Uptake of urea C and urea N by the coastal marine diatom Thalassiosira pseudonana

Abstract

Urea uptake rates of Thalassiusiru pseudonana (clone 3H) were determined using [14C]urea,[15N]urea, and by measuring disappearance of dissolved urea from the medium after adding 10 lg-atoms urea-N liter-'. In nitrate-sufficient cultures, the average [14C]urea uptake rate was 60% of the urea disappearance rate. Nitrate uptake continued in the presence of urea at a reduced rate, and only 15% of the urea N taken up was retained by the phytoplankton.The increase in PON during incubation was roughly equal to the total NO, and urea N taken up. Average urea uptake rates measured by all methods in 24-h nitrate-starved cultures were in excellent agreement with the rate of increase in PON during a l-h incubation. Uptake rate of [14C]urea and disappearance rate of urea were constant and equal. In neither nitrate-sufficient nor nitrate-starved cultures were [15N]urea uptake rates constant, with maximal rates measured 5.-l 5 min after the addition of urea.Ammonium was released by T. pseudonana following uptake of urea and was then taken up. A model of urea uptake and assimilation by T. pseudonana that involves efflux of urea N as NH, and its rapid reabsorption is proposed. These results can explain previous observations and have implications for utilization and cycling of urea N by phytoplankton in nature.Radioactive ( 14C) and stable (l 5N) isotope tracers of urea are used to measure urea uptake rates of phytoplankton in the laboratory and in nature, but uptake rates determined with these two isotopes are not always equal. Harrison et al. (1985) found that [ 14C]urea uptake rates were greater than [15N]urea uptake rates in the eastern Canadian Arctic, and Price and Harrison (in prep.) obtained similar results for urea uptake by Sargasso Sea plankton. In the Sargasso Sea, the time-courses of uptake of both labeled substrates were also different:[15N]urea incorporation by phytoplankton was roughly constant during a 24-h incubation period, while [ 14C]urea uptake was most rapid over the first 6 h of the incubation. Discrepancy between [14C]urea uptake and the change in the concentration of dissolved urea in seawater has also been re-L Present address: R. M. Parsons Laboratory, Bldg. 48-2 13,

Incorporation of nitrogen from amino acids and urea by benthic microbes: role of bacteria versus algae and coupled incorporation of carbon

“…4). These results are comparable with those of Lund & Blackburn (1989), who measured production of 14 C-DIC after addition of 14 C-urea to a coastal marine sediment but no incorporation of 14 C. Release of 13 C-or 14 C-DIC following addition of 13 C-or 14 C-urea was also observed for algal cultures (Price & Harrison 1988, Antia et al 1991 and references therein), bacteria from estuarine waters (Jørgensen 2006) and a natural planktonic microbial community (Tamminen & Irmisch 1996). Moreover, uncoupled uptake of 15 N and 13 C from dual-labeled urea has been reported for coastal waters (Mulholland et al 2004, Fan & Glibert 2005, Andersson et al 2006.…”

“…In order for bacteria and algae to use urea as a nitrogen source for growth, urea first needs to be broken down to NH 4 + and DIC. Subsequently, the resulting NH 4 + can be used to synthesize amino acids for production of proteinaceous biomass, while the DIC may be used as a carbon source for photosynthesis (Price & Harrison 1988, Antia et al 1991. This urea metabolization pathway appears to be widespread because urease, the enzyme responsible for the breakdown of urea, is a common enzyme used by algae as well as bacteria (Price & Harrison 1988, Mobley & Hausinger 1989, Antia et al 1991, Zehr & Ward 2002.…”

“…Studies on algal cultures have shown that incorporation of urea-N is uncoupled from urea-C (Price & Harrison 1988, Antia et al 1991, while both coupled and uncoupled uptake of nitrogen and carbon has been reported for amino acids (Algeus 1948, Stephens & North 1971, Palenik & Morel 1990, Antia et al 1991. For natural aquatic microbial communities, uptake of N and C from organic compounds has been studied using duallabeled urea (Mulholland et al 2004, Fan & Glibert 2005, Andersson et al 2006) and amino acids (Schell 1974, Zehr et al 1985, Mulholland et al 2002, Andersson et al 2006).…”

Seasonal variation in sediment urea turnover in a shallow estuary

“…The organic nitrogen compound, urea, has long been recognized as an important nitrogen source for primary producers in marine environments (Remsen 1971, Eppley et al 1973, McCarthy et al 1977, Harrison et al 1985, Sorensson & Sahlsten 1987, Price & Harrison 1988, Antia et al 1991, Cochlan & Harrison 1991. Urea is excreted as an endproduct of nitrogen metabolism by a variety of marine invertebrates (Wright 1975, Regnault 1987, Boucher & BoucherRodoni 1988, Stickle 1988, Lomstein et al 1989 including zooplankton (Eppley et al 1973).…”

Urea and nickel utilization in marine cyanobacteria as evaluated by incubation, proteomic, and uptake techniques

“…In fact, urea uptake rates in the western Sargasso Sea have been estimated to be near the maximum potential (Price and Harrison 1988a) and dual stable isotope ( 14 C-and 15 N-urea) labeling experiments in the Arctic Ocean show that urea-carbon is respired as CO 2 , while most of the urea-nitrogen is incorporated by phytoplankton (Harrison et al 1985). Culture experiments with the diatom Thalassiosira pseudonana revealed that significant portions of urea-nitrogen taken up may be released back to the environment as ammonia followed by rapid reabsorption (Price and Harrison 1988b). In both coastal and open ocean regimes urea uptake typically accounts for a third of the total nitrogen uptake (McCarthy 1972;Price and Harrison 1988a).…”