This review focuses on the hydrodynamic and mass transfer characteristics of various three-phase, gaslift fluidized bioreactors. The factors affecting the mixing and volumetric mass transfer coefficient (k(L)a), such as liquid properties, solid particle properties, liquid circulation velocity, superficial gas velocity, bioreactor geometry, are reviewed and discussed. Measurement methods, modeling and empirical correlations are reviewed and compared. To the authors' knowledge, there is no 'generalized' correlation to calculate the volumetric mass transfer coefficient, instead, only 'type-specific' correlations are available in the literature. This is due to the difficulty in modeling the gaslift bioreactor, caused by the variation in geometry, fluid dynamics, and phase interactions. The most important design parameters reported in the literature are: gas hold-up, liquid circulation velocity, 'true' superficial gas velocity, mixing, shear rate, aeration rate and volumetric mass transfer coefficient, k(L)a.
Summary. Lagenidium giganteum (Oomycetes: Lagenidiales), a facultative parasite of mosquito larvae, infects the larval stage of most species of mosquitoes and a very limited number of alternate hosts. Host infection by this and other members of Oomycetes is initiated by motile, laterally biflagellate zoospores. Chemical bases for the various degrees of host specificity exhibited by these parasites is not known, but presumably involves receptors on the zoospore surface recognizing compounds either secreted by or on the surface of their hosts. Surface topography had no detectable effect on L. giganteum encystment or appressorium formation. Scanning electron microscopy documented the detachment of flagella during zoospore encystment. Bulbous knobs at the basal end of the detached flagellum were interpreted as encysting zoospores dropping the axoneme and/or the basal body and associated structures to which flagella are attached. Multiple signals appear to be involved in the initial steps ofL. giganteum host invasion. Zoospores of this parasite did not encyst on powdered preparations of chitin or chitosan (deacetylated chitin). Upon dissolution of chitosan in dilute acid followed by drying these solutions to form thin, transparent films, zoospores readily encysted. The degree of reacetylation of these films and the spacing of acetylated and deacetylated residues had no significant effect on zoospore encystment. Zoospores of a strain of Lagenidiurn myophilurn isolated from marine shrimp, that also infects mosquito larvae, encysted on chitosan films. No encystment of spores of the plant parasite Phytophthora capsici was observed on chitin or chitosan films. Simulation of cuticle sclerotization by incubating chitosan films with different catecholamines and tyrosinase significantly reduced zoospore encystment. Zoospores that encysted on chitosan films did not germinate in distilled water. Germination could be induced by adding microgram quantities of bovine serum albumin or proteins secreted by motile zoospores into the water, and to a lesser degree by some amino acids, but not by various cations. Zoospores encysted and germinated on the pupal stage of some mos-*Correspondence and reprints: Botany Department, Box 351330, University of Washington, Seattle, WA 98195, U.S.A. quito species. Appressoria were occasionally fort-ned, but most subsequently sent out another mycelial branch, apparently without attempting to pierce the pupal cuticle. Methylation of pupal exuviae with ethereal diazomethane or methanol/HC1 significantly increased zoospore encystment. Modification of chitin by catecholamines, lipids and protein on the epicuticular larval surface all affected host invasion.
The action of hydrogen atoms -generated in an electrodeless high frequency gas dischargeon calf thymus DNA in aqueous solution was investigated. The loss of priming activity was com pared with the appearance of single strand breaks in native and denatured DNA, double strand breaks, denatured zones, base damage and rupture of hydrogen bonds. The primary lesions after exposure to H atoms and gamma radiation, respectively, are single strand breaks and base damage. Double strand breaks originating from accumulation of single breaks, and rupture of hydrogen bonds caused by single breaks and base damage, were identified as secondary lesions. In relation to strand breaks arising from radical attack on the sugar-phosphate backbone of the DNA molecule, base damage is about 12.5 times more frequent after H-exposure than after /-irradiation. It is con cluded from this observation, that single strand breaks are the predominant critical lesions respon sible for the loss of the functional activity of DNA.Radiation damage to cells and, consequently, also to higher organisms is mainly caused by alter ations in their genetic material. Thus, one of the principal tasks of molecular radiation biology con sists in investigating the effects which occur in irradiated nucleic acids. Since cellular radiation damage is caused at approximately equal parts by "direct" and "indirect" effects1, it is neces sary to include both types of radiation effects in the investigations. Both the direct and the indirect effects are initiated by a number of different pri mary processes: In dry systems excitations, ioniza tions, elastic nuclear collisions, and thermal hydro gen atoms are of importance, whilst the changes observed in irradiated aqueous solutions of bio molecules are caused by attack of H and OH radi cals and hydrated electrons. Having examined the biological effectiveness of the various primary proc esses of energy absorption in highly purified prepa rations of lyophilized DNA in a series of earlier experiments 2~4, the present paper deals with the action of hydrogen atoms on diluted aqueous solu tions of DNA describing both the change of various
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