DISTRIBUTION OF INTRACELLULAR NITROGEN IN MARINE MICROALGAE - BASIS FOR THE CALCULATION OF SPECIFIC NITROGEN-TO-PROTEIN CONVERSION FACTORS

The utilization of nitrogen-to-protein conversion factors (N-Prot fact ors) is a widely accepted and practical way to determine total protein content. The accuracy of protein determination depends on the establi shment of specific N-Prot factors, since the conventional factor of 6. 25 may be unsuitable for all species. This study was designed to deter mine the concentrations of the main nitrogenous compounds and to estab lish N-Prot factors specific for the following marine microalgae: Chlo rella minutissima, Dunaliella tertiolecta, Hillea sp., Isochrysis galb ana, Nannochloropsis oculata, Phaeodactylum tricornutum, Prorocentrum minimum, Skeletonema costatum, Synechococcus subsalsus, and Tetraselmi s gracilis. Cultures were maintained under a 12-h photoperiod (300 mu mol photons.m(-2).s(-1)) at temperatures of 20.0 degrees +/- 1.0 degre es C (dark) to 23.0 degrees +/- 2.0 degrees C (light) in Walne's cultu re medium without additional external carbon sources. The distribution of intracellular nitrogen was studied by determining total nitrogen ( TN, by CMV [carbon, hydrogen, and nitrogen] analysis), protein N (PN, by analysis of total amino acids), and nonprotein N (NPN, determined b y analysis of DNA, RNA, chlorophylls (chl) a, b, and c, and intracellu lar inorganic nitrogen-NO3-, NO2-, and NH3 + NH4+) in logarithmic and stationary growth phases of cultures. Variations occurred in both accu mulation and distribution of PN and NPN among the species, as well as in each species during the different growth phases. Inorganic nitrogen compounds were observed to be the most important NPN source (from 6.4 +/- 0.1% to 41.8 +/- 4.2% of total N) in all species (except D. terti olecta), followed by nucleic acids (from 0.8 +/- 0.1% to 26.1 +/- 2.4% of TN) and chlorophylls (from 0.2 +/- 0.0% to 3.1 +/- 0.3% of TN). To tal amino acid residues ranged from 63.1 +/- 4.6% up to 88.1 +/- 11.2% of TN, which is in agreement with the presence of high NPN concentrat ions. N-Prot factors are proposed for each growth phase in tice studie d species, based on the ratio of amino acid residues to TN, establishi ng specific N-prot factors ranging from 3.60 +/- 0.27 to 4.99 +/- 0.64 . The mean N-Prot factor for all species/growth phases was 4.58 +/- 0. 11. The present study shows that the use of the traditional factor 6.2 5 is not suitable for these marine microalgae, and possibly for other species, because it overestimates their actual protein content.