Ultrastructure and enzyme complement of proplastids from heterotrophically grown cells of the red alga Galdieria sulphuraria

The unicellular thermoacidophilic red alga Galdieria sulphuraria is known to grow autotrophically as well as heterotrophically on a wide variety of organic carbon sources. Strain 107.79 becomes nearly colourless when grown heterotrophically on lactose. Under autotrophic conditions, a dominant chloroplast occupies most of the cell volume, while very small, yellow to colourless plastid-like organelles were observed in heterotrophic cells. Their exact number could not be established with certainty. To investigate the role of these algal proplastids, we developed a method to isolate intact organelles from heterotrophic cells via sucrose density-gradient centrifugation. Using electron microscopy, these organelles were shown to have a morphology similar to proplastids with a double membrane envelope, very few thylakoids and one to several dense bodies, apparently plastoglobuli. Investigation of the enzymatic capacities of isolated proplastids showed activity of triose phosphate isomerase and NAD-glyceraldehyde ...