Utilization of inorganic and organic nitrogen by bacteria in marine systems1

The relative contribution of various inorganic and organic forms of nitrogen to the nitrogen requirements of picoplankton was examined with 15N tracers. Size fractionation was used to measure uptake by < l-pm size microorganisms, and inhibitors of protein synthesis were used to separate procaryotic from eucaryotic nitrogen uptake, Picoplankton utilized mainly ammonium and amino acids and only negligible amounts of nitrate and urea. Nearly all amino acid uptake was by procaryotes, while both procaryotes and eucaryotes utilized ammonium. About 78% of total ammonium uptake was by procaryotes, and a significant portion of this was due specifically to heterotrophic bacteria. Regeneration of ammonium was correlated with eucaryotic rather than procaryotic activity. Ammonium accounted for at least 20-60% of the summed ammonium plus amino acid utilization by bacteria. The results suggest that a significant portion of ammonium uptake in the euphotic zone was by heterotrophic bacteria rather than solely by phytoplankton. This may invalidate the use of the Rcdfield C : N ratio for estimating rates of nitrogen assimilation in the euphotic zone from carbon assimilation rates. Historically, it has been assumed that phytoplankton is responsible for most of the uptake of inorganic nitrogen in the euphotic zone of the sea (e.g. Dugdale and Goering 1967). Exceptions to this generalization have been noted, and recent indirect evidence supports the hypothesis that heterotrophic bacteria may play a significant role in the utilization of inorganic nitrogen in the euphotic zone. Eppley et al. (1977) found that uptake rates of inorganic nitrogen were high compared with 14C0, fixation rates during winter in the central North Pacific Ocean and attributed this discrepancy to nitrogen uptake by heterotrophic bacteria. Laws et al. (1985) compared ammonium uptake rates with rates of nitrogen uptake estimated from light-dependent 14C02 incorporation into protein. They observed that ammonium uptake was substantially higher than estimated from 14C0, incorporation rates and concluded that ammonium uptake by heterotrophic bacteria must have been high. Several studies have examined the uptake I This work was supported by the NSF Visiting Professorships for Women Program (grant RTI-83 10369) and by NSF grants OCE 83-08753 (to P. A. Wheeler) and OCE 8 1-17834 (to R. E. Hodson). 2 Present address: College of Oceanography, Oregon State University, Corvallis 9733 1. 3 Present address: College of Marine Studies, University of Delaware, Lewes 19958. of inorganic nitrogen by particles of different sizes. Ammonium uptake by the < Ipm fraction was small ( < 20%) in three marine environments (Harrison et al. 1977; Harrison 1978; Glibert 1982; Probyn 1985). However, Probyn and Painting ( 198 5) found that ammonium uptake by the < l-pm organisms was 50% of total uptake at two out of five stations in Antarctic surface waters; nitrate uptake was negligible at two stations and < 50% of total uptake at four stations. Uptake by picoplankton in the < l-pm size fraction is a maximum estimate of uptake by trapped heterotrophic bacteria, since this size fraction also contains procaryotic (cyanobacteria) and eucaryotic phototrophs (Johnson and Sieburth 1982). The relative use of inorganic and organic forms of nitrogen by picoplankton has not been compared for procaryotes and eucaryotes or for phototrophs and heterotrophs. Various experimental approaches have been used to demonstrate that heterotrophic bacteria are the major sink for dissolved organic nitrogen (DON) in marine environments (see Billen 1984). However, the sufficiency of the supply of DON for meeting the nitrogen requirements for growth of heterotrophic bacteria has not been established. Bacterial production appears to be substantially higher than the maximum measured uptake rates of dissolved free amino acids. Bacterial produc-