Highly ordered vertical structure of Synechococcus populations within the one-millimeter-thick photic zone of a hot spring cyanobacterial mat

A variety of contemporary techniques were used to investigate the vertical distribution of thermophilic unicellular cyanobacteria, Synechococcus spp,, and their activity within the upper l-mm-thick photic zone of the mat comm unity found in an alkaline siliceous hot spring in Yellowstone National Par k in Wyoming. Detailed measurements were made over a diel cycle at a 61 deg rees C site. Net oxygenic photosynthesis measured with oxygen microelectrod es was highest within the uppermost 100- to 200-mu m-thick layer until midm orning, but as the day progressed, the peak of net activity shifted to deep er layers, stabilizing at a depth of 300 mu m from midday throughout the af ternoon. Examination of vertical thin sections by bright-field and autofluo rescence microscopy revealed the existence of different populations of Syne chococcus which form discrete bands at different vertical positions. Denatu ring gradient gel electrophoresis analysis of PCR-amplified 16S rRNA gene s egments from horizontal cryosections obtained at 100-mu m-thick vertical in tervals also suggested vertical stratification of cyanobacterial, green sul fur bacterium-like, and green nonsulfur bacterium-like populations. There w as no evidence of diel migration. How-ever, image analysis of vertical thin sections revealed the presence of a narrow band of rod-shaped Synechococcu s cells in which the cells assumed an upright position. These upright cells , located 400 to 800 mu m below the surface, were observed only in mat samp les obtained around noon. In mat samples obtained at other time points, the cells were randomly oriented throughout the mat. These combined observatio ns reveal the existence of a highly ordered structure within the very thin photic zone of this hot spring microbial mat, consisting of morphologically similar Synechococcus populations that are likely to be differentially ada pted, some co-occurring with green sulfur bacterium-like populations, and a ll overlying green nonsulfur bacterium-like populations.