METABOLIC RESPONSES OF THE DIATOM ACHNANTHES BREVIPES (BACILLARIOPHYCEAE) TO NUTRIENT LIMITATION

The diatom Achnanthes brevipes C.A. Ag. was cultured in the presence of limiting concentrations of nitrogen (N) or inorganic phosphate (P_i). Growth, in terms of final yield, was more affected by N limitation than P_i limitation; N limitation had a greater effect also on protein and chlorophyll content. Carbohydrate concentrations increased under both nutrient starvation treatments, but N or P_i limitation had different effects. Total (intracellular plus extracellular) sugar content increased when cells were exposed to both types of nutrient limitation, but the extracellular polysaccharide fraction increased only in the presence of P_i starvation. Analyses were performed to identify the metabolic changes occurring in cells exposed to low phosphate because this was the main condition that affected carbohydrate extrusion. Activities of several enzymes involved in carbohydrate metabolism showed that under P_i limitation there was no activation of alternative reactions that were found to result in P_i liberation, instead of its consumption, in some higher plants and in the green alga Selenastrum minutum Naeg. Collins. Results showed that activities of pyruvate kinase, phosphorylating NAD-dependent 3-phosphate-glyceraldehyde dehydrogenase, and 3-phospho-glycerate kinase were inhibited under P_i-limited conditions compared with control cells, indicating limited glucose catabolism. Activity of uridine diphosphate glucose pyrophosphorylase, a key enzyme for the biosynthesis of the storage compound crysolaminarin, was also partly inhibited in P_i-stressed cells. Our findings suggest that carbohydrate catabolism in A. brevipes is limited under P_i deficiency, whereas extracellular extrusion of carbohydrate is favored.