Uptake of glycine by natural phytoplankton communities

Uptake of ["C] glycine by various size fractions of natural plankton communities was measured to determine if phytoplankton can compete effectively with other members of the plankton community for the free amino acids in seawater. The simultaneous occurrence of glycine uptake, plant pigments, and photosynthetic activity in the same size fractions, as well as autoradiography of the plankton samples, indicated that in some cases phytoplankton were responsible for 50% or more of glycine uptake. Comparison of prefiltration and postffltration procedures for this type of study provided an explanation for the discrepancy between these results and those of previous studies. The use of organic nitrogen by naarine phytoplankton has received considerable attention recently. There is increasing evidence that organic nitrogen•^both as urea (Carpenter et al. 1972; McCarthy 1972; Remsen et al. 1972) and as amino acids• may be a significant source of nitrogen for phytoplankton. This report is concerned with the use of glycine by natural ^gihytoplankton in Newport Bay, California. Many phytoplankters can be grown in culture with amino acids as the sole source of nitrogen. Some species can take up amino acids from extremely dilute solution. Studies of amino acid utilization by the green flagellate, Platymonas (North and Stephens 1967, 1971), and the diatoms Nitzschia (North and Stephens 1972) and Cyclotella (Liu and Hellebust 1974), show that these organisms can obtain sufficient nitrogen to support growth from amino acids supplied at concentrations as low as 5 X 10"'' M, commonly found in the normal habitat of inshore phytoplankters ( Bohling 1970; Clark et al. 1972; Riley and Segar 1970). Wheeler et al. (1974) extended these observations to 25 species of marine ' Present address: Kerckhoff Marine Laboratory, California Institute of Technology, 101 Dahlia St., Corona del Mar 92625. phytoplankters in culture of which 14 were able to accumulate one or more ^*C-labeled amino acids from dilute solution at rates potentially significant for their nitrogen requirements. Most of these were species from inshore and tide pool habitats. In contrast, those species that tend to occur in offshore regions showed little or no ability to take up amino acids in the laboratory. The availability of naturally occurring amino acid nitrogen for phytoplankton growth has also been studied. North (1975) showed that an axenic culture of Platymonas could remove primary amines from surface waters taken from a bay where this organism is a common member of the plankton population; about 50% of the primary amines in the samples was taken up rapidly by Platymonas, at a rate which could supply enough nitrogen to support a generation time of 30 h. In our previous work we used axenic cultures of phytoplankters to study the nutritional value of amino acids. The current investigation was undertaken to determine whether natural phytoplankton populations can compete effectively with other members of the plankton community for the free amino acids in seawater. The plankton community was separated into LIMNOLOGY AND OCEANOGEAPHY 900 SEPTEMBER 1977, V. 22(5)