Aerobic and Anoxic Growth and Nitrate Removal Capacity of a Marine Denitrifying Bacterium Isolated from a Recirculation Aquaculture System

Bacterial biofilters used in marine recirculation aquaculture systems need improvements to enhance nitrogen removal efficiency. Relatively little is known about biofilter autochthonous population structure and function. The present study was aimed at isolating and characterizing an autochthonous denitrifying bacterium from a marine biofilter installed at a recirculation aquaculture system. Colonization of four different media in a marine fish farm was followed by isolation of various denitrifying strains and molecular classification of the most promising one, strain T2, as a novel member of the Pseudomonas fluorescens cluster. This strain exhibits high metabolic versatility regarding N and C source utilization and environmental conditions for growth. It removed nitrate through aerobic assimilatory metabolism at a specific rate of 116.2 mg NO₃-N g dw⁻¹ h⁻¹. Dissimilatory NO₃-N removal was observed under oxic conditions at a limited rate, where transient NO₂-N formed represented 22% (0.17 mg L⁻¹) of the maximum transient NO₂-N observed under anoxic conditions. Dissimilatory NO₃-N removal under anoxic conditions occurred at a specific rate of 53.5 mg NO₃-N g dw⁻¹ h⁻¹. The isolated denitrifying strain was able to colonize different materials, such as granular activated carbon (GAC), Filtralite and Bioflow plastic rings, which allow the development of a prototype bioreactor for strain characterization under dynamic conditions and mimicking fish-farm operating conditions.     

Screening and characterization of an aerobic nitrifying-denitrifying bacterium from activated sludge

Taking advantage of the good biocompatibility and high efficiency of nitrogen removal with microbes, nitrifying and denitrifying bacteria, are becoming increasingly more widely used for wastewater treatment and prevention of eutrophication. In this research, an aerobic nitrifying-denitrifying bacterium was successfully screened from activated sludge and identified as Pseudomonas sp. (CCTCC No M2010209) by the 16S rDNA sequence. The activity verification confirmed its nitrifying-denitrifying capability of removing ammonium, nitrate and nitrite nitrogen. The types of carbon sources and carbon-nitrogen ratio greatly influenced the removal efficiency of NH₄ ⁺-N and NO₃ ⁻-N. When the initial concentrations of NH₄ ⁺-N and NO₃ ⁻-N in synthetic wastewater were less than 70 and 50 mg/L, the nitrogen removal rates reached 94 and 90% in 9 h, respectively. Preliminary comparisons of nitrogen removal capacity between this isolate and other commercial preparations in the treatment of synthetic wastewater revealed its promising potential to be used in the actual wastewater treatment.     

Identification of novel denitrifying bacteria Stenotrophomonas sp. ZZ15 and Oceanimonas sp. YC13 and application for removal of nitrate from industrial wastewater

Two novel denitrifying bacteria were successfully isolated from industrial wastewater and soil samples. Using morphological, biochemical/biophysical and 16S rRNA gene analyses, these two bacteria were identified as Stenotrophomonas sp. ZZ15 and Oceanimonas sp. YC13, respectively. Both of these two bacteria showed efficient NO₃ ⁻-N removing abilities under a semi-anaerobic condition without obvious accumulation of NO₂ ⁻-N, N₂O-N and NH₄ ⁺-N. NO₃ ⁻-N removal from paper mill wastewater was also successful by treatments with either a denitrifier or an immobilization method. Therefore, this study provides valuable denitrifying bacteria in biotreatment of industrial wastewater and other environmental pollution caused by NO₃ ⁻/NO₂ ⁻.     

