Volcanic areas with highly acidic solfatara soils and temperatures of up to 56 °C are inhabited by the red algal genus Galdieria. We examined three highly acidic but non-volcanic habitats in the western part of the Czech Republic for the occurrence of this red alga. In soil samples from the National Nature Reserve of Soos we found, together with Euglena mutabilis, Pseudococcomyxa simplex and species of Chlorella, a new strain of Galdieria. In contrast to all other Galdieria strains described so far, the strain from Soos exhibited a low temperature optimum for growth of about 30 °C. Other properties, such as the substrate spectrum for heterotrophic growth, ultrastructure, fatty acid composition, thermostability of enzymes and the nitrogen source, showed no obvious differences from other strains of Galdieria. Within a phylogenetic tree based on 18S rRNA sequence data, the strain from Soos occupied a position at the base of the ‘Galdieria’-branch. Our findings indicate that the genus Galdieria is not restricted to volcanic and mining areas and that strains of Galdieria are able to compete successfully with green algae in habitats like Soos.

The only eukaryotes found in highly acidic environments (pH 0·5–3) with elevated temperatures (up to 56 °C) are three species of unicellular red algae: Cyanidioschyzon merolae, Cyanidium caldarium and Galdieria sulphuraria. These habitats are scattered all over the world and are usually very small. Because all three species are strictly acidophilic and will not tolerate desiccation, distribution by wind or water seems very unlikely. The populations in the various habitats might have been isolated for very long times, providing a unique opportunity to observe a significant degree of independent evolution under strong selective pressure. We investigated the biogeography of 18 isolates of these red algae by comparison of partial sequences of the 18S rRNA. A gene tree based on 18S rRNA assigns 15 strains to one branch with high bootstrap values. These isolates share the feature of facultative heterotrophy, whether or not they were originally placed in the genus Galdieria. The remaining three strains – Galdieria maxima, C. merolae and 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 evolutionary 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 eukaryotic organisms the limits of phylogenetic analysis may be reached.

An image analysis system was used to count and measure acetoorcein-stained mitotic chromosomes of the acidothermophilic unicellular red algae Galdieria maxima, G. partita, G. sulphuraria and Cyanidium caldarium (Cyanidiophyceae). Chromosome numbers fell into two groups: n = 2 for all three species of Galdieria and n = 5(7) for C. caldarium. It seems that a separation of C. caldarium from Galdieria is karyologically justified. A chromosome number of 2 appears to be indicative of the genus Galdieria and could be used as a marker to distinguish this taxon from Cyanidium. The two smaller chromosomes in the karyotype of C. caldarium were about 0·4 μm long whereas the other three were 0·5–0·7 μm long. In karyotypes of Galdieria species, the two chromosomes differed in length, the smaller chromosome ranging from 0·8 to 1·8 μm and the larger one from 1·2 to 2·3 μm. The visualization of these extremely small chromosomes was possible due to pretreatment of the cells with the DNA intercalator 9-aminoacridine. The mean absolute length of each chromosome of the three members of Galdieria had statistically significant interspecies differences. Nuclear 1C DNA contents were estimated in the algal cells by the Feulgen technique. All species investigated had genome sizes of 1·50–2·25×10−2 pg. Thus it seems that the members of Cyanidiophyceae have the smallest known genomes of all photosynthetic eukaryotes.

The unicellular, acido- and thermophilic red alga Galdieria sulphuraria is unique in its ability to grow heterotrophically on at least 27 different sugars and polyols. The enzymatic machinery necessary for metabolizing this variety of compounds is constitutively expressed in the alga. The uptake system, however, has to be induced. From in vivo studies we conclude that the uptake system consists of several transporters with a partly overlapping substrate specificity. These transporters can be grouped into hexose, polyol and pentose transporters. The mode of induction depends on the substrate supplied for heterotrophic growth. When autotrophic cells were supplied with hexoses, hexose and polyol transporters were induced from the onset of heterotrophic conditions. Depletion of substrate in the medium triggered the induction of other transporters. Dulcitol, as well as other polyols and pentoses, led to a co-induction of all transporters as judged from induction and competition experiments. The heterotrophic capabilities of Galdieria are intriguing, because the natural habitat of the alga contains only traces of organic carbon. The substrates for heterotrophic growth apparently originate from decaying cells releasing hydrolysed cell material. Trace amounts of glucose were sufficient for the induction of sugar uptake in Galdieria. The co-induction of transporters obviously enables the cells in endolithic mats to import a wide spectrum of organic carbon in order to survive periods of light limitation.

The habitat of the acido- and thermophilic red algae Galdieria sulphuraria and Cyanidium caldarium was examined in acidic hot sulphur springs in the vicinity of Naples (Italy). These species grew on soil and rocks, but a large part of the populations was cryptoendolithic. The endolithic algal layer (1–3 mm in thickness) was covered by amorphous silica (1–2 mm in thickness) containing traces of hydrotroilite (FeS.nH2O) and elemental sulphur. Organotrophic bacteria and fungi were not found in the algal layer. Light penetration measurements showed that 0·1–1% of the sunlight reached the upper part of the algal layer. Thus, low-light-adapted algae should be able to perform some photosynthesis in this endolithic habitat. Under conditions where light is even more limited, e.g. in winter or after darkening of the covering layer, many of the cells might not survive. Aqueous extracts of these algae are excellent growth substrates for Galdieria sulphuraria. Therefore, we propose that compounds released from dead cells in the endolithic layer are used by the surrounding Galdieria cells for heterotrophic metabolism. This would increase their chance of surviving prolonged periods under detrimental conditions.