Strategies for the allocation of resources under sulfur limitation in the green alga Dunaliella salina

The effect of sulfur limitation on the partitioning of carbon, nitrogen, an d sulfur was investigated in Dunaliella salina. D. salina was able to adapt to 6 muM sulfate; under these conditions, the cells showed reduced growth and photosynthetic rates. Whereas intracellular sulfate was depleted, phosp hate, nitrate, and ammonium increased. Amino acids showed a general increas e, and alanine became the most abundant amino acid. The activities of four key enzymes of carbon, sulfur, and nitrogen metabolism were differentially regulated: Adenosine 5' triphosphate sulfurylase, activity increased 4-fold , nitrate reductase and phosphoenolpyruvate (PEP) carboxylase activities de creased 4- and 11-fold, respectively, whereas carbonic anhydrase activity r emained unchanged. Sulfur limitation elicited specific increase or decrease of the abundance of several proteins, such us Rubisco, PEP carboxylase, an d a light harvesting complex protein. The accumulation of potentially toxic ammonium indicates an insufficient availability of carbon skeletons. Sulfu r deficiency thus induces an imbalance between carbon and nitrogen. The dra matic reduction in PEP carboxylase activity suggests that carbon was divert ed away from anaplerosis and possibly channeled into C3 metabolism. These r esults indicate that it is the coordination of key steps and components of carbon, nitrogen, and sulfur metabolism that allows D. salina to adapt to p rolonged sulfur limitation.