130 "Plantae" Emended Based on the Common History of the Primary Plastid Endosymbiosis

Travertine terraces have been deposited by calcareous hot springs in Yellowstone from as early as 365,000 years to the present. Most of these porous and non-porous CaCO3 rocks (old or new) contain a 1–2 mm thick greenish band about 1–3 mm below the upper surface. These bands are composed of cyanobacteria and, sometimes, unicellular green algae. Although some moisture may be retained for much of the year, all undergo freezing in winter and desiccation in summer. DGGE (denaturing gradient gel electrophoresis), with subsequent 16S rDNA sequence analyses of bands, has shown that relatively few phylotypes of cyanobacteria are present, but some occur in travertine of very different ages, indicating secondary establishment of the communities. Clonal cultures of predominant types have also been established and sequenced. All those tested are able to survive extreme desiccation. Preliminary sequence analyses of cultures show that some strains are nearly identical to known cyanobacterial strains while others show little similarity. One sequence is 100% identical to the cyanobacterium Cyanobium gracile. This cyanobacterium is known to be distributed worldwide in lakes and brackish seas, but not in a cryptoendolithic environment. Another sequence shows 99% identity to two cyanobacteria isolated from Antarctic freshwater ponds. Both Antarctic ponds and Yellowstone travertine are environments in which adaptations for desiccation and/or freezing tolerance could be crucial. The lack of ecological similarity among some of these strains indicates that genes other than 16S rDNA must be used for differentiation. These results will be discussed along with the ecology of travertine habitats.