Amino acid uptake by marine phytoplankters

Axenic cultures of 25 species of unicellular marine algae were tested for their ability to utilize nine common amino acids, supplied at high concentrations in batch culture, as a nitrogen source; most species were able to use several amino acids, although growth was often slower than on nitrate nitrogen. The algae were also tested for their ability to t,ake up ‘“C-labclcd amino acids supplied at low, natural concentrations. In most cases, species that could grow on an amino acid at high concentration in culture could also take up amino acids at low concentrations. Uptake rates were higher in cells that had been deprived of nitrogen during growth. In some cases, uptake rates at low concentrations-if sustained-would bc sufficient to support moderate growth rates. The ability to accumulate amino acids from dilute solution occurs in many phytoplankters, particularly in spccics that normally occur in inshore and littoral habitats. The growth of phytoplankton in the sea often appears to be controlled by the availability of nitrogen (Dugdalc and Goering 1967). Ambient concentrations, uptake rates, assimilation and metabolism of the inorganic forms of nitrogen-nitrate and ammonia-have been studied to explore the relation between nitrogen nutrition and phytoplankton ecology, but the role of organic nitrogen has received much less attention. We wish to focus attention on the potential significance of amino acids as a nitrogen source for phytoplankters under natural conditions. Concentrations of dissolved free amino acids are usually in the range of 0.2 to 2.0 pg-atoms N liter-” for total amino acids (Bohling 1970; Clark et al. 1972; Riley and Segar 1970). A number of phytoplankters have been grown axenically with various organic sources of nitrogen, including amino acids ( Guillard 1963). However, in such growth experiments nitrogenous compounds are usually 1,000 times more concentrated than in natural waters. The ability of phytoplankton to use amino acids as nitrogen sources at high concentrations does not necessarily provide any insight into their potential significance at low concentrations. ‘This work was supported by NSF Grant GA 33904x. ’ N. Kaji and V. Cooke provided technical assistance and help in manuscript preparation. D. Staley assisted in the uptake measurements. To study growth directly in laboratory culture at natural concentrations it would be necessary to maintain axenic conditions in a large volume, to supply nutrients continuously to mimic a steady state, and to devise methods of assessing slow growth in a sparse suspension of cells. We have chosen to approach the issue by studying the short-tclm rate of amino acid uptake by phytoplankters from dilute solution in conjunction with growth studies at higher concentrations. Analysis of the kinetics of uptake allows estimation of entry rates under natural conditions, while growth experiments establish the substrate as a metabolically adequate nutrient. We have demonstrated uptake of amino acids from low concentrations and their oxidation and assimilation into synthetic pathways by the unicellular algae Plutymonas subcorcliformis and Nitxschiu oualis (e.g. North and Stephens 1967, 1969, and later). Amino acid transport systems in phytoplankters are extremely labile. Restriction of nitrogen availability in the culture decreases the nitrogen content of the cells and slows their growth rate; it also greatly accelerates the rate of amino acid uptake. In some cases, no uptake is observed until nitrogen in the growth medium is restricted. The possible contribution of amino acids to the nitrogen required for growth appears to depend on the nutritional history of the cells. Amino acids supplied at low levels LIMNOLOGY AND OCEANOGRAPHY 249 MARC11 1974, V. 19(2)