[en] The ability of Chlamydomonas reinhardtii to grow under heterotrophic conditions, without light and solely with an exogeneous organic carbon source (acetate) in its cultivation medium, is one the particular features that have brought this green microalga to be considered as a model organism for the last 50 years, especially for studies on photosynthesis. Assimilation of acetate as an energy source is enabled by the glyoxylate cycle (GC), whose steps are quite similar to the ones of the Krebs cycle but comprises two specific enzymes, namely isocitrate lyase (ICL) and malate synthase (MAS). This cycle is well-known in other model organisms such as the yeast S. cerevisiae and the land plant A. thaliana but has been only poorly studied in Chlamydomonas since its discovery back in 1957, and more particularly when cells are cultivated under alternating light and dark phases, conditions under which it is thought to play a crucial role for the development of the alga.
During this project, we started by precisely identifying the isoforms of the enzymes comprised in the GC in addition to their subcellular localisation. It came out that all of them are located inside the peroxisomes with the notable exception of ICL which is located in the cytosol. We then studied the evolution with time during two consecutives day/night cycles (12h/12h) of some of the main central carbon metabolism molecules which are tightly interconnected with the glyoxylate cycle. This was performed with control strains at first and then with a mutant strain deficient for the ICL gene, allowing us to observe that most of these metabolites accumulation profiles are influenced by the cell cycle and division rate, and that the GC is essential for the growth of C. reinhardtii under such alternate light conditions with acetate supplemented in the cultivation medium. Finally, in order to have a better comprehension of the cellular regulation mechanisms linked to these phenotypical observations, we performed a transcriptional analysis of data sampled from the same cultivation conditions using a method named “Surprisal Analysis”. Application of this original method enabled to show that control cells are highly influenced by acetate metabolism during day phase and by replication mechanisms during night phase while mutant cells, which are unable to divide, are markedly influenced by stress-related pathways during both phases but more particularly when light is turned off and they have therefore access to no source of energy to maintain their metabolism.
