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 c omplete (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 i n the oldest -P cultures contained the same amount of inorganic orthophosph ate 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. Phosph orus deficiency enhanced production, excretion and metabolism of glycolate during the whole investigated period. In the initial phase of -P culture gr owth, medium acidification and low DIC concentration were conducive to glyc olate 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 concen tration might be the main reason of enhancement of glycolate metabolism. Th e results support the suggestion that involvement of glycolate metabolism i n acclimation to low phosphorus supply improves regeneration of inorganic o rthophosphate and protects chloroplasts against photoinhibitory damage by c onsumption of excess of absorbed light energy. (C) 2000 Editions scientifiq ues et medicales Elsevier SAS.