C-phycocyanin as a storage protein in the blue-green alga Spirulina platensis

The possibility that c-phycocyanin serves as a nitrogen source in Spirulina platensis during nitrogen starvation was studied. The following evidence was obtained in support of this idea. 1. Under favourable conditions for growth, c-phycocyanin existed in large excess in the algal cells. 2. When the supply of nitrogen was low, about 30-50% of the c-phycocyanin disappeared without any effect on the maximal growth rate. 3. A culture which was deprived of nitrogen continued to grow unaffectedly for a period, the duration of which depended on the c-phycocyanin content in the cell before nitrogen starvation was initiated. 4. c-phycocyanin was the only nitrogenous compound that was depleted during the course of nitrogen starvation when growth was yet unaffected. 5. When protein synthesis was inhibited either by nitrogen starvation or by methionine sulfoximine (MSO), phycocyanin content began to decline immediately and growth continued at normal rates as long as c-phycocyanin did not decline below 50%. 6. The decrease in c-phycocyanin content during nitrogen starvation was accompanied by an increase in proteolytic activity.