Members of the rhodophytan order Cyanidiales are unique among phototrophs in their ability to live in extreme environments that combine low pH levels (ϳ0.2 to 4.0) and moderately high temperatures of 40 to 56°C. These unicellular algae occur in far-flung volcanic areas throughout the earth. Three genera (Cyanidium, Galdieria, and Cyanidioschyzon) are recognized. The phylogenetic diversity of culture isolates of the Cyanidiales from habitats throughout Yellowstone National Park (YNP), three areas in Japan, and seven regions in New Zealand was examined by using the chloroplast RuBisCO large subunit gene (rbcL) and the 18S rRNA gene. Based on the nucleotide sequences of both genes, the YNP isolates fall into two groups, one with high identity to Galdieria sulphuraria (type II) and another that is by far the most common and extensively distributed Yellowstone type (type IA). The latter is a spherical, walled cell that reproduces by internal divisions, with a subsequent release of smaller daughter cells. This type, nevertheless, shows a 99 to 100% identity to Cyanidioschyzon merolae (type IB), which lacks a wall, divides by "fission"-like cytokinesis into two daughter cells, and has less than 5% of the cell volume of type IA. The evolutionary and taxonomic ramifications of this disparity are discussed. Although the 18S rRNA and rbcL genes did not reveal diversity among the numerous isolates of type IA, chloroplast short sequence repeats did show some variation by location within YNP. In contrast, Japanese and New Zealand strains showed considerable diversity when we examined only the sequences of 18S and rbcL genes. Most exhibited identities closer to Galdieria maxima than to other strains, but these identities were commonly as low as 91 to 93%. Some of these Japanese and New Zealand strains probably represent undescribed species that diverged after long-term geographic isolation.The Cyanidiales are an order of asexual, unicellular red algae that are able to grow in low-pH environments and at moderately high temperatures throughout the globe (4, 31, 32). These algae are not red, but blue-green, due to their predominant pigments, c-phycocyanin and chlorophyll a. Many members of this order are known to grow at temperatures as high as 56°C and at pH levels from 0.2 to 4.0. No other photosynthetic microorganisms are known to inhabit this combination of conditions, and these algae often form well-developed mats in acidic geothermal locations. Surprisingly though, little is known about the ecology, biodiversity, and geographical distribution of these organisms. In these acidic habitats, no prokaryotic phototrophs are known to exist below a pH level of ϳ4 and the number of species reaching levels below pH 5 is small (37).The order Cyanidiales consists of three recognized genera, Cyanidium, Galdieria, and Cyanidioschyzon (6, 13, 16), referred to colloquially as "cyanidia" in this work. The genera Cyanidium and Cyanidioschyzon are thought to include a single species each, Cyanidium caldarium and Cyanidioschyzon merolae, respe...

The unicellular eukaryotic algae Cyanidium, Galdieria, and Cyanidioschyzon (herein referred to as ''cyanidia'') are the only photoautotrophs occurring in acidic (pH<4.0) geothermal environments at temperatures above 401C. In Yellowstone National Park (YNP), we examined an annual event we refer to as ''mat decline,'' where cyanidial mats undergo a seasonably defined color fading. Monthly sampling of chemical, physical, and biological features revealed that spring aqueous chemistry was essentially invariant over the 1-year sampling period. However, multiple regression analysis suggested that a significant proportion of algal most probable number (MPN) count variation could be explained by water temperature and UV-visible (VIS) light exposure. Irradiance manipulations (filtering) were then coupled with 14 CO 2 incorporation experiments to directly demonstrate UV inhibition of photosynthesis. Population dynamics were also evident in 18S rDNA PCR clone libraries, which were different in composition at MPN maxima and minima, and again evident in PCR-amplified chloroplast genomic short sequence repeat (SSR) analysis. PCR-cloned SSRs of the YNP isolates and mats were very similar to Cyanidioschyzon merolae Luca, Taddei et Varano, although distance analysis could distinguish the YNP cyanidia from the genome sequenced C. merolae that was isolated in Italy. Unexpectedly, while phylogenetic analysis of 18S rDNA sequences and SSR sequences derived from YNP cyanidial mats and pure cultures suggested these algae are most closely related to C. merolae (99.7% identity), cell morphology was consistent with the genera Galdieria and Cyanidium.

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