Hillslope Nutrient Dynamics Following Upland Riparian Vegetation Disturbance

We investigated the effects of removing near-stream Rhododendron and of the natural blowdown of canopy trees on nutrient export to streams in the southern Appalachians. Transects were instrumented on adjacent hillslopes in a first-order watershed at the Coweeta Hydrologic Laboratory (35°03′N, 83°25′W). Dissolved organic carbon (DOC), K⁺, Na⁺, Ca²⁺, Mg²⁺, NO₃ ⁻-N, NH₄ ⁺-N, PO₄ ³⁻-P, and SO₄ ²⁻ were measured for 2 years prior to disturbance. In August 1995, riparian Rhododendron on one hillslope was cut, removing 30% of total woody biomass. In October 1995, Hurricane Opal uprooted nine canopy trees on the other hillslope, downing 81% of the total woody biomass. Over the 3 years following the disturbance, soilwater concentrations of NO₃ ⁻-N tripled on the cut hillslope. There were also small changes in soilwater DOC, SO₄ ²⁻, Ca²⁺, and Mg²⁺. However, no significant changes occurred in groundwater nutrient concentrations following Rhododendron removal. In contrast, soilwater NO₃ ⁻-N on the storm-affected hillslope showed persistent 500-fold increases, groundwater NO₃ ⁻-N increased four fold, and streamwater NO₃ ⁻-N doubled. Significant changes also occurred in soilwater pH, DOC, SO₄ ²⁻, Ca²⁺, and Mg²⁺. There were no significant changes in microbial immobilization of soil nutrients or water outflow on the storm-affected hillslope. Our results suggest that Rhododendron thickets play a relatively minor role in controlling nutrient export to headwater streams. They further suggest that nutrient uptake by canopy trees is a key control on NO₃ ⁻-N export in upland riparian zones, and that disruption of the root–soil connection in canopy trees via uprooting promotes significant nutrient loss to streams. Received 30 January 2001; accepted 25 July 2002.     

Spatial and Temporal Variability in Sediment Denitrification Within an Agriculturally Influenced Reservoir

Reservoirs are intrinsically linked to the rivers that feed them, creating a river–reservoir continuum in which water and sediment inputs are a function of the surrounding watershed land use. We examined the spatial and temporal variability of sediment denitrification rates by sampling longitudinally along an agriculturally influenced river–reservoir continuum monthly for 13 months. Sediment denitrification rates ranged from 0 to 63 μg N₂O g ash free dry mass of sediments (AFDM)⁻¹ h⁻¹ or 0–2.7 μg N₂O g dry mass of sediments (DM)⁻¹ h⁻¹ at reservoir sites, vs. 0–12 μg N₂O gAFDM⁻¹ h⁻¹ or 0–0.27 μg N₂O gDM⁻¹ h⁻¹ at riverine sites. Temporally, highest denitrification activity traveled through the reservoir from upper reservoir sites to the dam, following the load of high nitrate (NO₃⁻-N) water associated with spring runoff. Annual mean sediment denitrification rates at different reservoir sites were consistently higher than at riverine sites, yet significant relationships among theses sites differed when denitrification rates were expressed per gDM vs. per gAFDM. There was a significant positive relationship between sediment denitrification rates and NO₃⁻-N concentration up to a threshold of 0.88 mg NO₃⁻ -N l⁻¹, above which it appeared NO₃⁻-N was no longer limiting. Denitrification assays were amended seasonally with NO₃⁻-N and an organic carbon source (glucose) to determine nutrient limitation of sediment denitrification. While organic carbon never limited sediment denitrification, all sites were significantly limited by NO₃⁻-N during fall and winter when ambient NO ₃⁻-N was low.     

Bioassay Method in Evaluation of Trophic Conditions and Nutrient Limitation in the Danube Wetland Waters (1388–1426 r. km)

