Applied and Environmental Microbiology, 2000
A variety of contemporary techniques were used to investigate the vertical distribution of thermophilic
unicellular cyanobacteria, Synechococcus spp., and their activity within the upper 1-mm-thick photic zone of the
mat community found in an alkaline siliceous hot spring in Yellowstone National Park in Wyoming. Detailed
measurements were made over a diel cycle at a 61°C site. Net oxygenic photosynthesis measured with oxygen
microelectrodes was highest within the uppermost 100- to 200-µm-thick layer until midmorning, but as the day
progressed, the peak of net activity shifted to deeper layers, stabilizing at a depth of 300 µm from midday
throughout the afternoon. Examination of vertical thin sections by bright-field and autofluorescence microscopy
revealed the existence of different populations of Synechococcus which form discrete bands at different
vertical positions. Denaturing gradient gel electrophoresis analysis of PCR-amplified 16S rRNA gene segments
from horizontal cryosections obtained at 100-µm-thick vertical intervals also suggested vertical stratification
of cyanobacterial, green sulfur bacterium-like, and green nonsulfur bacterium-like populations. There was no
evidence of diel migration. However, image analysis of vertical thin sections revealed the presence of a narrow
band of rod-shaped Synechococcus cells in which the cells assumed an upright position. These upright cells,
located 400 to 800 µm below the surface, were observed only in mat samples obtained around noon. In mat
samples obtained at other time points, the cells were randomly oriented throughout the mat. These combined
observations 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 adapted, some co-occurring with green sulfur bacterium-like populations, and all overlying green
nonsulfur bacterium-like populations.
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