Primary production of particulate protein amino acids : algal protein metabolism and its relationship to the composition of particulate organic matter

Abstract

The biochemical and physiological bases underlying primary production of particulate protein amino acids were investigated in an effort to understand the relationship between algal protein metabolism and particulate organic matter composition. In order to examine biochemical processes associated with conversion of inorganic carbon and nitrogen into protein, the effects of NH+4 limitation on free amino acid and protein composition and incorporation of inorganic 14C were studied in the marine chlorophyte, Nannochloris sp. (clone GSB Nanna). Free amino acid metabolism was sensitive to changes in steady state NH+4-limited growth rates. Reduced carbon and nitrogen flux into protein resulting from nitrogen limitation of growth was associated with reductions in proportions of cellular carbon and nitrogen in the intracellular free amino acid (IFAA) pool. Growth rate-dependent changes IFAA pool composition reflected changes in rate limiting steps which were intermediate between assimilation of inorganic nitrogen and the incorporation of nitrogen into macromolecules. The proportion of cellular carbon in both protein and pools of free amino acids decreased with decreasing growth rate. Distributions of incorporated inorganic 14C among free amino acids and protein provided qualitative descriptions of growth related compositional variations. Saturation rates of (IFAA) carbon with dissolved inorganic 14C did not significantly change as growth rates decreased. In contrast, saturation rates of free glutamate, glycine + alanine and valine did decrease with growth rate. At low growth rate, specific activities of the newly assimilated glutamate, valine, and glycine + alanine in protein were higher than specific activities of their corresponding free amino acid pools. This was likely a consequence of metabolic segregation and more rapid saturation of protein precursor pools. Enrichment of NH+4-limited steady state cultures of Nannochloris sp. with NH+4 led to a dramatic time dependent increase in free glutamine concentrations accompanied by differential increases in other free amino acids. Patterns of isotope incorporation into the free amino acids reflected real changes in concentrations. Increases were associated with the diversion of photosynthetically fixed carbon from lipophilic material towards amino acid biosynthesls, and signalled the onset of increased protein synthesis. Preliminary investigations of two other marine phytoplankton species, Dunaliella tertiolecta (clone Dun) and Thalassiosira weissflogii (clone Actin), suggested that the nature and timing of the biochemical response to NH+4 enrichment is different among different species. At high light intensity, increased NH+4 limitation of Nannochloris sp. was associated with decreases in cellular protein, protein to carbon ratios, and protein to chlorophyll a ratios. At low light intensities, cellular protein and protein to carbon ratios did not decrease with increasing NH+4 limitation. Chlorophyll a to protein ratios were…