Use of 13N as tracer for bacterial and algal uptake of ammonium from sea-water

Abstract

The relative importance of phytoplankton and heterotrophlc bacteria in the ut~lization of ammonium in a temperate coastal pelaglc environment (Long Island Sound. New York, USA) was examined uslng the short-lived radioisotope, I3N Uptake of ' 3 ~ ~ : Into different slze fractions under simulated in situ temperature and light condihons was compared to size fractionations of bacterial abundance, chlorophyll, and uptake of tritlated thymidme and 14C-b~carbonate In January and Apnl, little ' 3 ~ entered the bactenal slze fract~on, but when bacteria were more metabolically a c t ~ v e in May and July, about '/3 of the total I3N uptake appeared in the size fraction conta~ning most of the bactena but httle of the chlorophyll (0 2 to 0 6 ,pm) Exposure to up to 8 mC1 I-' of I3N radioactivity d ~ d not ~ n h ~ b ~ t bacterial or phytoplankton activity, and hlled controls showed virtually no uptake Ammonium uptake rates measured by I3N were comparable to those measured by the stable isotope 15N in 2 of 3 experiments The size fract~onation results were consistent wlth estimates of bactenal and phytoplank-ton demand tor nitrogen for growth Turnover rates for the dissolved a m m o n ~ u m pool ranged from 0 6 %