Autori: Banciu HL.

Editorial: Optima Grafische Communicatie BV, Rotterdam, p.154, 2004.

Rezumat:

The inorganic sulfur oxidation by obligate haloalkaliphilic chemolithoautotrophs was only recently discovered and investigated. These autotrophic sulfur oxidizing bacteria (SOB), capable of oxidation of inorganic sulfur compounds at moderate to high salt concentration and at high pH, can be divided into three genera belonging to the γ subdivision of the Proteobacteria: Thioalkalimicrobium, Thioalkalivibrio and Thioalkalispira. Their taxonomy, metabolic diversity and the potential application in biological removal of toxic sulfur compounds of haloalkaliphilic SOB were reviewed in Chapter 1 (Introduction) of this thesis.
The scope of this thesis was to characterize the physiology and growth kinetics of the newly isolated haloalkaliphilic chemolithotrophic SOB with respect to their potential use in the biotechnology of H2S removal under haloalkaline conditions. A second aim of this thesis was to obtain more information about the specific adaptation to high salt and alkaline conditions from the population level down to the biomolecular level.
The growth physiology of representatives of the genus Thioalkalimicrobium and Thioalkalivibrio is compared in Chapter 2. The competitive interaction between these two groups of organisms and their survival strategy with direct implications in their ecology is also described. The aim was to study what are the rationales for the environmental occurrence of one or another group in the soda lakes. The Thioalkalimicrobium strains demonstrated relatively high specific growth rates, low growth yield, high maintenance and extremely high rates of thiosulfate and sulfide oxidation. In contrast, the Thioalkalivibrio strains, in general, were slowly growing and high-yield organisms with lower maintenance and much lower rates of oxidation of sulfide and thiosulfate. Moreover, the latter survived starvation much better than the Thioalkalimicrobium. In mixed thiosulfate-limited chemostat cultures at low salinity a Thioalkalimicrobium strain outcompeted a Thioalkalivibrio strain at D>0.02 h-1. The overall results suggest that Thioalkalimicrobium and Thioalkalivibrio represent two different ecological strategies.
The Chapter 3 describes the sodium salt requirement for the growth and sulfur-oxidizing potential at alkaline conditions in Thioalkalivibrio versutus strains isolated from soda lakes. A clear distinction was made between the NaCl- and Na2CO3/NaHCO3-requiring and tolerant strains. The different sodium salt requirement in the strains belonging to the same species allows us to conclude that there is a clear physiological difference between the alkaliphilic, high–salt organisms: “natronophiles” which require and tolerate high Na2CO3/NaHCO3 but not NaCl and the “halophiles” requiring and tolerating high concentrations of NaCl for their growth.
The comparative growth kinetics at different Na+ concentration (Chapter 4) showed that Thioalkalivibrio versutus strain ALJ 15 is capable of relatively high rates of sulfur oxidation as compared to the neutrophilic sulfur-oxidizing bacteria. The growth experiments with strain ALJ 15 under thiosulfate, sulfide and polysulfide limitation showed that the oxidation pathway for thiosulfate and polysulfide may be linked or common, whilst the sulfide oxidation occurs via different pathway.
The growth physiology of an extremely salt-tolerant and facultative alkaliphile, Thioalkalivibrio halophilus sp. nov., is presented in Chapter 5. This organism tolerated high concentrations of sodium carbonate and sodium chloride. It was also capable of growing well at pH 7.5 and 9.8. The hypothesis that two aqueous solutions with same Na+ concentration but containing different anionic species (HCO3- /CO32- and Cl-), resembling two types of saline environments, would have different osmotic pressure was verified theoretically and experimentally. This difference clearly has a direct consequence for the production of organic compatible solutes in the same organism.
The research concerning the salt-dependent compatible solutes production and membrane lipid composition of haloalkaliphilic sulfur-oxidizing bacteria is the subject of Chapter 6 of this thesis. Significant biochemical differences were revealed in the two groups of haloalkaliphilic SOB. The Thioalkalimicrobium species primarily use ectoine as the compatible solute, whereas the Thioalkalivibrio species primarily use glycine betain and low concentrations of sucrose. The estimation of the membrane surface potential using a fluorescent lipophilic probe supported the hypothesis of a very negative surface potential in the membranes of Thioalkalivibrio species.
In the last chapter (Chapter 7) the results are integrated and general conclusions are presented, pointing to the uniqueness and importance of the chemolithoautotrophic haloalkaliphilic SOB for fundamental as well as for applied research.
Overall, alkaliphilic SOB isolated from soda lakes proved to be highly specialized for the doubly extreme conditions of high salt concentration and high pH. They can be isolated from soda lake environments only. Some strains tolerate the entire possible range of pH and salt concentrations that might transiently appear in their natural habitat. Based on these properties, the haloalkaliphilic SOB from soda lakes can be usefully exploited in bioreactors with stable soda alkalinity and supplied with inorganic sulfur compounds for sulfur removal from waste streams.

Cuvinte cheie: Biotechnology, Haloalkalipihilic, Soda lakes, Sulfur oxidation, Sulfide removal