The algal growth potential (AGP) in water samples of the Danube wetland waters (1388–1426 r. km) as well as the effect of nitrogen (in final concentration of 0.16 g l⁻¹) and phosphorus enrichment (in final concentration of 0.02 g l⁻¹) on the AGP was investigated by miniaturized bioassay method. Values of the total biomass of Chlorella kessleri up to the 14th day of incubation were suitable for the evaluation of trophic conditions according to the classification of AGP. On the basis of the AGP results, trophic conditions in 55% of the samples were oligo-mesotrophic and in 46% of the samples meso-eutrophic. A statistically significant correlation (r = 0.34) was established between the AGP of C. kessleri in original water samples and NO₃ concentrations in situ. The TN/TP ratio in the wetland waters indicated a greater limitation due to nitrogen than phosphorous. Significantly lower TSI_TN than TSI_SD, TSI_Chla and TSI_TP indicated nitrogen limited conditions. In order to quantify established nutrient limitation by the bioassay method, the effect of added N and P concentrations on the growth rate of C. kessleri was expressed as the degree of nutrient limitation (Δr d⁻¹) during 7 days of incubation. In the Danube wetland waters only N limitation was established in June and July; N and P limitation in May and September while in August and October 2003 neither of the tested nutrients were limiting. From May to October 2003 the significantly highest degree of nitrogen (Δr = 0.736 d⁻¹) and phosphorus limitation (Δr = 0.474 d⁻¹) was determined in Lake Sakadaš.     

Simultaneous nutrients and carbon removal during pretreated swine slurry degradation in a tubular biofilm photobioreactor

The biodegradation potential of an innovative enclosed tubular biofilm photobioreactor inoculated with a Chlorella sorokiniana strain and an acclimated activated sludge consortium was evaluated under continuous illumination and increasing pretreated (centrifuged) swine slurry loading rates. This photobioreactor configuration provided simultaneous and efficient carbon, nitrogen, and phosphorous treatment in a single-stage process at sustained nitrogen and phosphorous removals efficiencies ranging from 94% to 100% and 70–90%, respectively. Maximum total organic carbon (TOC), NH₄ ⁺, and PO₄ ³⁻ removal rates of 80 ± 5 g C m_r ⁻³ day⁻¹, 89 ± 5 g N m_r ⁻³ day⁻¹, and 13 ± 3 g P m_r ⁻³ day⁻¹, respectively, were recorded at the highest swine slurry loadings (TOC of 1,247 ± 62 mg L⁻¹, N–NH₄ ⁺ of 656 ± 37 mg L⁻¹, P–PO₄ ³⁺ of 117 ± 19 mg L⁻¹, and 7 days of hydraulic retention time). The unusual substrates diffusional pathways established within the phototrophic biofilm (photosynthetic O₂ and TOC/NH₄ ⁺ diffusing from opposite sides of the biofilm) allowed both the occurrence of a simultaneous denitrification/nitrification process at the highest swine slurry loading rate and the protection of microalgae from any potential inhibitory effect mediated by the combination of high pH and high NH₃ concentrations. In addition, this biofilm-based photobioreactor supported efficient biomass retention (>92% of the biomass generated during the pretreated swine slurry biodegradation).     

Nitrate removal in a packed bed reactor using volatile fatty acids from anaerobic acidogenesis of food wastes

A packed bed reactor (PBR) was fed with nitrate containing synthetic wastewater or effluent from a sequencing batch reactor used for nitrification. The C source introduced into the PBR consisted of volatile fatty acids (VFAs) produced from anaerobic acidogenesis of food wastes. When nitrate loading rates ranged from 0.50 to 1.01 kg N/m³·d, the PBR exhibited 100∼98.8% NO₃ ⁻-N removal efficiencies and nitrite concentrations in the effluent ranged from 0 to 0.6 NO₂ ⁻-N mg/L. When the PBR was further investigated to determine nitrate removal activity along the bed height using a nitrate loading rate less than 1.01 kg N/m³·d, 100% nitrate removal efficiency was observed. Approximately 83.2% nitrate removal efficiency was observed in the lower 50% of the packed-bed height. When reactor performance at a C/N ratio of 4 and a C/N ratio of 5 was compared, the PBR showed better removal efficiency (96.5%) of nitrate and less nitrite concentration in the effluent at the C/N ratio of 5. VFAs were found to be a good alternative to methanol as a carbon source for denitrification of a municipal wastewater containing 40 mg-N/L.     

