CORRELATED BIOCHEMICAL AND ULTRASTRUCTURAL-CHANGES IN NITROGEN-STARVED EUGLENA-GRACILIS

Growth of Euglena gracilis Z Pringsheim under photoheterotrophic condi tions in a nitrogen-deprived medium resulted in progressive loss of ch loroplastic material until total bleaching of the cells occurred. Bioc hemical analysis and ultrastructural observation of the first stages o f the starvation process demonstrated an early lag phase (from 0 to 9 h) in which cells increased in size, followed by a period of cell divi sion, apparently supported by the mobilization of some chloroplastic p roteins such as the photosynthetic CO2-fixing enzyme ribulose-1,5-bisp hosphate carboxylase/oxygenase. The degradation of the enzyme started after 9 h of starvation and was preceded by a transient concentration of this protein in pyrenoidal structures. Protein nitrogen and photosy nthetic pigments as well as number of chloroplasts per cell decreased during proliferation through mere distribution among daughter cells. H owever, after 24 h, when cell division had almost ceased, there was a slow but steady decline of photosynthetic pigments. This was parallele d by observable ultrastructural changes including progressive loss of chloroplast structure and accumulation of paramylon granules and lipid globules in the cytoplasm. These findings reinforce the role of chlor oplastic materials as a nitrogen source during starvation of E. gracil is in a carbon-rich medium. The excess of ribulose-1,5-bisphosphate ca rboxylase/oxygenase acts as a first reservoir that, once exhausted, is superseded by the generalized disassembly of the photosynthetic struc tures, if the adverse environment persists more than 24 h.