J. Exp. Mar. Biol. Ecol., 1990, Vol. 139, pp. 151-166 Elsevier 151 JEMBE 01430 Composition of intracellular and extracellular pools of amino acids, and amino acid utilization of microalgae of different sizes Kevin J. Flynn Dunstafiage Marine Laboratory, Oban, ArgyN, UK (Received 26 October 1989; revision received 19 February 1990; accepted 25 February 1990) Abstract: 15 axenic species and strains of unicellular algae of micro, nano, and pica sizes were examined. The total concentration of a-amino-N inside cells of all species decliied on N deprivation but the changes in proportion of individual amino acids are generally too complex for use as a metabolic fingerprint. The only exception was that the mole ratio glutamine/glutamate was consistently high for exponentially growing cells and low for N-deprived cultures. Generally, the smaller algae showed little significant ability to use amino acids supplied at near-natural concentrations. This lack of ability contrasts with reports of amino acid uptake by diatoms. Some of these algae also release amino acids, whilst others release and use little and may play little part in amino acid cycling. The explanation for the apparent limited used of amino acids by nano- and picoalgae may lie in the tight coupling of use and regeneration of inorganic nutrients within microbial loop communities negating any competitive advantage in evolving a capability to use a wide range of amino acids. Key words: Amino acid, extra- and intracellular; Amino acid, release and uptake; Glutamine/glutamate; Microbial loop; Nanoalga; Picoalga INTRODUCTION Recently, there has been much interest in the intra- and extracellular pools of amino acids (InAA and ExAA, respectively) of phytoplankters. ExAA are an important component of dissolved organic nitrogen in the marine environment, with roles in both nutrition of microbes (Flynn & Butler, 1986) and in chemosensory responses by preda- tors (Sibbald et al., 1987; Gill & Poulet, 1988). Algae play a role in ExAA cycling through processes of release and uptake. The composition of InAA is a function of C-N status for which Flynn et al. (1989) suggest that the mole ratio of intracellular glutamine/glutamate (GLN/GLU) gives a high value when N stress is least significant and a low ratio (< 0.1) when cells have been starved of N. In addition, it has been suggested that the composition of InAA can be used as a “metabolic fingerprint” of the N status of the organisms (Admiraal et al., 1986). Correspondence address: K. J. Flynn, Dunstatfnage Marine Laboratory, PO BOX 3, Oban, Argyll PA34 4AD, UK. 0022-0981/90/$03.50 0 1990 Elsevier Science Publishers B.V. (Biomedical Division)