Abiotic reaction of nitrite with dissolved organic carbon? Testing the Ferrous Wheel Hypothesis

The Ferrous Wheel Hypothesis (Davidson et al. 2003) postulates the abiotic formation of dissolved organic N (DON) in forest floors, by the fast reaction of NO₂ ⁻ with dissolved organic C (DOC). We investigated the abiotic reaction of NO₂ ⁻ with dissolved organic matter extracted from six different forest floors under oxic conditions. Solutions differed in DOC concentrations (15–60 mg L⁻¹), NO₂ ⁻ concentrations (0, 2, 20 mg NO₂ ⁻-N L⁻¹) and DOC/DON ratio (13.4–25.4). Concentrations of added NO₂ ⁻ never decreased within 60 min, therefore, no DON formation from added NO₂ ⁻ took place in any of the samples. Our results suggest that the reaction of NO₂ ⁻ with natural DOC in forest floors is rather unlikely.     

Seasonal variation in nutrient limitation of microbial biofilms colonizing organic and inorganic substrata in streams

Humans have increased the availability of nutrients including nitrogen and phosphorus worldwide; therefore, understanding how microbes process nutrients is critical for environmental conservation. We examined nutrient limitation of biofilms colonizing inorganic (fritted glass) and organic (cellulose sponge) substrata in spring, summer, and autumn in three streams in Michigan, USA. Biofilms were enriched with nitrate (NO₃ ⁻), phosphate (PO₄ ³⁻), ammonium (NH₄ ⁺), NO₃ ⁻ + PO₄ ³⁻, NH₄ ⁺ + PO₄ ³⁻, or none (control). We quantified biofilm structure and function as chlorophyll a (i.e., primary producer biomass) and community respiration on all substrata. In one stream, we characterized bacterial and fungal communities on cellulose in autumn using clone library sequencing and denaturing gradient gel electrophoresis to determine if community structure was linked to nutrient limitation status. Despite oligotrophic conditions, primary producer biomass was infrequently nutrient limited. In contrast, respiration on organic substrata was frequently limited by N + P combinations. We found no difference between biofilm response to NH₄ ⁺ versus NO₃ ⁻ enrichment, although the response to both N-species was positively related to water column PO₄ ³⁻ concentrations and temperature. Molecular analysis for fungal community composition suggested no relationship to nutrient limitation, but the dominant members of the bacterial community on cellulose were different on NO₃ ⁻, PO₄³, and NO₃ ⁻ + PO₄ ³⁻ treatments relative to control, NH₄ ⁺, and NH₄ ⁺ + PO₄ ³⁻ treatments, which matched patterns for biofilm respiration rates from each treatment. Our results show discrete patterns of nutrient limitation dependent upon substratum type and season, and imply changes in bacterial community structure and function may be linked following nutrient enrichment in streams.     

Organic matter release by dominant hermatypic corals of the Northern Red Sea

Particulate organic matter (POM) and dissolved organic carbon (DOC) release by six dominant hermatypic coral genera (Acropora, Fungia, Goniastrea, Millepora, Pocillopora and Stylophora) were measured under undisturbed conditions by laboratory incubations during four seasonal expeditions to the Northern Red Sea. In addition, the influence of environmental factors (water temperature, light availability and ambient inorganic nutrient concentrations) was evaluated. Particulate organic carbon (POC) and particulate nitrogen (PN) release were always detectable and genus-specific, with Stylophora releasing most POM (6.5 mg POC and 0.5 mg PN m⁻² coral surface area h⁻¹) during all seasons. The fire coral Millepora released significantly less POM (0.3 mg POC and 0.04 mg PN m⁻² coral surface area h⁻¹) than all investigated anthozoan genera. The average POC:PN ratio of POM released by all coral genera was 12 ± 1, indicating high carbon/low nitrogen content of coral-derived organic matter. POM release showed little seasonal variation, but average values of POC and PN release rates correlated with water temperature, light availability and ambient nitrate concentrations. DOC net release and elevated DOC:POC ratios were detectable for Acropora, Goniastrea and Millepora, revealing maximum values for Acropora (30.7 mg DOC m⁻² coral surface area h⁻¹), whilst predominant DOC uptake was observed for Pocillopora, Fungia and Stylophora. Depth-mediated light availability influenced DOC fluxes of Acropora and Fungia, while fluctuations in water temperature and ambient inorganic nutrient concentrations showed no correlation. These comprehensive data provide an important basis for the understanding of coral reef organic matter dynamics and relevant environmental factors.     

