Peculiar proteome of dark-cultivated Euglena gracilis

Euglena gracilis is a flagellate photosynthetic microalga that, thanks to its metabolic adaptability, can grow under both autotrophic and heterotrophic conditions. We focused on the proteome of E. gracilis cultivated in Cramer-Myers medium in dark and light conditions. Cultures grown in the light showed a characteristic green coloration, while cultures incubated in the dark were bright, almost colorless, and had 60% fewer cells per volume. 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. Phenol-extracted proteins were digested and purified using the SP3 protocol. We revealed 162 proteins, classified into 12 functional groups, which differentially accumulated in light- and dark-grown cells. The most represented categories in the case of dark cultivation were defense mechanisms, metabolism, and energy. Of note, in the category metabolism of lipids and sterols, we detected 4 proteins involved in the β-oxidation of fatty acids more abundant in the dark. Accumulation os stress- and detoxification-related proteins in microalgal cells cultivated in darkness supported interpretation of such condition as challenging stress factor. Euglena gracilis cultured in the dark is useful for basic research, applied biotechnology, and environmental protection.