Effect of temperature on growth, chemical composition and fatty acid composition of tropical Australian microalgae grown in batch cultures

Abstract

The growth and nutritional content of four tropical Australian microalgal species, diatom Chaetoceros sp. (CS256), two cryptomonads, Rhodomonas sp. (NTI5) and Cryptomonas sp. (CRFI01) and unidentified prymnesiophyte (NT19), cultured at five different temperatures, were studied. Commercially available Isochrysis sp. (clone TISO) was included in the study for comparison. Microalgae were grown in laboratory 1.5-1 batch cultures in F/2 medium at 25, 27, 30, 33 and 35°C (salinity 25‰; pH 8.3; photon flux density 80 μmol photon m-2 s-1; 12:12 h light:dark cycle). Microalgal cells were harvested in late logarithmic growth phase and analysed for protein, carbohydrate, lipid, chlorophyll a, inorganic matter (ash) and fatty acid composition. The optimum temperature for growth was 25-27°C for Rhodomonas sp. (specific growth rate, μ=0.27 day-1), and 27-30°C for prymnesiophyte NT19, Cryptomonas sp., Chaetoceros sp. and Isochrysis sp. (μ=0.56, 0.33, 0.87 and 0.97 day-1, respectively). Only Chaetoceros sp. grew well at 33 and 35°C (μ>0.78 day-1). All tropical Australian species had significantly lower percentages of protein when cells were grown at temperatures above 27°C, but there was no consistent trend in the percentages of carbohydrate. Chaetoceros sp. had highest percentage of lipid (16.8% dry weight; P<0.01), when cells were cultured at 25°C, while Rhodomonas sp., Cryptomonas sp., NT19 and Isochrysis sp., had significantly higher amounts of lipid at temperatures within the range 27-30°C (15.5, 12.7, 21.4, and 21.7% dw, respectively; P<0.05 in each case). Considering all species together, there was no overall relationship between percentage of protein, carbohydrate or lipid and temperature, but there was an overall, linear relationship between percentage of ash (inorganic matter) and temperature (r2=0.42, P≤O.05). Highest calculated energy values were found in Chaetoceros sp. (21.9 kJ g-1) and Tahitian Isochrysis sp. (22.5 kJ g-1) cultured at 27-30°C. There was no significant change in chlorophyll a (range 1.2-1.68 pg cell-1) for any species over the temperature range studied. The highly unsaturated fatty acid (HUFA), eicosapentaenoic acid, 20:5n-3, was present in all species, with highest amounts in prymnesiophyte NT19 (19.9% total fatty acids). Percentages of 20:5n-3 were slightly lower at highest growth temperatures for all species. The control, Isochrysis sp., had the highest amount of docosahexaenoic acid, 22:6n-3 (6.6% total fatty acids). All species had lower percentages of 22:6n-3 at higher growth temperatures. Chaetoceros sp. and NT19 had moderate amounts of arachidonic acid, 20:4n-6 in the fatty acid profile (2.7-5.4% total fatty acids). Highest percentages were associated with growth temperatures within the range 27-30°C. Only Chaetoceros sp. grew well at 35°C, maintaining moderate percentages of protein, carbohydrate, lipid, PUFA and HUFA (9.6% total fatty acids), at that temperature. All tropical Australian species performed better than Isochrysis sp., in terms of percentage of HUFA, over the range of growth temperatures. © 2002 Elsevier Science B.V. All rights reserved.