Diverse Microhabitats Experienced by Halomonas variabilis on Salt-Secreting Leaves

The leaf surfaces of the salt-excreting treeTamarix aphylla harbor a wide diversity of halophilic microorganisms, includingHalomonas sp., but little is known of the factors that shape community composition in this extreme habitat. We isolated a strain ofHalomonas variabilis from the leaf surface ofT. aphylla and used it to determine the heterogeneity of salt concentrations experienced by bacteria in this environment. This halophilic strain was transformed with aproU ::gfp reporter gene fusion, the fluorescence of which was responsive to NaCl concentrations up to 200 g liter−1 . These bioreporting cells were applied toT. aphylla leaves and were subsequently recovered from dew droplets adhering to the leaf surface. Although cells from within a given dew droplet exhibited similar green fluorescent protein fluorescence, the fluorescence intensity varied between droplets and was correlated with the salt concentration measured in each drop. Growth ofH. variabilis was observed in all droplets, regardless of the salt concentration. However, cells found in desiccated microniches between dew drops were low in abundance and generally dead. Other bacteria recovered fromT. aphylla displayed higher desiccation tolerance thanH. variabilis , both in culture and on inoculated plants, despite having lower osmotic tolerance. Thus, theTamarix leaf surface can be described as a salty desert with occasional oases where water droplets form under humid conditions. While halotolerant bacteria such asHalomonas grow in high concentrations of salt in such wet microniches, other organisms are better suited to survive desiccation in sites that are not wetted.