Removal of <Emphasis Type="Italic">p</Emphasis>-chlorophenol by the marine microalga <Emphasis Type="Italic">Tetraselmis marina</Emphasis>

The ability of Tetraselmis marina, a green coastal microalga, to remove chlorophenols under photoautotrophic conditions was investigated. T.marina was able to grow in the presence of 20 mg L⁻¹ of the phenolic compounds tested. The EC₅₀ (growth rate) value of p-chlorophenol (p-CP) to T.marina was found to be 25.5 mg L⁻¹. The microalga was able to remove chlorophenols, showing higher efficiency for p-CP. The effect of photoregime and NaHCO₃ concentration on p-CP removal was investigated. Under continuous illumination with 1 g L⁻¹ NaHCO₃ initial concentration T.marina removed 65% of 20 mg L⁻¹ in a 10-day cultivation period.     

Bioaugmented sulfur-oxidizing denitrification system with <Emphasis Type="Italic">Alcaligenes defragrans</Emphasis> B21 for high nitrate containing wastewater treatment

The performance of enriched sludge augmented with the B21 strain of Alcaligenes defragrans was compared with that of enriched sludge, as well as with pure Alcaligenes defragrans B21, in the context of a sulfur-oxidizing denitrification (SOD) process. In synthetic wastewater treatment containing 100–1,000 mg NO₃⁻-N/L, the single strain-seeded system exhibited superior performance, featuring higher efficiency and a shorter startup period, provided nitrate loading rate was less than 0.2 kg NO₃⁻-N/m³ per day. At nitrate loading rate of more than 0.5 kg NO₃⁻-N/m³ per day, the bioaugmented sludge system showed higher resistance to shock loading than two other systems. However, no advantage of the bioaugmented system over the enriched sludge system without B21 strain was observed in overall efficiency of denitrification. Both the bioaugmented sludge and enriched sludge systems obtained stable denitrification performance of more than 80% at nitrate loading rate of up to 2 kg NO₃⁻-N/m³ per day.     

Characteristics of net ecosystem carbon dioxide exchange (NEE) from August to October of Alpine meadow on the Tibetan Plateau, China

The Alpine meadow is one of the vegetation types widely distributed on the Tibetan Plateau in China with an area of about 1.2 million square kilometers. The Damxung rangeland station, located in the hinterland of the Tibetan Plateau, is covered with an typical vegetation. The continuous carbon flux data (from August to middle October, 2003) measured with the open-path eddy covariance system was used to analyze the diurnal variation pattern of net ecosystem carbon dioxide exchange (NEE) and its relationship with the environmental factors, such as photosynthetically active radiation (PAR), precipitation, and temperature. Results showed that NEE presented obvious diurnal variation pattern with single-peak of diurnal maximum carbon assimilation at 11: 00–12: 00 (local time) with an average of −0.268 mg CO₂·m⁻²·s⁻¹, i.e., −6.08 μmol CO₂·m⁻²·s⁻¹. During the daytime, NEE fitted fairly well with PAR in a rectangular hyperbola function with the apparent quantum yield (0.020 3 μmol CO₂ μmol⁻¹ PAR) and maximum ecosystem assimilation (9.741 1 μmol CO₂·m⁻²·s⁻¹). During the night-time, NEE showed a good exponential relation with the soil temperature at 5 cm depth. __________ Translated from Acta Ecologica Sinica 2005, 25(8): 1948–1952 [译自: 生态学报, 2005, 25(8): 1948–1952]     

Carbon and hydrogen isotope fractionation under continuous light: implications for paleoenvironmental interpretations of the High Arctic during Paleogene warming

