Photodynamic processes and the synthesis of 5-aminolevulinic acid in chlorella cells treated with amino acids and 1,10-phenanthroline

Treatment of chlorella (Chlorella sp.) cells for 2 h in darkness with tetrapyrrole-dependent photodynamic herbicides (TDPH) derived on the basis of 0.3 mM 1,10-phenanthroline (Ph) combined with 0.6 mM Glu or 0.6 mM Gln induced the accumulation of sensitizers of photodynamic processes: magnesium protoporphyrin IX (MgPP) and MgPP monomethyl ester (MgPPE). Within the first day after chlorella cells treated with TDPH were illuminated, photodestruction of MgPP(E) was observed, and production of the first specific precursor of chlorophyll (Chl), 5-aminolevulinic acid (ALA), in the cells declined. Then the accumulation of ALA was stimulated, and the level of heme, which is a retroinhibitor of ALA synthesis, simultaneously fell. During the first two days of illumination, the content of Chl and carotenoids in the algae treated with TDPH did not differ from their levels in control culture, which suggests a high resistance of photosynthetic pigments to photodynamic process induced by porphyrins. Subsequently, a slight but rising in time accumulation of pheophytin (Pheo) was observed, as well as photodestruction of Chl and carotenoids. After five days of illumination, the difference in the content of Chl between the culture treated with TDPH and control material was 10–30% depending on the illuminance. Chlorella cells treated with TDPH remained capable of producing Chl from exogenous ALA in the dark for at least eight days. In the experiments simultaneously conducted with a higher plant, cucumber (Cucumis sativa L.), which accumulated in the dark essentially the same content of porphyrins in response to TDPH as algae did, the residual level of Chl after five days of illumination was only 10–20% of control plants. It was assumed that a high tolerance of the chlorella pigment pool to photooxidative stress induced by the accumulation of MgPP(E) and Pheo depended on a highly active state of the antioxidant protective system and the ability of ALA molecules additionally formed under the influence of TDPH to be converted into Chl, thereby participating in its de novo synthesis.