Denaturing Gradient Gel Electrophoresis (DGGE) and subsequent sequencing of partial 16S rRNA gene sequences was used to investigate the molecular biodiversity of cyanobacterial communities inhabiting various lithified morpho-structures in a Yellowstone National Park hot spring. These morpho-structures — flat-topped columns, columnar cones, and ridged cones — resemble Precambrian stromatolites (Conophyton), which are presumed to be biogenic. The top, middle and bottom sections of these lithified morpho-structures, were analyzed to determine the vertical and spatial distribution of cyanobacterial communities. Results from DGGE indicate that the cyanobacterial community composition of lithified morpho-structures (flat-topped columns, columnar cones, and ridged cones) were largely similar in vertical distribution as well as among the morpho-structures studied. Analysis of partial 16S rRNA gene sequences obtained from these community profiles show that the closest relatives of these lithifying microorganisms are cyanobacteria detected in or isolated from higher temperature hot springs. Preliminary data also indicate that the phylogeny of cyanobacteria in these lithified morpho-structures may be different from those in surrounding non-lithified mats. These results provide additional support to the theory that certain cyanobacterial species are involved in the morphogenesis of lithifying morpho-structures in hot springs and may have played a role in the formation of extant lithified stromatolites as well as ancient ones.

Key words: 16S rDNA, DGGE, hot spring, lithifying cyanobacteria, microbial mat, phylogeny