Involvement of glycolate metabolism in acclimation of Chlorella vulgaris cultures to low phosphate supply

Chlorella vulgaris (Beijer.) was grown for 8 d under air in cultures with complete (Control) or with phosphorus deficient (-P) medium limiting culture growth. The cells assimilated only 5-17 % of orthophosphate supplied from the complete medium, whereas from medium of -P cultures, orthophosphate was almost totally exhausted. Despite limited phosphorus availability, cells in the oldest -P cultures contained the same amount of inorganic orthophosphate as the control cells and only slightly less organic phosphates. The -P cells showed normal chlorophyll concentration and increased V-max and 1/K-0.5 dissolved inorganic carbon (DIC) of photosynthetic O-2 evolution. Phosphorus deficiency enhanced production, excretion and metabolism of glycolate during the whole investigated period. In the initial phase of -P culture growth, medium acidification and low DIC concentration were conducive to glycolate production. With subsequent medium alkalization, DIC content and cell carbonic anhydrase activity increased the photosynthetic O-2 evolution of -P cells two-fold. At that period, the elevated intrachloroplast O-2 concentration might be the main reason of enhancement of glycolate metabolism. The results support the suggestion that involvement of glycolate metabolism in acclimation to low phosphorus supply improves regeneration of inorganic orthophosphate and protects chloroplasts against photoinhibitory damage by consumption of excess of absorbed light energy. (C) 2000 Editions scientifiques et medicales Elsevier SAS.