestrial aerophytic or atmophytic habitats. The community dominated by Scytonema julianum thrived in areas protected from rain. Trentepohlia sp. and many crustose lichens, which had this alga as a photobiont, were abundant. b) Intermediate level, with moderate abiotic oscillations and low light (25-0.5 mV) that was not yet a stress factor.A mixture of species formed biofilms. Cyanobacteria were their most visible constituents, occasionally mixed with green algae and diatoms. Biofilm abundance diminished with decreasing irradiance. c) Deep level (\u3c 1 mV) until light extinction, with stable abiotic factors (10°C and dew point humidity). Only a few species were able to colonize this dim light zone. The presence of remains and empty sheaths increased with decreasing irradiance, and except for Geitleria calcarea and Loriella osteophila, the algae were heavily colonized by bacteria and filamentous fungi