A hundred years of Dunaliella research: 1905–2005

A hundred years have passed since the description of the genus Dunaliella, the unicellular green alga which is responsible for most of the primary production in hypersaline environments worldwide. The present paper provides an historical survey of research on Dunaliella, from the early work in the 19(th )century to the thorough taxonomic studies by Teodoresco, Hamburger, Lerche and others from the beginnig of the 20(th )century onwards. It attempts to trace the origin of some of the most important breakthroughs that have contributed to our present understanding of this alga that plays such a key role in many hypersaline environments

Microbial diversity and microbial abundance in salt-saturated brines: Why are the waters of hypersaline lakes red?

Salt-saturated lakes such as the North Arm of Great Salt Lake and saltern crystallizer ponds contain 10-10 and more red microorganisms ml-1. Even the Dead Sea occasionally turns red due to microbial blooms. Three types of organisms may contribute to the coloration: the alga Dunaliella salina rich in carotene, halophilic Archaea (family Halobacteriaceae) containing 50-carbon bacterioruberin carotenoids and sometimes also retinal proteins (bacteriorhodopsin, halorhodopsin), and the recently discovered Salinibacter (Bacteroidetes) which contains pigment salinixanthin (an unusual acylated C40-carotenoid glucoside) as well as different retinal pigments. Bacteriorhodopsin and halorhodopsin enable the cells to directly use light energy for respectively the outward pumping of protons driving ATP generation and for the inward transport of chloride ions. The carotenoid pigments (carotene, bacterioruberin and derivatives, salinixanthin) primarily appear to protect the cells against photooxidative damage. Salinixanthin also acts as a light harvesting antenna for xanthorhodopsin, the proton pumping retinal pigment of Salinibacter. Quantitative assessment of the relative importance of the different pigments in the coloration of red brines of natural salt lakes and solar saltern crystallizer ponds suggests that bacterioruberin and other carotenoids contributed by members of the Halobacteriaceae are generally responsible for most of the color of the waters. The quantity of carotene present in dunaliella cells often greatly exceeds that of the haloarchaeal bacterioruberin pigments. However, the large amounts of carotene contribute only little to the optical properties of the brines because of the dense packing of the pigment in little globules within the chloroplast. Presence of salinixanthin and of bacteriorhodopsin and derivatives in the biomass can often be demonstrated as well, but these pigments have never been shown to contribute greatly to the overall optical properties of the waters. Thus, carotenoids of the bacterioruberin group appear to be the main factor causing the characteristic red color of hypersaline brines worldwide

Microbial life at high salt concentrations: phylogenetic and metabolic diversity

Halophiles are found in all three domains of life. Within the Bacteria we know halophiles within the phyla Cyanobacteria, Proteobacteria, Firmicutes, Actinobacteria, Spirochaetes, and Bacteroidetes. Within the Archaea the most salt-requiring microorganisms are found in the class Halobacteria. Halobacterium and most of its relatives require over 100–150 g/l salt for growth and structural stability. Also within the order Methanococci we encounter halophilic species. Halophiles and non-halophilic relatives are often found together in the phylogenetic tree, and many genera, families and orders have representatives with greatly different salt requirement and tolerance. A few phylogenetically coherent groups consist of halophiles only: the order Halobacteriales, family Halobacteriaceae (Euryarchaeota) and the anaerobic fermentative bacteria of the order Halanaerobiales (Firmicutes). The family Halomonadaceae (Gammaproteobacteria) almost exclusively contains halophiles. Halophilic microorganisms use two strategies to balance their cytoplasm osmotically with their medium. The first involves accumulation of molar concentrations of KCl. This strategy requires adaptation of the intracellular enzymatic machinery, as proteins should maintain their proper conformation and activity at near-saturating salt concentrations. The proteome of such organisms is highly acidic, and most proteins denature when suspended in low salt. Such microorganisms generally cannot survive in low salt media. The second strategy is to exclude salt from the cytoplasm and to synthesize and/or accumulate organic 'compatible' solutes that do not interfere with enzymatic activity. Few adaptations of the cells' proteome are needed, and organisms using the 'organic-solutes-in strategy' often adapt to a surprisingly broad salt concentration range. Most halophilic Bacteria, but also the halophilic methanogenic Archaea use such organic solutes. A variety of such solutes are known, including glycine betaine, ectoine and other amino acid derivatives, sugars and sugar alcohols. The 'high-salt-in strategy' is not limited to the Halobacteriaceae. The Halanaerobiales (Firmicutes) also accumulate salt rather than organic solutes. A third, phylogenetically unrelated organism accumulates KCl: the red extremely halophilic Salinibacter (Bacteroidetes), recently isolated from saltern crystallizer brines. Analysis of its genome showed many points of resemblance with the Halobacteriaceae, probably resulting from extensive horizontal gene transfer. The case of Salinibacter shows that more unusual types of halophiles may be waiting to be discovered