The effect of low intensity continuous light, e.g., in the High Arctic summer, on plant carbon and hydrogen isotope fractionations is unknown. We conducted greenhouse experiments to test the impact of light quantity and duration on both carbon and hydrogen isotope compositions of three deciduous conifers whose fossil counterparts were components of Paleogene Arctic floras: Metasequoia glyptostroboides, Taxodium distichum, and Larix laricina. We found that plant leaf bulk carbon isotopic values of the examined species were 1.75–4.63‰ more negative under continuous light (CL) than under diurnal light (DL). Hydrogen isotope values of leaf n-alkanes under continuous light conditions revealed a D-enriched hydrogen isotope composition of up to 40‰ higher than in diurnal light conditions. The isotope offsets between the two light regimes is explained by a higher ratio of intercellular to atmospheric CO₂ concentration (C _i/C _a) and more water loss for plants under continuous light conditions during a 24-h transpiration cycle. Apparent hydrogen isotope fractionations between source water and individual lipids (ε_{lipid–water}) range from −62‰ (Metasequoia C₂₇ and C₂₉) to −87‰ (Larix C₂₉) in leaves under continuous light. We applied these hydrogen fractionation factors to hydrogen isotope compositions of in situ n-alkanes from well-preserved Paleogene deciduous conifer fossils from the Arctic region to estimate the δD value in ancient precipitation. Precipitation in the summer growing season yielded a δD of −186‰ for late Paleocene, −157‰ for early middle Eocene, and −182‰ for late middle Eocene. We propose that high-latitude summer precipitation in this region was supplemented by moisture derived from regionally recycled transpiration of the polar forests that grew during the Paleogene warming.     

Investigation of lipid production by nitrogen-starved <Emphasis Type="Italic">Parachlorella kessleri</Emphasis> under continuous illumination and day/night cycles for biodiesel application

This study aims to investigate the triacylglycerol (TAG) productivity of Parachlorella kessleri grown under continuous illumination and to investigate its metabolism in simulated day/night cycles in order to estimate the feasibility of a large-scale production in outdoor solar photobioreactors. The strain was chosen for its ability to accumulate large amounts of triacylglycerol during nitrogen starvation. Several protocols of nitrogen starvation were tested in continuous illumination as well as in simulated day/night cycles. Sudden and progressive nitrogen starvation conditions have enhanced the TAG concentration and productivity of P. kessleri reaching up to 48 dry wt% and 4.4 × 10^?3 kg m^?2 day^?1, respectively. Microalgal cell metabolism was significantly affected by the day/night illumination cycles. The energy-rich compounds (TAGs and carbohydrates) were accumulated by P. kessleri during the photoperiods and partly consumed during the dark to sustain the microalgae vitality. This TAG oxidation ultimately led to a 26% decrease in TAG productivity in cultures exposed to day/night cycles compared to ones exposed to continuous illumination of equal 24-h average photon flux density. The results can dictate the optimal time for harvesting cells for recovering the largest amount of TAGs.     

Nitrate removal with bacterial cells attached to quartz sand and zeolite from salty wastewaters

A mixed bacterial culture was acclimated to the removal of high nitrate-N concentrations (100–750 mg NO₃ ⁻-N L⁻¹) from salty wastewaters. The experiments were carried out under anoxic conditions in the presence of 0.5, 1.5 and 3% (w/v) NaCl at different temperatures. The acclimated mixed bacterial culture was attached to quartz sand and zeolite. Denitrification was monitored in a continuous-flow bioreactor at different hydraulic retention times (HRT). Nitrate removal with cells attached to quartz sand and zeolite was completed at HRT of 167 h and 25 h respectively. Then brine denitrification with bacterial cells attached to zeolite was monitored for 85 days. Under the increased nitrate loading rate, nitrate removal was above 90%. Furthermore, during denitrification, not more than 0.5 mg NO₂ ⁻-N L⁻¹ could be produced. It can be concluded that nitrate removal with the cells attached to zeolite is economically and operationally a promising solution to denitrification of brine wastewaters.     

