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In solfataric fields in southwestern Iceland, neutral and sulfide-rich hot springs are characterized by thick bacterial mats at 60 to 80°C that are white or yellow from precipitated sulfur (sulfur mats). In low-sulfide hot springs in the same area, grey or pink streamers are formed at 80 to 90°C, and a Chloroflexus mat is formed at 65 to 70°C. We have studied the microbial diversity of one sulfur mat (high-sulfide) hot spring and one Chloroflexus mat (low-sulfide) hot spring by cloning and sequencing of small-subunit rRNA genes obtained by PCR amplification from mat DNA. Using 98% sequence identity as a cutoff value, a total of 14 bacterial operational taxonomic units (OTUs) and 5 archaeal OTUs were detected in the sulfur mat; 18 bacterial OTUs were detected in the Chloroflexus mat. Although representatives of novel divisions were found, the majority of the sequences were >95% related to currently known sequences. The molecular diversity analysis showed that Chloroflexus was the dominant mat organism in the low-sulfide spring (1 mg liter ؊1 ) below 70°C, whereas Aquificales were dominant in the high-sulfide spring (12 mg liter ؊1 ) at the same temperature. Comparison of the present data to published data indicated that there is a relationship between mat type and composition of Aquificales on the one hand and temperature and sulfide concentration on the other hand.Sulfide-rich hot springs with neutral or alkaline pH are relatively rare in most geothermal areas in the world. However, these types of hot springs are rather common in Iceland due to high ground water level and climatic conditions, i.e., from melting snow and rain. Bacteria that thrive in such springs often form long streamers or mats, but the appearance of the mats and the types of bacteria in them seem to vary depending on the sulfide concentration, pH, temperature, and other chemical and physical factors (6,7,10,11,12,20,33). Many Icelandic hot springs have sulfide concentrations as high as 30 mg liter Ϫ1 and, under such conditions, thick bacterial mats which can be spectacularly white or bright yellow from precipitated sulfur are formed.The diversity of many microbial ecosystems has now been studied with different molecular methods, such as analysis of small-subunit (SSU) rRNA by sequencing, denaturing gradient gel electrophoresis, or restriction fragment length polymorphism analyses. These studies show that the diversity of microbial ecosystems is typically 100 to 1,000 times greater than that shown by cultivation alone (14,15,22,23,30,31). The sequencing of rRNA genes from environmental samples is very informative, since it provides information on both the phylogenetic relationship and the population structure of the microbial community. With increased understanding of the role and importance of microbes in many ecosystems, the benefit of microbial diversity studies is being recognized. The practical value of these methods is already widespread, as they can be used to study the performance of wastewater treatment plants (27), monitor changes upo...

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299 ~~~ ~ ~Thermop hi I ic, redd is h-coloured he tero t rop hic bacteria different from Thermus were isolated from submarine alkaline hot springs in Iceland. The bacteria were obligately aerobic, moderately halophilic, Gram-negative rods, about 0.5 pm in diameter and 2-2.5 pm long. Neither spores, flagella nor lipid granules were observed, but a slime capsule was formed on carbohydrate-rich medium. Optimum growth was at 65 "C, pH 7.0, and at about 2% (w/v) NaC1. The bacteria were oxidase negative, catalase positive and contained a carotenoid pigment with the main absorbance peak at 476 nm and shoulders at 456 and 502 nm. The GC content of the DNA was about 64mol %. Electron micrographs clearly showed an outer membrane, about 9 nm thick, and the cytoplasmic membrane together with the peptidoglycan layer was about 14 nm in thickness. The isolates were nutritionally different from Thermus. They utilized several common sugars but glutamate and aspartate were the only amino acids that most strains used. These bacteria are considered to represent a new genus which we name Rhodothermus, with the type species Rhodothermus marinus.

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