Assessing amino acid uptake by phototrophic nanoflagellates in nonaxenic cultures using flow cytometric sorting

Biologically available concentrations of individual dissolved amino acids in the open ocean are generally <1 nM. Despite this, the microbial turnover of amino acids is usually measured in hours indicating high demand. It is thought that the majority of uptake is due to bacterioplankton, although protists, particularly phototrophic protists, are also expected to take up amino acids. In order to assess the ability of protists to compete with prokaryotes for amino acids at subnanomolar concentrations, we examined the direct uptake of 3H-leucine by phototrophic nanoflagellates (prasinophytes, pelagophytes and trebouxiophytes) and by associated bacteria using flow cytometric cell sorting. In contrast to 3H-leucine-assimilating bacterial copopulations, none of the six studied nanoflagellates showed measurable direct uptake of 3H-leucine, suggesting that the studied phototrophic protists were unable to utilize dissolved 3H-leucine at natural oceanic concentrations. More practically, the flow-sorting technique allowed rapid and unequivocal differentiation of organic nitrogen uptake between prokaryotic cells and eukaryotic cells in mixed microbial populations, reducing the need to establish and maintain axenic algal cultures.