Landscape Controls on Organic and Inorganic Nitrogen Leaching across an Alpine/Subalpine Ecotone,Green Lakes Valley,Colorado Front Range

Here we report measurements of organic and inorganic nitrogen (N) fluxes from the high-elevation Green Lakes Valley catchment in the Colorado Front Range for two snowmelt seasons (1998 and 1999). Surface water and soil samples were collected along an elevational gradient extending from the lightly vegetated alpine to the forested subalpine to assess how changes in land cover and basin area affect yields and concentrations of ammonium-N (NH₄-N), nitrate-N (NO₃-N), dissolved organic N (DON), and particulate organic N (PON). Streamwater yields of NO₃-N decreased downstream from 4.3 kg ha⁻¹ in the alpine to 0.75 kg ha⁻¹ at treeline, while yields of DON were much less variable (0.40–0.34 kg ha⁻¹). Yields of NH₄-N and PON were low and showed little variation with basin area. NO₃-N accounted for 40%–90% of total N along the sample transect and was the dominant form of N at all but the lowest elevation site. Concentrations of DON ranged from approximately 10% of total N in the alpine to 45% in the subalpine. For all sites, volume-weighted mean concentrations of total dissolved nitrogen (TDN) were significantly related to the DIN:DON ratio (R ² = 0.81, P < 0.001) Concentrations of NO₃-N were significantly higher at forested sites that received streamflow from the lightly vegetated alpine reaches of the catchment than in a control catchment that was entirely subalpine forest, suggesting that the alpine may subsidize downstream forested systems with inorganic N. KCl-extractable inorganic N and microbial biomass N showed no relationship to changes in soil properties and vegetative cover moving downstream in catchment. In contrast, soil carbon–nitrogen (C:N) ratios increased with increasing vegetative cover in catchment and were significantly higher in the subalpine compared to the alpine (P < 0.0001) Soil C:N ratios along the sample transect explained 78% of the variation in dissolved organic carbon (DOC) concentrations and 70% of the variation in DON concentrations. These findings suggest that DON is an important vector for N loss in high-elevation ecosystems and that streamwater losses of DON are at least partially dependent on catchment soil organic matter stoichiometry. Received 26 July 2001; accepted 6 May 2002.     

Effects of Organic Resource Quality on Soil Profile N Dynamics and Maize Yields on Sandy Soils in Zimbabwe

Optimising the use efficiency of nitrogen (N) derived from different quality organic resources and mineral fertilizers on sandy soils with <100 g clay kg⁻¹ is a major challenge for smallholder farmers in Southern Africa. The dominant sandy soils have a poor capacity to store and supply crop nutrients due to low organic matter contents and inherent infertility. A study was conducted in Zimbabwe to determine the differential N supply effects of different quality and quantities of organic nutrient sources on maize productivity. Crotalaria juncea L., Calliandra calothyrsus Meissn., cattle manure, maize (Zea mays L.) stover and Pinus patula Schiede & Schltdl. & Cham. sawdust which represented high to low quality materials respectively, were each incorporated into soil at 1.2 and 4 t C ha⁻¹ at Makoholi Experiment Station (rainfall: 450–650 mm yr⁻¹) and tested against a sole mineral N fertilizer and control treatments. In a separate experiment conducted in farmers’ fields under different rainfall zones of Zimuto (450–650 mm yr⁻¹), Chinyika (650–750 mm yr⁻¹) and Chikwaka (>750 mm yr⁻¹), commonly available organic materials, including manure and composted miombo leaf litter, applied in varying amounts by farmers were evaluated. Nitrogen release patterns were consistent with differences in resource quality. At 3 weeks after incorporation into soil at the onset of the rains, C. juncea and C. calothyrsus had released as high as 24% and 13% of added N respectively, compared with no more than 5–6% for the rest of the amended treatments. Most of the N released was lost through leaching as evidenced by progressive movement of NO₃⁻-N bulges beyond maize rooting depth following major rainfall events. Maize yields were significantly related to the size of profile mineral N fluxes, with the best linear relationship (R² = 0.86) obtained with N available in the top 30 cm of soil at maize flowering. High grain yields of ~3 t ha⁻¹ were only achieved with C. juncea applied at 4 t C ha⁻¹, which also had highest NO₃⁻-N leaching losses. Conversely, the same application rate increased N immobilization by 30% and 42% under maize stover and sawdust, respectively, relative to the control. Results from farmers’ fields showed that organic resources traditionally used on smallholder farms are invariably of low quality relative to C. juncea and C. calothyrsus. However, they exhibited shorter N immobilization effects than was shown for maize stover and sawdust at Makoholi, suggesting that pre-application treatments, such as composting, employed by farmers enhance seasonal N benefits from these materials. Maize yields increased linearly with total N added in these resources in combination with N fertilizer, justifying the high organic matter loading strategy (e.g. >20 t ha⁻¹ for manure, fresh litter and composted litter) used by farmers who often achieve high crop yields on such coarse sandy soils in Zimbabwe.     

