Changes in intracellular and extracellular amino acids during the predation of the chlorophyte Dunaliella primolecta by the heterotrophic dinoflagellate Oxyrrhis marina and the use of the glutamine/glutamate ratio as an indicator of nutrient status in mixed populations

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Abstract
Concentrations of intracellular and extracellular (free) amino acids (IAA and FAA respectively) were determined during predation of prey-deplete and prey-replete Oxyrrhis marina on dead, N-deplete and N-replete Dunaliella primolecta, and also during the growth of mixed populations of the 2 organisms over 7 d. Cultures were axenic. Dunaliella did not tak up any of 20 FAA tested on Oxyrrhis took up (amongst others) methionine, leucine, valine, and alanine. Uptake of FAA by Oxyrrhis could contribute a significant proportion of nitrogen demand. The composition of FAA in media containing mixed populations differed depending on the nutrient status of both organisms but generally those amino acids present were those for which Oxyrrhis did not show an affinity, although levels of, in particular, alanine increased until Oxyrrhis became prey-depleted and then decreased. Glutamate, alanine, arginine, glutamine and serine (and taurine in Oxyrrhis) were the major components of IAA but the porportions varied over a wide range depending on the nutrient status of the organisms. The value of the glutamine/glutamate (GLN/GLU) ratio appeared consistent with the incorporation of NH4+ by the population, indicating assimilation by Dunaliella of NH4+ regenerated by Oxyrrhis which was not detected in the medium. Elevated GLN/GLU was also indicative of prey-replete Oxyrrhis. However, the evelated levels of GLU, and correspondingly low GLN/GLU, in prey-deplete Oxyrrhis and during initial feeding, could mask the GLN/GLU of Dunaliella if the predator biomass was high. Both prey-deplete Oxyrrhis and N-depleted Dunaliella had GLN/GLU of ca 0.01.