Exchange of Metabolites in Cyanophora paradoxa and its Cyanelles*

Abstract

AbstractThe flagellate Cyanophora paradoxa contains blue‐greenish, organelle‐like inclusions termed cyanelles which perform photosynthetic CO2‐fixation in place of chloroplasts. By the use of the HPLC‐technique, Cyanophora was shown to form glucose, sucrose, maltose, mannitol, ribose, glycerol and trehalose. Extracts from the whole organism and from the eucaryotic host, but not from the cyanelles, convert 14C‐labelled UDP‐glucose to polyglucan. Synthesis of sucrose from UDP‐glucose and fructose‐6‐P or fructose could not be demonstrated in any extract from Cyanophora. The transfer of metabolites into cyanelles was monitored by the silicone oil filtering technique. The solute spaces for 14C‐labelled sorbitol and 3H2O were the same indicating that sorbitol freely penetrated the plasma membrane of cyanelles in contrast to the situation found in chloroplasts. The measurements of the solute spaces for the different compounds showed that maltose and sucrose were not accumulated by isolated cyanelles. Other compounds like fructose, fucose, glutamine or glycine had intermediate sizes of their solute spaces. Cyanelles apparently possess a rapidly transporting glucose carrier and not a malate/oxaloacetate shuttle and also not an ATP/ADP translocator. The carrier composition at the plasma membrane of cyanelles and at the inner envelope membrane of chloroplasts seems to be totally different.