The effects of drying on sediment nitrogen content in a Mediterranean intermittent stream: a microcosms study

Mediterranean climates predispose aquatic systems to both flood and drought periods, therefore, stream sediments may be exposed to desiccation periods. Changes in oxygen concentrations and sediment water content influence the biotic processes implicated in nitrogen dynamics. The objectives of this study were to identify (1) the changes of inorganic nitrogen in stream sediments during the transition from wet to dry conditions, and (2) the underlying processes in N dynamics and its regulation. Extractable sediment NO₃ ⁻-N and NH₄ ⁺-N, organic matter and extractable organic carbon content were assessed during natural desiccation in microcosms with sediments from an intermittent Mediterranean stream. In agreement with our initial hypothesis, our results showed how the NO₃ ⁻-N content of the sediment was enhanced during the first 10 days of sediment drying, whereas NH₄ ⁺-N was lost by 14 days post-drying. During the first 10 days, sediment desiccation seemed to stimulate the net N-mineralization and net nitrification from sediments. Afterwards, the extractable NO₃ ⁻-N concentration sharply dropped, which may be attributed to lower ammonium-oxidation rates as ammonium and organic matter are depleted, and to an increase in NO₃ ⁻-N consumption by microbial populations. Denitrification was inhibited, with a significant decrease as % water-filled pore space lowered. We hypothesize that the sediment inorganic N content enhanced during sediment desiccation could be released as part of the N pulse observed after sediment rewetting. However, the stream N availability after rewetting dried sediments would differ depending on desiccation period duration.     

Natural 15N abundance in two nitrogen saturated forest ecosystems

Natural ¹⁵N abundance values were measured in needles, twigs, wood, soil, bulk precipitation, throughfall and soil water in a Douglas fir (Pseudotsuga menziesii (Mirb.) and a Scots pine (Pinus sylvestris L.) stand receiving high loads of nitrogen in throughfall (>50 kg N ha⁻¹ year⁻¹). In the Douglas fir stand δ¹⁵N values of the vegetation ranged between −5.7 and −4.2‰ with little variation between different compartments. The vegetation of the Scots pine stand was less depleted in ¹⁵N and varied from −3.3 to −1.2‰δ¹⁵N. At both sites δ¹⁵N values increased with soil depth, from −5.7‰ and −1.2‰ in the organic layer to +4.1‰ and +4.7‰ at 70 cm soil depth in the Douglas fir and Scots pine stand, respectively. The δ¹⁵N values of inorganic nitrogen in bulk precipitation showed a seasonal variation with a mean in NH₄ ⁺-N of −0.6‰ at the Douglas fir stand and +10.8‰ at the Scots pine stand. In soil water below the organic layer NH₄ ⁺-N was enriched and NO₃ ⁻-N depleted in ¹⁵N, which was interpreted as being caused by isotope fractionation accompanying high nitrification rates in the organic layers. Mean δ¹⁵N values of NH₄ ⁺ and NO₃ ⁻ were very similar in the drainage water at 90 cm soil depth at both sites (−7.1 to −3.8‰). A dynamic N cycling model was used to test the sensitivity of the natural abundance values for the amount of N deposition, the ¹⁵N ratio of atmospheric N deposited and for the intrinsic isotope discrimination factors associated with N transformation processes. Simulated δ¹⁵N values for the N saturated ecosystems appeared particularly sensitive to the ¹⁵N ratio of atmospheric N inputs and discrimination factors during nitrification and mineralization. The N-saturated coniferous forest ecosystems studied were not characterized by elevated natural ¹⁵N abundance values. The results indicated that the natural ¹⁵N abundance values can only be used as indicators for the stage of nitrogen saturation of an ecosystem if the δ¹⁵N values of the deposited N and isotope fractionation factors are taken into consideration. Combining dynamic isotope models and natural ¹⁵N abundance values seems a promising technique for interpreting natural ¹⁵N abundance values found in these forest ecosystems. Received: 5 May 1996 / Accepted: 10 April 1997     

