Morphological, phylogenetic and physiological diversity of cyanobacteria in the hot springs of Zerka Ma

Th e freshwater thermal springs of Zerka Ma’in, located in Jordan in the mountains of Moab east of the Dead Sea, are densely inhabited by cyanobacteria up to the highest temperature of 63 °C. We have investigated the cyanobacterial diversity of these springs and their outfl ow channels by microscopic examination, culture-dependent and culture-independent phylogenetic analysis, and by physiological studies of selected isolates of special interest. Both unicellular and fi lamentous types of cyanobacteria are present, and we identifi ed morphological types such as Th ermosynechococcus, Chroogloeocystis, Fischerella (Mastigocladus), Scytonema (occurring as large masses at lower temperatures), and others. Although morphologically similar cyanobacteria have been identifi ed in hot springs world-wide, the Zerka Ma’in strains were phylogenetically distinct based on 16S rRNA gene sequence analysis. Considerable diversity was detected also in the gene sequences of nifH (nitrogenase reductase), encoding one of the key enzymes involved in nitrogen fi xation. Nitrogen fi xation in a Mastigocladus isolate obtained from the springs was investigated in further depth. Th e heterocystous strain could fi x nitrogen (as assayed by acetylene reduction) at temperatures up to 53 °C.