The extremely halophilic archaea (family Halobacteriaceae) are the dominant heterotrophic organisms in hypersaline environments in which salt concentrations exceed 250–300 g l−1. During the last decades our knowledge on the taxonomy, physiology and biochemistry of the Halobacterium group has greatly increased. However, our understanding of the ecology of the halophilic archaea lags far behind the progess made in the study of other aspects of their biology. A few hypersaline environments, such as the Dead Sea and solar salterns, have been studied more in depth, using techniques such as lipid analysis to obtain information on the types of organisms present and measurement of uptake of labeled substrates to quantify the dynamics of bacterial processes. The results of these studies, in combination with the information obtained from laboratory studies of representative isolates of the Halobacteriaceae, enable the beginning of an understanding of the functioning of the halophilic archaea in nature.

The ecology of the extremely halophilic archaea

The Order Halobacteriales

Summary: Pigmented pleomorphic rods morphologically similar to Halobacterium volcanii were isolated from a salt pond near Alicante, Spain. The salt concentration range for growth (10 to 35 %, w/v, total salts) was more moderate than that typical of Halobacterium. Twenty-two strains were characterized as members of the genus Halobacterium, but were distinguished from currently recognized species by the absence of complex nutrient requirements, a rapid growth rate, nutritional versatility and salt-dependent pigment production. A typical strain (R-4) has been deposited in the Czechoslovak Collection of Microorganisms, no. CCM 3361.

SHORT COMMUNICATION Isolation of Extremely Halophilic Bacteria Able to Grow in Defined Inorganic Media with Single Carbon Sources

A novel extremely halophilic member of the Archaea, strain 40T, was isolated from Egypt (Aswan). This isolate requires at least 1.6 M sodium chloride for growth and exhibits optimal growth between 37 and 42 degrees C. Determination of the entire 16S rRNA gene sequence revealed the highest similarity to the type strain of Natrialba asiatica (> 99%). Polar lipid analysis indicated that strain 40T and Natrialba asiatica have essentially identical compositions, indicating that the former is a member of genus Natrialba. However, physiological and biochemical data provided evidence that Natrialba asiatica strains B1T and 172P1T, as well as strain 40T, are sufficiently different to be divided in three different species. The G+C content of strain 40T was 61.5+/-0.6 mol%. In addition, DNA-DNA hybridization data supported the placement of the isolate in a new species in the genus Natrialba, Natrialba aegyptiaca sp. nov., and indicated that Natrialba asiatica strain B1T should also be placed in a separate species, Natrialba taiwanensis sp. nov. Morphological studies of strain 40T indicated clearly that this isolate appears in three completely different cell shapes (cocci, rods, tetrads) under different conditions of growth, including different sodium chloride concentrations and different growth temperatures. Another interesting property of strain 40T is the ability to produce an extracellular polymer, which was found to be composed predominantly of glutamic acid (85% w/w), representing poly(glutamic acid), carbohydrates (12.5% w/w) and unidentified compounds (2.5% w/w). Among the Archaea, production of an extracellular polysaccharide has been described for some members of the genera Haloferax and Haloarcula.

/pdf/transfer-of-natrialba-asiatica-b1t-to-natrialba-taiwanensis-1oqwzmsh8y.pdf

Transfer of Natrialba asiatica B1T to Natrialba taiwanensis sp. nov. and description of Natrialba aegyptiaca sp. nov., a novel extremely halophilic, aerobic, non-pigmented member of the Archaea from Egypt that produces extracellular poly(glutamic acid).

Publisher Summary It is noted that for the purposes of establishing the phylogenetic status of the archaebacteria, much emphasis has been placed on the molecular biology of these organisms and on the chemical nature of their cell walls and membranes. However, it is also becoming clear that the pathways of metabolism in archaebacteria and their constituent enzymes are equally fruitful areas for investigation. This chapter focuses on the enzymology of archaebacteria, not in isolation but in comparison with that of eubacteria and eukaryotes. The enzymes of the central metabolic pathways have been reviewed because these pathways are thought to be some of the first cellularly established metabolic routes and are the most studied and well-characterized systems in non-archaebacterial species. Archaebacteria grow in extreme environments and therefore their macro- molecules will be structurally adapted to function under such conditions. Thermoacidophilic archaebacteria grow at temperatures between 55 and 110°C and in pH values as low as pH 1-2. Although there can be no temperature differential between the outside and inside of a cell, it is thought that cytoplasmic pH values are approximately neutral.

Archaebacteria: the comparative enzymology of their central metabolic pathways

Four previously unrecognized strains of extremely halophilic bacteria that utilize carbohydrates have been isolated. Gas production proved an unreliable index of carbohydrate metabolism; therefore, carbohydrate utilization was measured by determining acid formation and sugar disappearance during growth. By these procedures, carbohydrate utilization was readily detected. The results suggest that carbohydrate dissimilation by extremely halophilic bacteria may be more common than previously thought and that the apparent rarity of carbohydrate-metabolizing halophiles may be an artifact of the isolation procedures used.

Geraldine A. Tomlinson

Papers

Isolation of carbohydrate-metabolizing, extremely halophilic bacteria