(1) During the process of life cycle of Chlorella, as modified by the limited supply of sulfur and nitrogen source, changes of lipid and carbohydrate contents of the algal cells were followed.
(2) It was found that an abnormally large amount of fat accumulated in the cells during the phase of sulfur starvation, while the content of carbohydrate decreased.
(3) when the S-starved cells were transferred to a medium containing sulfate alone, the depleted level of carbohydrate in the cells returned to normal, The fat content of the cells, however, showed a tendency even to increase further. The abonormally accumulated fat disappeared rapidly when the S-starved cells were incubated in the medium containing both sulfate and nitrate.
(4) It was concluded that in the S-deficiency some mechanism was in operation which was in favor of lipid synthesis and suppressive for the net formation of carbohydrate, and that a certain biochemical step requiring both S- and N-sources was essentially involved in the normal course of lipid metabolism.
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