Fig. 10: Schematic representation of the remodeling of G. partita cells between the photoautotrophic and heterotrophic states.

A A schematic diagram comparing the photoautotrophic and heterotrophic states of G. partita. The photoautotrophic and heterotrophic states of G. partita differ mainly in (1) the source of carbohydrates (photosynthate or external organic carbon), (2) the activities of fatty acid and membrane lipid synthesis (glycolipids or phospholipids), and (3) protein synthesis activities (in the plastid or cytosol and mitochondrion). In order to enable photoautotrophic growth, the cell requires more nitrogen, protein, and fatty acids, especially glycolipids, than heterotrophic cells. When exogenous organic carbon is available, the cell ceases the synthesis of proteins and lipids required for photosynthesis, and intracellular resources are allocated more toward cell growth and proliferation. B The putative mechanism that triggers trophic remodeling in G. partita is as follows. The intracellular concentration of certain substrate(s) (e.g., a specific sugar phosphate) in sugar or carbohydrate metabolic pathways, metabolized from either organic substrate(s) obtained through photosynthesis or from outside the cell, is sensed by G. partita cells. Depending on the degree of the shortage of the sensed substrate(s), the cells transcribe genes encoding proteins required for photosynthesis. When the level of the sensed substrate(s) is relatively low, the cell synthesizes proteins and lipids for photosynthesis based on the degree of the shortage (greening or maintaining a steady photosynthetic state). However, when abundant exogenous organic carbon sources are available and the concentration of the substrate(s) becomes relatively high, the cell represses the synthesis of proteins and lipids for photosynthesis, while continuing to grow and proliferate, resulting in the dilution of photosynthetic machinery (bleaching or maintaining a steady heterotrophic state).