Comparative characteristic of mitochondria ultrastructural organization in Chlorella cells under altered gravity conditions

Abstract

Results from experiments that used cells from the unicellular alga Chlorella vulgaris (strain Larg-1) grown on a clinostat, demonstrated the occurrence of rearrangements in cellular organelles, including changes in the mitochondrial ultrastructure compared to controls. Changes in mitochondrial structure were observed in auto- and heterotrophic regimes of cells grown in altered gravity conditions, especially in long-term experiments. The mitochondrial rearrangements become apparent during cell proliferation, which resulted in an increase in the relative volume of mitochondria per cell: up to 2.7±0.3% in short-term clino-rotation (2.2±0.1% in the control) and up to 5.3±0.4% and 5.1±0.4% in long-term clino-rotation (2.3±0.2% in the control). The size of the mitochondria and their cristae increased in cells grown under long-time clino-rotation. In addition, hypertrophied organelles, not typical for this strain, were observed. These changes in the cells were accompanied by increased electron density of the matrix and a well-ordered topography of the cristae. To examine the separation of oxidative phosphorylation and respiration, an inhibitory agent 2,4-dinitrophenol (2,4-DNP) was applied to cells which resulted in insignificant volume changes of the mitochondria (2.5±0.4% versus 2.1±0.2% in the control). The increase of mitochondrial size with regularly arranged cristae, with more condensed matrix and extension of cristae areas of clino-rotated cells, may demonstrate higher functional activity of the mitochondria under altered gravity conditions. Changes observed early in clino-rotated cells, in particular the increased level of respiration, adenylate content (especially ATP) and more intensive electron-cytochemical reactions of Mg 2+-ATPase and succinat dehydrogenase (SDH) in mitochondria (including hypertrophic organelles), also suggest increased activity of mitochondria from cells grown under altered gravity conditions compared to controls.