Lithic cyanobacteria as bioindicators for dewless habitats within a dew desert

Abstract

While previous measurements in a small drainage basin in the Negev Desert (Sede Boqer) discovered dewless habitats (<0.03 mm of non-rainfall water, NRW), which are inhabited by lithic cyanobacteria, it is not clear whether these findings can be generalized to other locations in the Negev. The question is especially relevant for high elevation sites (1000 m above msl) where previous measurements recorded high amounts of dew and fog and lush cover of lichens. Towards this end, measurements along an elevational gradient were carried out, at Nizzana (NIZ), 250 m above msl, Sede Boqer (SB), 550 m above msl, and Har Harif (HH), 1000 m above msl. The measurements included (a) cover and biomass (chlorophyll a) of the cyanobacteria and lichens on bedrocks along a 30 m transect at the south-facing slopes, (b) simultaneous monitoring of the surface temperatures of bedrocks inhabited by cyanobacteria and lichens, and (c) periodic measurements of NRW at these habitats using the cloth-plate method (CPM) and cloths attached to rock surfaces inhabited by both lithobionts. A significant decrease in minimum temperatures and increase in NRW (as recorded by the CPM) with elevation was recorded following the pattern: NIZ (0.12 mm) < SB (0.20 mm) < HH (0.29 mm). A significant increase in NRW with elevation was also recorded at the lichen habitat (with NRW of up to 0.51 mm), but yet NRW at the cyanobacteria habitat was always <0.04 mm, i.e., below the necessary threshold required for their activation. The findings point to the fact that variability in rock temperatures dictates the amount of NRW, and regardless of the general increase in NRW with elevation, cyanobacteria are confined to dewless habitats. A prokaryote–eukaryote partition in accordance with NRW is thus apparent, with cyanobacteria serving as useful bioindicators for dewless habitats in the Negev.