Biogeography of the Cyanidiaceae (Rhodophyta) based on 18S ribosomal RNA sequence data

The only eukaryotes found in highly acidic environments (pH 0.5-3) with ele vated temperatures (up to 56 degreesC) are three species of unicellular red algae: Cyanidioschyzon merolae, Cyanidium caldarium and Galdieria sulphura ria. These habitats are scattered all over the world and are usually very s mall. Because all three species are strictly acidophilic and will not toler ate desiccation, distribution by wind or water seems very unlikely. The pop ulations in the various habitats might have been isolated for very long tim es, providing a unique opportunity to observe a significant degree of indep endent evolution under strong, selective pressure. We investigated the biog eography of 18 isolates of these red algae by comparison of partial sequenc es of the 18S rRNA. A gene tree based on ISS rRNA assigns 15 strains to one branch with high bootstrap values. These isolates share the feature of fac ultative heterotrophy, whether or not they were originally placed in the ge nus Galdieria. The remaining three strains - Galdieria maxima, C. merolae a nd C. caldarium - form a sister clade to this group. The exact position of these two groups in relation to other red algae remains unresolved. The evo lutionary distance between individual Galdieria strains is high, indicating that within this genus several races have developed significantly altered 18S sequences. Our comparison of 18S sequences from thermo-acidophilic red algae indicates that even within a seemingly homogeneous group of eukaryoti c organisms the limits of phylogenetic analysis may be reached.