Microbial communities in salt lakes: Phylogenetic diversity, metabolic diversity, and in situ activities

A roundtable discussion was held on Monday, 12 May 2008, following the sessions on \u27Microorganisms in Hypersaline Environments\u27 at the 10th Conference on Salt Lake Research & 2008 FRIENDS of Great Salt Lake Issues Forum, University of Utah, Salt Lake City. Among the aspects discussed were: 1. The gap between our knowledge of the microorganisms isolated in culture and the true microbial diversity as apparent when using culture-independent techniques, in most cases based on the characterization of small subunit ribosomal RNA genes; 2. The metabolic diversity of the microorganisms inhabiting saline and hypersaline lakes and the lack of information to what extent the metabolic potential of the microbiota as apparent from culture studies or detection of functional genes is realized in the environment; 3. The limited understanding of the diversity of algae, bacteriophages and protozoa in hypersaline lakes and their relative importance of such microbial predators and grazing animals on the regulation of the microbial community sizes in such lakes; 4. The impact of high throughput -omics technologies for assessing the diversity and metabolism of hypersaline environments. In recent years a number of comprehensive studies were performed in selected hypersaline environments. In recent years a number of comprehensive studies were performed in selected hypersaline environments by large interdisciplinary teams of scientists. Such studies contribute invaluable information to define the nature and function of the microbial communities in such environments. However, the inability to independently grow specific organisms compared to the genetic diversity revealed by non-cultivation techniques indicates that additional work is needed to develop and define in vitro cultivation conditions. More of such studies are needed, with the appropriate funding, to solve the basic questions relating to the importance of microorganisms in saline lakes and other hypersaline ecosystems

Buoyancy studies in natural communities of square gas-vacuolate archaea in saltern crystallizer ponds

BACKGROUND: Possession of gas vesicles is generally considered to be advantageous to halophilic archaea: the vesicles are assumed to enable the cells to float, and thus reach high oxygen concentrations at the surface of the brine. RESULTS: We studied the possible ecological advantage of gas vesicles in a dense community of flat square extremely halophilic archaea in the saltern crystallizer ponds of Eilat, Israel. We found that in this environment, the cells' content of gas vesicles was insufficient to provide positive buoyancy. Instead, sinking/floating velocities were too low to permit vertical redistribution. CONCLUSION: The hypothesis that the gas vesicles enable the square archaea to float to the surface of the brines in which they live was not supported by experimental evidence. Presence of the vesicles, which are mainly located close to the cell periphery, may provide an advantage as they may aid the cells to position themselves parallel to the surface, thereby increasing the efficiency of light harvesting by the retinal pigments in the membrane

Teaching on Zoom In the Eyes of the Lecturer: An Eye Tracking Study

The shift to “Zoom” online learning in higher education, has aroused questions regarding the professional vision of lecturers- which is the core of this study. Since teaching on “Zoom” is screen based, it was possible to use an eye-tracking device to study lecturers’ vision gazes during teaching. The study describes ten lecturers vision behaviours by fixations and lecturer’s personal perception of vision. Main results revealed that sidelong variability between lecturers’ eye gazes, lecturers mainly focused on their presentations. They felt high vision interaction with participated students yet, more than fixations revealed. These results and more discussed in the study, emphasis the importance of understanding lecturers’ professional vision on “Zoom”, for improving the quality of teaching on this rather new method of teaching