CHANGES OF AMINO ACID COMPOSITION OF CHLORELLA CELLS DURING THEIR LIFE CYCLE

The cells of Chlorella ellipsoidea were grown synchronously,and at different stages of their life cycle, the cells wereanalysed for their contents in amino acids existing in freeforms as well as in the fractions of bulk protein and peptides.Throughout the algal life cycle, the content of bulk protein(per unit dry weight of cells) remained relatively constant,being about 20 to 40 times those of peptides and free aminoacids. The amino acid composition of the protein fraction alsoremained fairly constant, the predominant amino acids beingalanine, glutamic acid, glycine and leucine. The contents inthe bulk peptides increased appreciably during the periods ofgrowth and "ripening" (light period), and decreased markedlyduring the periods of "post-ripening" and cellular division(dark period). Similar modes of change in content were alsoobserved in most of the individual amino acids contained inthe peptide fraction. The most abundant component in the peptidefraction was arginine followed by glutamic acid, glycine andcyst(e)ine. Rather irregular was the mode of change of the levelsof individual free amino acids, although, as a whole, theirbehavior was similar to that of bulk peptides, increasing duringthe light period and decreasing during the dark period. Themost predominant free amino acids were glutamic acid and alaninefollowed by proline. Experimental evidence showed that the processes of formationof free amino acids and peptides are for the most part lightdependent, while the synthesis of protein, which is thoughtto be effected using as building blocks mostly free amino acids—formeddirectly or indirectly from early photosynthates or derivedfrom pre-formed peptides—is essentially a light-independentprocess. Peptides, as a whole, seem to have significance asreservoirs of building blocks for the syntheses in the darkof protein and other nitrogenous cellular substances. The synthesisof protein in the dark takes place not only by consuming thefree amino acids and peptides that have been accumulated duringthe light period, but also by assimilating the exogenous nitrogensource (nitrate). The distribution of individual amino acidsin the three main fractions mentioned above as it changed duringthe course of algal cell cycle was followed in detail, and theresults obtained were discussed in relation to various relevantdata reported by other workers. (Received June 29, 1964; )