Heterotrophic production of the microalgae Cyclotella cryptica: feed for aquaculture. (2010)

Feed cost and feed availability is a major bottleneck for the aquaculture industry and has prompted significant research into identifying new microalgal strains and/or growth conditions that reduce production costs, increase the ease of cultivation and improve the nutritional value of the cultivated biomass. The nutritional value of microalgal biomass depends on several physiological and biochemical attributes including microalgal cell size and shape, digestibility, non-toxicity and biochemical composition. The growth rate and biochemical composition of microalgal biomass can be influenced by culture and environmental conditions.&#13;\nHeterotrophic cultivation systems are a potential alternative to the more traditional photoautotrophic cultivation systems, which typically suffer from low biomass concentrations, high production costs and unexplained ‘crashes’. This thesis presents the investigations undertaken into the heterotrophic cultivation of the diatom Cyclotella cryptica (UTEX 1269). C. cryptica was chosen as a model organism as this species is capable of heterotrophic growth, has previously been used within aquaculture and was recommended as a species worthy of further investigations. The effects of major nutritional and environmental factors on the growth and biochemical characteristics were studied in 250 mL Erlenmeyer flasks, 500 mL Schott bottles and 19 L carboys. The results from this investigation are unique as the optimal heterotrophic growth&#13;\nconditions were previously either not known or reported.&#13;\nThe major contributions from this research are the increased knowledge of the effects of the concentration of glucose, sodium metasilicate and nitrogen source (sodium nitrate, urea and ammonium chloride) on the growth dynamics and biomass productivity of C. cryptica and the effects of the nitrogen availability (sodium nitrate, urea and ammonium chloride) on the fatty acid composition and total fatty acid content. In addition, there is now an increased knowledge of the effects of the cultivation temperature, salinity, pH and degree of mixing or agitation on the growth dynamics and biomass productivity of&#13;\nC. cryptica and the effects of the cultivation temperature and salinity on the fatty acid composition and total fatty acid content. The calculated specific growth rates in heterotrophic culture were in the order of 0.05 h⁻¹, which are comparable to the photoautotrophic growth rates reported in the literature. The most predominant fatty acids synthesised by C. cryptica under heterotrophic growth conditions were palmitic acid (16:0), palmitoleic acid (16:1 n-7), stearidonic acid (18:4 n-3, SDA),&#13;\neicosapentaenoic acid (20:5 n-3, EPA) and docosahexaenoic acid (22:6 n-3, DHA). These fatty acids are similar to the fatty acids synthesised under photoautotrophic conditions. An economic assessment confirmed that the biomass production costs were sensitive to the biomass concentration. The cost of producing C. cryptica biomass under heterotrophic cultivations in a 100 L commercial bioreactor, at a steady state biomass concentration of 30 g.L⁻¹, was estimated at approximately AU $180 per kg. This cost, although high, is reasonable given photosynthetic microalgal production costs in small hatcheries is up to AU $750 - $1000 per kg. Unit production costs were sensitive to the steady-state biomass concentration and a steady-state biomass concentration of 2 g.L⁻¹ resulted in the unit production cost increasing to approximately AU $2,675 per kg.