A novel terrestrial halophilic environment: The phylloplane of Atriplex halimus , a salt‐excreting plant

This paper describes the microbial ecosystem found on the leaves of Atriplex halimus , a salt‐excreting plant in the central Negev highlands of Israel. Because of the regular nightly occurence of dew at this location, these leaves undergo a diurnal wetting so that phylloplane microorganisms experience large fluctuations in salinity and water activity, as well as tolerate repeated desiccation. During the dry season, in the late spring and summer, a significant amount of salts and organic material coats the leaf surface. During dew events the salt concentration at the leaf surface was calculated to be > 0.4 M. Direct counts of the respiring bacteria on the leaf surface ranged from 1.06×10 4 to 5.06×10 5 per cm 2 . Using a variety of media it was shown that there was limited bacterial diversity which could be cultured, with greater than 90% of the isolates being orange colored Gram‐negative rods. Viable counts ranged from 0.32 to 2.32×10 4 bacteria per cm 2 of A. halimus leaf surface. No bacteria capable of nucleating ice were recovered in these studies. The dominant orange pigmented bacterium, identified as a halotolerant Pseudomonas sp., grew optimally at 30°C and at 5% NaCl and was capable of growth in media containing up to 20% NaCl. This bacterium could grow on a variety of organic compounds, including some associated with plant materials. The leaf bacteria were desiccation‐tolerant when on the leaf surface or when directly washed off the leaves, but much less so when in isolatd culture. A major component of the tolerance to desiccation is probably related to the compounds on the leaf surface.