Peculiar proteome of dark-cultivated Euglena gracilis

Abstract

Euglena gracilis is a flagellate photosynthetic microalga that, thanks to its metabolic adaptability, can grow under both autotrophic and heterotrophic conditions. This adaptability makes euglena an interesting species for applied biotechnology. We focused on the proteome of E. gracilis cultivated in Cramer-Myers medium (supplemented with ethanol) in dark and light conditions. Cultures grown in the light showed a characteristic green coloration, while cultures incubated in the dark were bright yellow. When cultured in the dark, microalga showed reduced concentration of chlorophylls (a, b, and total) and carotenoids compared to cells cultured in the light. Conversely, there was an increase in proline content in the dark compared to light cultivation. Using proteomic approach, we revealed 162 differentially accumulated proteins in light- and dark-grown cells classified into 12 functional groups. Notably, alterations in the metabolism of fatty acids and amino acids, secondary metabolism, and accumulation of stress- and detoxification-related proteins in microalgal cells cultivated in darkness with ethanol as a carbon source may help euglena adapt to these conditions. Based on our results and literature, we hypothesize that vitamin B12 potentially plays an important role in light/dark metabolic switch, similarly as in bacteria.

Keywords: Adenosylcobalamin; Mixotrophy; Photosynthetic pigments; Proline; Stress; Β-oxidation of fatty acids.

Fig. 1

Phenotype of Euglena gracilis. Cultures in light (A) and dark (B) conditions after 7 days of incubation in Cramer-Meyers medium. Cells cultured in the light showed a characteristic green color, while cells cultured in the dark were yellowish. Microscopy images with 400 × magnification in light (C) and dark (D) cultivation conditions. Cells cultured in the dark changed shape from oblong to spherical.

Fig. 2

Concentrations of chlorophylls n = 7 (A), carotenoids n = 7 (B), and proline n = 3 (C) in Euglena gracilis cultivated in light (light grey) and dark (black) conditions after 7 days of incubation in Cramer-Meyers medium.

Fig. 3

Principal component analysis of quantified proteins showing separation and distribution of the microalgal samples cultivated in light (green, L1-L5) and dark (blue, D1-D5) conditions. The figure shows 5 independent samples for each variant. The first component, explaining a major share of the total data variance, aligned with different experimental conditions—cultivation in light versus darkness. The second component explained much lower proteome-level variance between biological replicates, particularly in light cultivation.

Fig. 4

KEGG-reconstruct assisted visualization of proteomic map of fatty acid metabolism in dark and in light cultivated Euglena gracilis. Green arrows represent proteins identified in our study.

Fig. 5

Functional distribution of differentially abundant proteins accumulated in light (light grey) and dark (black) cultivated Euglena gracilis.