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 1.06×10⁴ to 5.06×10⁵ per cm². 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 2.32×10⁴ bacteria per cm² 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.