Uptake and regeneration of free amino acids in marine waters of Southeast Alaska1

The nutritive importance of dissolved organic nitrogen to phytoplankton populations was assessed in Auke Bay, Alaska. Nitrate, ammonia, total dissolved organic nitrogen, and dissolved free amino acids were monitored and related to phytoplankton growth. Uptake rates of lGN-labeled glycine and glutamic acid ranged from 0.001 to 0.002 pg-atom N/hr+g-atom particulate N, compared to 0.03 to 0.05 for nitrate and ammonia. Uptake experiments using both 15Nand l”C-labeled glycine and glutamic acid showed that the nitrogen of glycine is preferentially incorporated, the carbon being respired; the reverse is true for glutamic acid, the nitrogen presumably being released as ammonia. Phytoplankton supplied with I’N-labeled nitrate released about 10% of the 15N back into the seawater as dissolved organic nitrogen within 48 hr. This value is probably conservative by virtue of the experimental procedure. The least understood and perhaps most intricate of the pathways involved in the nitrogen cycle in the sea are those concerned with dissolved organic nitrogen ( DON). Since the dissolved organic nitrogen pool can contain many complex components, past work has largely centered on description of the total DON (Krogh and Keyes 1934; VonBrand and Rakestraw 1941; Duursma 1961). Recently, however, improved analytical techniques for the isolation of specific nitrogenous compounds (Chau and Riley 1966; Webb and Wood 1967) and for the determination of the total dissolved organic nitrogen fraction in seawater (Armstrong et al. 1966; Armstrong and Tibbetts 1968) have stimulated work on the significance of dissolved organic nitrogen in marine phytoplankton ecology. The green alga Platymonas can assimilate glycine rapidly and efficiently at concentrations similar to those found in natural waters (North and Stephens 1967). Hobbie et al. (1968) found large differences between the uptake rates of different amino acids in a temperate estuary 1 Contribution No. 199 from the Institute of Marinc Scicncc, University of Alaska. This study was funded by National Science Foundation Grant GB-2678 and was part of a thesis submitted for the Ph.D. degree at the University of Alaska. ’ Present address: Hawaii Institute of Marine Biology, University of Hawaii at Manoa, Kaneohe 96744. and concluded that the overall uptake #of dissolved free amino acids (DFAA) was due to bacterial populations. Further support for ascribing natural uptake to bacteria was Williams’ ( 1970) finding that the major part of labeled amino acids assimilated was present in the particulate fraction of < 2 p diameter. North and Stephens (1971) d emonstrated that the uptake of glycine by Platymonas was greatly stimulated by restricting inorganic nitrogen sources and yet was not inhibited by high concentrations of nitrate or ammonia, so that DFAA could be an important nitrogen source in marine communities. Thomas et al. ( 1971)) however, concluded that DON was of little importance to phytoplankton nutrition in the eastern tropical Pacific Ocean and was probably composed of heterocyclic compounds of high molecular weight refractory toward biological degradation. This paper describes the relative ecological significance of DFAA and inorganic nitrogen to the phytoplankton community of Aukc Bay, near Juneau, Alaska (58”1O’N, 134’4O’W) (Fig. 1). The study began in March 1968, under winter conditions with a thoroughly mixed water column and essentially no particulate nitrogen (< 2 ig it ended in June when increased input of freshwater from nearby rivers radically altered the marine habitat. All forms of available nitrogen including DFAA were measured and related to changes in the phytoplankton community. Experiments were conducted to compare the uptake rates of nitrate, ammonia, and amino acids at naturally occurring levels, to determine whether carbon and nitrogen of specific amino acids were incorporated at equal rates and to measure the rate of DFAA excretion by rapidly growing phy-