Abstract
BackgroundEuglena gracilis, a photosynthetic protist, produces protein, unsaturated fatty acids, wax esters, and a unique β-1,3-glucan called paramylon, along with other valuable compounds. The cell composition of E. gracilis was investigated in this study to understand how light and organic carbon (photo-, mixo- and heterotrophic conditions) affected growth and cell composition (especially lipids). Comparisons were primarily carried out in cultures grown at 23 °C, but the effect of growth at higher temperatures (27 or 30 °C) was also considered.Cell growthSpecific growth rates were slightly lower when E. gracilis was grown on glucose in either heterotrophic or mixotrophic conditions than when grown photoautotrophically, although the duration of exponential growth was longer. Temperature determined the rate of exponential growth in all cultures, but not the linear growth rate during light-limited growth in phototrophic conditions. Temperature had less effect on cell composition.Cell compositionAlthough E. gracilis was not expected to store large amounts of paramylon when grown phototrophically, we observed that phototrophic cells could contain up to 50% paramylon. These cells contained up to 33% protein and less than 20% lipophilic compounds, as expected. The biomass contained about 8% fatty acids (measured as fatty acid methyl esters), most of which were unsaturated. The fatty acid content of cells grown in mixotrophic conditions was similar to that observed in phototrophic cells, but was lower in cells grown heterotrophically. Heterotrophic cells contained less unsaturated fatty acids than phototrophic or mixotrophic cells. α-Linolenic acid was present at 5 to 18 mg g-1 dry biomass in cells grown in the presence of light, but at < 0.5 mg g-1 biomass in cells grown in the dark. Eicosapentaenoic and docosahexaenoic acids were detected at 1 to 5 mg g-1 biomass. Light was also important for the production of vitamin E and phytol.
Highlights
Microalgae, as a source of nutrients, bioactive compounds, biofuels and chemicals, have gained attention from both academia and industry [1,2,3,4,5,6,7,8]
Our result indicated that E. gracilis cells can be harvested to provide either protein or paramylon, but that the cultivation process should be optimised for the specific target product, to maximise the yield and productivity
E. gracilis grew well in photoautotrophic, mixotrophic and heterotrophic conditions at 23 ̊C, with mixo- and heterotrophic conditions primarily providing a benefit in producing high biomass concentrations
Summary
Microalgae, as a source of nutrients, bioactive compounds, biofuels and chemicals, have gained attention from both academia and industry [1,2,3,4,5,6,7,8]. E. gracilis accumulates paramylon (a type of β-1,3-glucan that is unique to euglenoids, Euglena species) for energy and carbon storage, especially when grown on organic carbon (i.e. in mixo- or heterotrophic conditions) [4,9]. Wax esters become the main storage compounds in E. gracilis [12]. These esters are composed of medium chain fatty acids and fatty alcohol, which can be converted into biofuels [13,14]. A photosynthetic protist, produces protein, unsaturated fatty acids, wax esters, and a unique β-1,3-glucan called paramylon, along with other valuable compounds. Comparisons were primarily carried out in cultures grown at 23 ̊C, but the effect of growth at higher temperatures (27 or 30 ̊C) was considered.
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