Mechanism of Conversion from Heterotrophy to Autotrophy in Euglena gracilis

Abstract

Summary Euglena grown to stationary phase in the dark without aeration accumulated lipids. When these high lipid cells are transferred to an inorganic medium and aerated, lipids were rapidly metabolized and the respiratory rate declined concomitant with the decline in cellular lipid content. Prolamellar bodies, propyrenoids and prothylakoids developed within the proplastid of dark aerated cells and the cells developed an increased capacity for chlorophyll synthesis manifested upon subsequent exposure to light. Lipid content did not decline in cells exposed to nitrogen and chlorophyll synthesis ability did not increase. The addition of an organic carbon source to cells at the start of aeration did not prevent lipid degradation. Organic carbon source addition and inhibitors of RNA and protein synthesis did however inhibit the development of an increased capacity for chlorophyll synthesis. These results suggest that oxygen triggers light independent proplastid development with the oxidative metabolism of lipids providing the carbon and energy for the synthesis of nucleic acids and proteins required for proplastid development in the dark. Growth of microorganisms requires carbon and energy for the synthesis of cellular components. Autotrophs utilize sunlight or the energy released by oxidization of inorganic compounds for growth while the breakdown of organic compounds provides the energy needed for the growth of heterotrophs. Most pathogenic bacteria, protozoa, and fungi are heterotrophs. The objective of this study was to analyze the mechanism of conversion from heterotrophy to autotrophy using the unicellular alga, Euglena gracilis. Key wordsEuglena gracilis grows heterotrophically on a variety of carbon sources in the dark showing a typical animal-type metabolism and when it is placed in the light on an inorganic medium it grows autotrophically exhibiting plant-type metabolism. Euglena is the only organism that can reversibly switch from animal-like heterotrophic metabolism to plant-like autotrophic metabolism making its taxonomic classification as a plant or animal problematic (Johnson 1968). Schiff's group (Osafune et al. 1990, Schiff andSchwartzbach 1982) has studied the transformation from a heterotroph to a phototroph, the greening of Euglena gracilis var. bacillaris, by transferring dark grown non-dividing cells, resting cells, into the light to induce chloroplast development. They found that the breakdown of the storage carbohydrate paramylum provided carbon and energy for the light induced synthesis of proteins, nucleic acids and lipids needed for chloroplast development (Schwartzbach et al. 1975, Rosenberg et al. 1964 found that lipids, mainly wax esters, accumulated in the cytoplasm of dark grown stationary Euglena and their breakdown was induced by light exposure providing an additional source of carbon and energy for light induced chloroplast development.

Photomovement in Euglena

“…Euglena gracilis is a photosynthetic unicellular flagellate (Buetow 1968a), but it can also live heterotrophically (Sumida et al 2007). The size ranges between 50 μm and 80 μm length and 8 μm to 12 μm width (Buetow 1968b).…”

Comparative toxicity of the pesticides carbofuran and malathion to the freshwater flagellate Euglena gracilis

“…Chlorophyll was extracted in 80% acetone (Osafune and Sumida 2006;Sumida et al 2007). Aliquots of cultures were centrifuged at 6,000 g for 10 min at 4°C.…”

Alterations of Membrane Lipid Content Correlated With Chloroplast and Mitochondria Development in Euglena gracilis

“…E . gracilis cells are known to accumulate a storage carbohydrate, paramylon (β-1,3-glucose polymer) under heterotrophic growth conditions ( Schwartzbach et al, 1975 ; Inui et al, 1982 ), and they utilize it during chloroplast development for synthesis of proteins, nucleic acids, and membrane lipids ( Rosenberg and Pecker, 1964 ; Schiff and Schwartzbach, 1982 ; Osafune et al, 1990 ; Sumida et al, 2007 ). Light-grown cells have also been placed in the dark and analyzed for degradation of chloroplasts ( Scheer and Parthier, 1982 ; Ferroni et al, 2009 ).…”