Organic Nitrogen Release Research Articles

A 15N tracer method for the measurement of dissolved organic nitrogen release by phytoplankton

We present a new method for separation of dissolved organic nitrogen (DON) from dissolved inorganic nitrogen (DIN) in marlne samples which permlts the measurement of DO'*N production during DII5N uptake and regeneration experiments. Ion retardat~on resin was used to separate DI1'N from DOI'N and the isolated D0I5N was subsequently analyzed by mass spectrometry Variat~on in D0I5N atom % enrichment in duplicate samples, determined w ~ t h the ion retardation column method, was less than 40 . We also separated the low n~olecular weight (LMW) from total D015N using ultrafiltration and found the ratio of these variables to be a useful index in determining what release processes were likely occurring in a given sample. Examples are presented from 2 types of experiments conducted in Chesapeake Bay waters: first, a 6 h time-course of D015N release from both NH,' and I5NO3uptake, and second, a series of short-term (0.5 h) release measurements from 15NH,* uptake over the course of a day. In the former, total DO1% release rates resulting from 15NH,* uptake were several-fold higher than those resulting from IsNO3uptake due to extremely low rates of incorporat~on of NO? into cellular organic material. The release of LMW DOI'N resulting from I5NH,+ uptake decreased from 78 to 21 % of total D0I5N release during the 6 h incubation which suggests that processes other than direct release by phytoplankton (e.g. sloppy feeding, cell lysis) likely became more important as the incubation progressed In the latter expenment, LMW DO'% release was 0 to 16 % of total D0I5N release, which indicates that, in this case, also, processes other than direct release by phytoplankton were dominating the flux of DON from phytoplankton.

Effect of litter nitrogen on decomposition and microbial biomass inSpartina alterniflora

The effect on decomposition of 4 different levels of nitrogen in aerial tissue ofSpartina alterniflora, collected at the end of its growing season litter, was studied in laboratory percolators for 56 days at 20‡C. The CO2 evolution and the release of organic nitrogen and organic carbon were monitored. From these data, the ash-free dry weight (AFDW), nitrogen (N) content, and carbon∶nitrogen (C/N) ratio were calculated at various times during decomposition. Fungal biomass, bacterial biomass, and the relative autoradiographic activity of bacteria were measured at the end of the study. Decomposition was significantly affected by the nitrogen content of the litter. A 55% increase in plant N increased overall weight loss and k by 50% and 40%, respectively. Furthermore, k (calculated from time course weight loss data) responded linearly to the 4 different levels of nitrogen inSpartina tissue. Fungi appear to dominate the microbial community. At the end of the experiment, fungal biomass was between 2.23 and 3.08% of the AFDW, and was calculated to contain 12 to 22% of the nitrogen in the litter. Bacterial biomass was 1/10 of the fungal biomass, and 12-17% of the bacteria were active. The total microbial biomass was not affected by increased plant nitrogen. In the course of decomposition, the organic nitrogen and carbon were highest in the effluent water in all treatments during the first 8 days. The respiration rate (CO2 evolution) first increased to a maximum at day 18 and then decreased to a constant rate (1-2 mg C/day/g detritus). Respiration was highest in the high N litter. The C/N ratio in all treatments increased from the start to day 8, then decreased to day 20. In low N litter, C/N then increased again as a result of increased total organic nitrogen (TON) loss relative to carbon mineralization. In the high N, this was reversed.

Effect of food availability on the metabolism of the ctenophore Mnemiopsis mccradyi

Measurements of respiration and excretion for the ctenophore Mnemiopsis mccradyi Mayer at 21°C at 4 concentrations of food (copepods: primarily Acartia tonsa) showed a marked effect of food availability on the metabolic rate. Starved ctenophores respired at a rate of 6.0 μg-at O2 h-1 g-1 dry wt, and excreted at 0.42 μg-at N−NH4 h-1 g-1 dry wt. Ctenophores fed at 500 prey l-1 respired at a rate of 26.4 μg-at O2 h-1 g-1 dry wt and excreted at a rate of 1.8 μg-at N−NH4 h-1 g-1 dry wt. Freshly collected ctenophores from Biscayne Bay, Florida, during November 1979, had intermediate metabolic rates. The atomic O:N ratio (based on ammonium alone) averaged 12:1 and was fairly constant for all conditions. The release of organic nitrogen was approximately equal to ammonium excretion, and urea was about 10% of the total. The elemental composition of carbon and nitrogen as a percentage of the dry weight varied directly with food availability. Total nitrogen turnover ranged from 8% per day for starved ctenophores, to 19% per day for the highest food concentration. Carbon turnover rates were higher. M. mccradyi exhibited a clear shift upwards in their excretion rate in response to the time they had been feeding, reaching a maximum after about 2h. When ctenophores were removed from food, the metabolic rate declined exponentially with time, the most rapid change occurring in the first few hours of starvation. Comparisons of carefully measured metabolic rates for freshly collected ctenophores with experimental results such as these may help to establish the nutritional state and extent of food limitation in situ.