Photosynthetic acclimation to photon irradiance and its relation to chlorophyll fluorescence and carbon assimilation in the halotolerant green alga Dunaliella viridis

This work describes the long-term acclimation of the halotolerant microalga Dunaliella viridis to different photon irradiance, ranging from darkness to 1500 μmol m⁻² s⁻¹. In order to assess the effects of long-term photoinhibition, changes in oxygen production rate, pigment composition, xanthophyll cycle and in vivo chlorophyll fluorescence using the saturating pulse method were measured. Growth rate was maximal at intermediate irradiance (250 and 700 μmol m⁻² s⁻¹). The increase in growth irradiance from 700 to 1500 μmol m⁻² s⁻¹ did not lead to further significant changes in pigment composition or EPS, indicating saturation in the pigment response to high light. Changes in Photosystem II optimum quantum yield (F_v/F_m) evidenced photoinhibition at 700 and especially at 1500 μmol m⁻² s⁻¹. The relation between photosynthetic electron flow rate and photosyntetic O₂ evolution was linear for cultures in darkness shifting to curvilinear as growth irradiance increased, suggesting the interference of the energy dissipation processes in oxygen evolution. Carbon assimilation efficiencies were studied in relation to changes in growth rate, internal carbon and nitrogen composition, and organic carbon released to the external medium. All illuminated cultures showed a high capability to maintain a C:N ratio between 6 and 7. The percentage of organic carbon released to the external medium increased to its maximum under high irradiance (1500 μmol m⁻² s⁻¹). These results suggest that the release of organic carbon could act as a secondary dissipation process when the xanthophyll cycle is saturated. This revised version was published online in June 2006 with corrections to the Cover Date.     

Characterization of microbial community and kinetics for spent sulfidic caustic applied autotrophic denitrification

Spent sulfidic caustic was applied to sulfur utilizing autotrophic denitrification as the simultaneous source of electron donor and alkalinity. The two experiment set-up of upflow anoxic hybrid growth reactor (UAHGR) and upflow anoxic suspended growth reactor (UASGR) was adopted and nitrate removals were similar in both reactors. Approximately 90% of the initial nitrate was denitrified at nitrate loading rate of 0.15∼0.40 kgNO₃ ⁻/m³·d. The experimental stoichiometric ratio of sulfate production to nitrate removal was ranged from 1.5 to 2.1 mgSO₄ ²⁻/mgNO₃ ⁻. During the operation period, denaturing gradient gel electrophoresis (DGGE) analysis of polymerase chain reaction (PCR)-amplified 16S rDNA fragments for the sludge sample of both reactors showed the change of microbial communities. Thiobacillus denitrificans-like microorganism occupied 28.5% (18 clones) of the 63 clones by cloning the PCR products from the sludge sample of UAHGR. Acidovorax avenae, which can reduce nitrate to nitrogen gas while oxidizing phenol (heterotrophic denitrifier), was also found in 7 clones (11.1%). Although an organic carbon source was not added to the medium, a microorganism (Kaistella koreensis) capable of oxidizing organic compounds was found in 7 clones (11.1%). Therefore, the microbial community of spent sulfidic caustic applied autotrophic denitrification process well corresponds to the substrate components of spent sulfidic caustic. Through the batch cultivation of microorganisms in UAHGR, the microbial kinetic coefficients of spent sulfidic caustic applied autotrophic denitrification were estimated to be μ _max = 0.097 h⁻¹, k _d = 0.0021 h⁻¹, K _s = 200 mgNO₃ ⁻/L, and Y = 0.31 mgMLVSS/mgNO₃ ⁻.