The cyanobacterial community of the Zerka Ma'in hot springs, Jordan: morphological and molecular diversity and nitrogen fixation

The area of the Dead Sea, bordering Jordan and Israel, contains many springs with different physical and chemical properties. Hot springs are found on the eastern shore of the lake: the springs of Zara (the ancient Callirrhoe) with tem- peratures up to 59 °C and those of Zerka Ma'in, 5 km inland. The latter springs, mentioned by Josephus (1st century C.E.), yield fresh water with a low sulfide content, near-neutral pH and temperatures up to 63 °C. Although the growth of green microorganisms was already noted in 1807 by the German explorer ulrich Jasper seetzen, the microflora of the springs has remained unexplored. In 2005 we began a series of surveys characterizing the microbial diversity of the springs. The sources of the Zerka Ma'in springs and their outflow channels are covered by green to orange mats, containing a diverse community of unicellular and filamen- tous cyanobacteria. Conspicuous types include Thermosynechococcus , unicellular Gloeocapsa types, and Spirulina-like filaments. Large colonies of Scytonema were also found. Several representative types of Thermosynechococcus , Gloeocapsa, and Mastigocladus/Fischerella were isolated and cultured. Amplification of cyano- bacterial 16S rRNA genes from DNA extracted from the mats showed a high di- versity of Thermosynechococcus in the springs. Low concentrations of ammonium and nitrate in the spring water and the presence of heterocystous cyanobacteria indicated that biological nitrogen fixation may be important in the spring ecosys-