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Growth, intracellular free amino acid pools and photosynthetic and respiratory activities in nutrient sufficient cells and in N-, K- and P-limited cells of Cyanidium caldarium (Tilden) Geitler, and responses to nutrient resupply were investigated. Addition of ammonium to N-limited cells and of phosphate to P-limited cells resulted in a stimulation of dark respiration and in a decrease in photosynthetic oxygen evolution. Addition of K to K-limited cells had no effect on rates of photosynthesis and respiration. Nutrient limited cells and sufficient cells exhibited different free amino acid profiles. Upon resupply of ammonium to N-limited cells levels of glutamine, citrulline, arginine, alanine, and serine increased. Also the levels of delta-aminolevulinic acid (delta-ALA) and putrescine increased notably. On adding phosphate to P-limited cells the level of glutamate decreased significantly whereas the level of alanine increased and the concentrations of other amino acids remained unaffected. On adding potassium to K-limited cells there was an increase in glutamate and citrulline concentrations, and a decrease in putrescine concentration, whereas concentrations of arginine and alanine remained at the very high levels observed already before addition. Resuspension of N- and K-limited cells in a complete growth medium resulted in resumption of growth, but maximum growth rate was reached only after 25-30 h. In P-limited cells resumption of growth in complete medium occurred progressively and reached the maximum rate 30 h later. P-, K- and N-limited cells resuspended into sufficient media showed different rates of ammonium and phosphate assimilation. The pattern of recovery from nutrient limitation is discussed according to the cellular role fulfilled by the nutrient which was growth rate-limiting.

Amino acid pools in Cyanidium caldarium (Tilden) Geitler, growing under conditions of sufficiency of nutrients (sufficient cells), or under conditions of nitrogen, potassium or phosphorus limitation (N-, K- or P-limited cells), and responses upon nutrient resupply were investigated. Glutamate was the dominant amino acid in sufficient cells [11.2-mu-mol ml-1 packed cell volume (pcv)], in N-limited cells (7.71-mu-mol ml-1 pcv) and in P-limited cells (7.46-mu-mol ml-1 pcv), whereas in K-limited cells the dominant amino acid was alanine (8.83-mu-mol ml-1 pcv). Other amino acids detected occurring at significantly high concentrations were: citrulline, alanine, aspartate, glutamine and delta-aminolevulinic acid in sufficient cells, alanine in N-limited cells; alanine, citrulline, serine, arginine and asparagine in P-limited cells; glutamate, arginine, glutamine, citrulline, asparagine, serine, aspartate and putrescine in K-limited cells. On adding NH4+ to N-limited cells the level of glutamate decreased and then increased to the starting value; there occurred an increase in the level of glutamine, aspartate, citrulline, arginine, alanine, serine, as well as of delta-ALA and putrescine. Resupply of Pi to P-limited cells caused a significant decrease of glutamate and a concomitant increase of alanine, but was without effect on the concentration of other amino acids. On adding K+ to K-limited cells there was an increase of glutamate and citrulline, and a decrease of putrescine, whereas arginine and alanine remained unvaried at the very high level occurring before addition.