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The fermentation characteristics of Saccharomyces cerevisiae strains which overexpress a constitutive OLE1 gene were studied to clarify the relationship between the fatty acid composition of this yeast and its ethanol productivity. The growth yield and ethanol productivity of these strains in the medium containing 15% dextrose at 10 degrees C were greater than those of the control strains under both aerobic and anaerobic conditions but this difference was not observed under other culture conditions. During repeated-batch fermentation, moreover, the growth yield and ethanol productivity of the wild-type S. cerevisiae increased gradually and then were similar to those of the OLE1-overexpressing transformant in the last batch fermentation. However, the unsaturated fatty acid content (77.6%) of the wild-type cells was lower than that (86.2%) of the OLE1-recombinant cells. These results suggested that other phenomena caused by the overexpression of the OLE1 gene, rather than high unsaturated fatty acid content, are essential to ethanol fermentation by this yeast.

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Two kinds of bioreactors, a bubble-column and an air-lift bioreactor, have been designed. The influence of operating conditions such as medium composition, light intensity, carbon dioxide concentration in the flushing gas, culture temperature, and gas flow rate, on photosynthesis of Dunaliella tertiolecta were studied using a chemometrics approach. The bubble-column bioreactor system was shown to be advantageous over the air-life because of a weaker intensity of hydrodynamic stress derived from gas bubble dispersion and culture broth mixing. Optimal conditions for carbon dioxide fixation or maximal growth rate were determined. The effect of hydrodynamic shear forces on the algal wall produced by gas bubbling was identified as one of the most significant factors for algal growth.

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Arabidopsis thaliana ⌬-12 fatty acid desaturase gene (FAD2) was overexpressed in Saccharomyces cerevisiae by using the GAL1 promoter. S. cerevisiae harboring the FAD2 gene was capable of forming hexadecadienoyl (16:2) and linoleoyl (18:2) residues in the membrane lipid when cultured in medium containing galactose. Gas-liquid chromatography analysis of total lipids indicated that the transformed S. cerevisiae accumulated these dienoic fatty acyl residues and that they accounted for approximately 50% of the total fatty acyl residues. Phospholipid analysis of this strain indicated that the oleoyl (18:1) residue binding phosphatidylcholine (PC) was mostly converted to the 18:2 residue binding PC, whereas 50% of the palmitoleoyl (16:1) residue binding PC was converted to the 16:2 residue binding PC. A marked effect on the unsaturation of 16:1 and 18:1 was observed when S. cerevisiae harboring the FAD2 gene was cultured at 8؇C. To assess the ethanol tolerance of S. cerevisiae producing polyunsaturated fatty acids, the cell viability of this strain in the presence of ethanol was examined. The results indicated that S. cerevisiae cells overexpressing the FAD2 gene had greater resistance to 15% (vol/vol) ethanol than did the control cells.

323The growth of the ciliate Tetrahymena pyriformis on non-growing Escherichia coli has been studied by following the time courses of population densities and protozoan mean cell volume in batch cultures. Viable, non-encysted protozoa always stopped feeding before the bacterial density was reduced to zero and non-feeding ciliates tended to swim faster than feeding ciliates. In addition, the number of bacteria and other particles of bacterial size consumed in the formation of one new ciliate, when averaged over the lag and reproductive phases of a culture, declined toward a limiting value of about 1.6 x lo4 particles per ciliate as the initial density of such particles was increased. I N T R O D U C T I O NThe feeding of protozoa on bacteria, unicellular algae and other protozoa constitutes predation on a microscopic scale. Such predation is important in wastewater treatment, since the protozoa collect and concentrate particulate matter in the water and also serve to regulate competition between decomposer populations. It is probably also important to the dynamics of aquatic ecosystems, where the protozoa constitute a link in the food webs. Finally, it is important in that it is an example of a basic natural process that can be isolated and studied under controlled laboratory conditions. A number of such studies have been reported, beginning with the classic work of Gause (1 934) -for a review, see Fredrickson (1977) and also Taylor (1978).Some of the foregoing studies have dealt with the effect of prey density on the feeding rate of the predators and the results have been particularly interesting. Thus, Salt (1967) in his study of predation of the large ciliate Woodruf$a metabolica on Paramecium aurelia found that the predators' feeding exhibited a threshold type of response to prey density. When the prey density was above a threshold value Woodrufja metabolica fed at a rate that was independent of prey density but when the prey density fell below the threshold value Woodruffia metabolica underwent a morphological change and formed non-motile, non-feeding cysts. Danso & Alexander (1975) observed similar phenomena when they added various protozoa to cultures of non-growing bacteria. The protozoa (Tetramitus rostratus, and strains of Hartmanella, Naegleria and Vahlkamp$a) would eat the bacteria (Rhizobium meliloti) down to a threshold level but no further. Dive (1975)