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A study of high lipid yielding microalgae with potential for large-scale production of lipids and polyunsaturated fatty acids

Mercz, Tom Imre (1994) A study of high lipid yielding microalgae with potential for large-scale production of lipids and polyunsaturated fatty acids. PhD thesis, Murdoch University.

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Abstract

In this study an enrichment and isolation programme was carried out to select for high temperature (30°C) and high irradiance tolerant microalgae from saline and hypersaline environments, for potential biomass and lipid production in large-scale mass culture. The 75 isolates were dominated by pennate benthic diatoms, motile chlorophytes and cyanophytes. Seventeen diatoms and 4 chlorophytes were screened to determine which isolates had the best growth characteristics, and the highest biomass and cellular lipid content, in both indoor laboratory and outdoor glasshouse conditions.

The most promising of these isolates, the 4 diatoms strains, Amphora coffaeofonnis Ag. M-158, M-160. M-174 and Nitzschia communis Rab. M-169, and 2 prasinophytes, Tetraselmis sp. M-165 and M-168, were cultured in 2L air-lift batch cultures, and their growth, biochemical composition and total fatty acid content monitored in the exponential and stationary growth phases. The effect of nutrient limitation, salinity and temperature, and semi-continuous culture was examined in A. coffaeoformis M-158 and Tetraselmis sp. M-168. The culture yield and maximum productivity (productivity during the exponential growth phase), of biomass, total extractable lipid, and the polyunsaturated fatty acids (PUFAs), 20:4(n-6) and 20:5(n-3), was determined to evaluate potential growth strategies to optimise their production.

In light-saturated batch culture, lipid accumulated in the stationary phase in the diatoms (up to 39.8% ash free dry weight AFDW), whereas in Tetraselmis sp. M-168 the lipid content was maximal (around 35.0% AFDW) during the exponential phase. Lipid yield (75.0 mg.L-!) and maximum productivity (28.8 mg.L_l.d"l) was greatest in Tetraselmis sp. M-168, due to its higher growth rate and biomass yields. High irradiance in the glasshouse stimulated both lipid and carbohydrate accumulation in the diatoms, and carbohydrate in Tetraselmis sp. M-168. High irradiance enhanced lipid productivity in the diatoms, but it reduced lipid culture yield in Tetraselmis sp. M-168. Growth-limiting nitrate concentration in both microalgae, and low phosphate concentration in Tetraselmis sp. M-168 reduced biomass and induced carbohydrate accumulation, at the expense of lipid, resulting in decreased lipid yield and productivity.

Silicate limitation in A. coffaeoformis M-158 induced rapid accumulation of lipid (40% AFDW) which was offset by the marked decline in biomass, resulting in similar lipid productivity during the exponential phase, compared to nutrient sufficient conditions. However, a further reduction in growth rate and biomass productivity under nitrate and silicate-limited semi-continuous culture, decreased the lipid productivity compared to nutrient sufficient conditions.

Polyunsaturated fatty acid (PUFA) and 20:5(n-3) content (% of total fatty acids) was maximal during the exponential phase, correlating with maximum growth rate and degree of unsaturation in both algae. High irradiance enhanced the relative content and production of 20:4(n-6) and 20:5(n-3) in A. coffaeoformis M-158 and N. communis M- 169, correlating with an increased degree of unsaturation, while the production of 20:4(n-6) and 20:5(n-3) decreased in Tetraselmis sp. M-168. mainly due to the decreased biomass production.

The culture yield of 20:5(n-3) was highest in both algae under nutrient sufficient conditions, decreasing 3-4.5 fold under low nutrient conditions, whereas the 20:4(n-6) content in A. coffaeoformis M-158 increased at the expense of 20:5(n-3), under low nitrate and low silicate conditions. A. coffaeoformis M-158 also increased its PUFA content in response to increasing salinity (0.88 to 7.0% NaCl) and lower temperature (20°C), resulting in the maximal cellular content (1.5% of biomass) and culture yield of 20:5(n-3) (2.98 mg.L-1) occurring at 20°C. Increasing salinity generally reduced the 20:4(n-6) and 20:5(n-3) yields in both algae, due to the decreasing biomass yields. In Tetraselmis sp. M-168, increasing salinity stress (3.5 to 15.0% NaCl) and sub-optimal temperature, reduced the PUFA content, resulting in the maximal cellular 20:4(n-6) and 20:5(n-3) content (0.75 and 0.34% of biomass, respectively) and yield of 20:4(n-6) and 20:5(n-3) (0.16 mg.L-1 and 0.74 rng.L'1, respectively) coinciding with growth conditions favouring maximum growth rate.

The results indicate that maximum productivity of lipid and 20:5(n-3) can be achieved by optimising growth conditions (nitrate and phosphate, temperature, salinity, irradiance) for maximum growth rate and biomass production. Tetraselmis sp. M-168 is the best candidate for lipid and 20:5(n-3) production in large-scale mass culture due to its high biomass production, and maximal PUFA content coinciding with maximal lipid content during the exponential growth phase. A. coffaeoformis M-158 is the best candidate for 20:4(n-6) production. The physiological significance of changes in the lipid and fatty acid composition in response to environmental factors, and the culture strategy for optimising total lipid and 20:4(n-6) and 20:5(n-3) production in batch and continuous culture are discussed.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: repository@murdoch.edu.au. Thank you.
Supervisor(s): Borowitzka, Michael
URI: http://researchrepository.murdoch.edu.au/id/eprint/51717
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