Primary production of protein: I. Comparison of net cellular carbon and protein synthesis with 14C-derived rate estimates in steady-state cultures of marine phytoplankton

Measurements of total inorganic I4C-carbon fixation and I4C-carbon incorporation into protein were compared with calculated net synthesis of total cellular carbon and protein carbon respectively in NH:-limited continuous cultures of 4 marine phytoplankton species: the marine chlorophyte NannocNoris atomis, the diatom Chaetoceros sjmplex, the chrysophyte Chattonella luteus, and the cryptomonad Chroomonas sahna. Net synthesis rates were estimated from dilution rates and steady-state concentrations of total cellular carbon and protein in the chemostat cultures. For 3 out of the 4 species, there was good agreement between the increase in total fixed I4C-carbon over 3 h and the calculated net synthesis of newly fixed cellular carbon. Observed increases in fhed IJC-carbon in cultures of C. s a h a exceeded calculated values indicating degradahve loss of unlabeled cell material. Extracellular release of fixed "C-carbon was significant for C. luteus as shown by a higher rate of production of acid non-volatile 'v relative to total particulate I4C (i.e. retained on Whatman GF/F glass fiber filter). As a result of incomplete equilibration of protein precursors with the exogenous "Clabeled inorganic carbon pool, incorporation of I4C-carbon into protein of all species was less than calculated net synthesis of protein carbon for up to 3 h after isotope addition. Comparisons of net synthesis and incorporation of I4C-carbon lnto individual protein amino acid residues revealed that precursor sources of protein-bound glycine and alanine rapidly attained isotopic equhbrium with the I4C-labeled inorganic carbon pool for all growth conditions and in all species examined. Precursor equilibration dynamics of protein-bound glutamate and aspartate were generally slower than glycine and alanine. Rates of incorporation of I4C-carbon into protein of N. atomis decreased more rapidly in response to interruption of the supply of fresh media to the chemostat culture than did rates of total 14Ccarbon fixation. C. simplex was also sensitive to interruption of media inflow, as indicated by continuous decreases in "C-carbon incorporation into protein-bound glycine and alanine. These results emphasize the importance of time-course sampling and keeping incubations as short as possible for measurements of 14C-carbon incorporation into protein. Characteristics of incorporation of I4Ccarbon into protein-bound glycine and alanine make them select tracers for measuring protein synthesis in muted autotrophic populations using short-term incubations. The relative uniformity of the amino acid composition of proteins provides a basis for extrapolating synthesis rates of glycine and alanine to total protein production rates, thereby leading to a determination of the proportion of primary production associated with protein synthesis. This will provide information about the physiological state of phytoplankton populations, and their role in influencing the trophodynamic flux and composition of particulate organic matter.