Seasonal Variations of Dissolved Nitrogen and DOC:DON Ratios in an Intermittent Mediterranean Stream

Seasonal variations of dissolved inorganic nitrogen (DIN) (NO₃–N and NH₄–N) and dissolved organic nitrogen (DON) were determined in Fuirosos, an intermittent stream draining an unpolluted Mediterranean forested catchment (10.5 km²) in Catalonia (Spain). The influence of flow on streamwater concentrations and seasonal differences in quality and origin of dissolved organic matter, inferred from dissolved organic carbon to nitrogen ratios (DOC:DON ratios), were examined. During baseflow conditions, nitrate and ammonium had opposite behaviour, probably controlled by biological processes such as vegetation uptake and mineralization activity. DON concentrations did not have a seasonal trend. During storms, nitrate and DON increased by several times but discharge was not a good predictor of nutrient concentrations. DOC:DON ratios in streamwater were around 26, except during the months following drought when DOC:DON ratios ranged between 42 and 20 during baseflow and stormflow conditions, respectively. Annual N export during 2000–2001 was 70 kg km⁻¹ year⁻¹, of which 75% was delivered during stormflow. The relative contribution of nitrogen forms to the total annual export was 57, 35 and 8% as NO₃–N, DON and NH₄–N, respectively.     

Large Loss of Dissolved Organic Nitrogen from Nitrogen-Saturated Forests in Subtropical China

Dissolved organic nitrogen (DON) has recently been recognized as an important component of terrestrial N cycling, especially under N-limited conditions; however, the effect of increased atmospheric N deposition on DON production and loss from forest soils remains controversial. Here we report DON and dissolved organic carbon (DOC) losses from forest soils receiving very high long-term ambient atmospheric N deposition with or without additional experimental N inputs, to investigate DON biogeochemistry under N-saturated conditions. We studied an old-growth forest, a young pine forest, and a young mixed pine/broadleaf forest in subtropical southern China. All three forests have previously been shown to have high nitrate (NO₃⁻) leaching losses, with the highest loss found in the old-growth forest. We hypothesized that DON leaching loss would be forest specific and that the strongest response to experimental N input would be in the N-saturated old-growth forest. Our results showed that under ambient deposition (35–50 kg N ha⁻¹ y⁻¹ as throughfall input), DON leaching below the major rooting zone in all three forests was high (6.5–16.9 kg N ha⁻¹ y⁻¹). DON leaching increased 35–162% following 2.5 years of experimental input of 50–150 kg N ha⁻¹ y⁻¹. The fertilizer-driven increase of DON leaching comprised 4–17% of the added N. A concurrent increase in DOC loss was observed only in the pine forest, even though DOC:DON ratios declined in all three forests. Our data showed that DON accounted for 23–38% of total dissolved N in leaching, highlighting that DON could be a significant pathway of N loss from forests moving toward N saturation. The most pronounced N treatment effect on DON fluxes was not found in the old-growth forest that had the highest DON loss under ambient conditions. DON leaching was highly correlated with NO₃⁻ leaching in all three forests. We hypothesize that abiotic incorporation of excess NO₃⁻ (through chemically reactive NO₂⁻) into soil organic matter and the consequent production of N-enriched dissolved organic matter is a major mechanism for the consistent and large DON loss in the N-saturated subtropical forests of southern China. Dr. YT Fang performed research, analyzed data, and wrote the paper; Prof. WX Zhu participated in the initial experimental design, analyzed data, and took part in writing the paper; Prof. P Gundersen conceived the study and took part in writing; Prof. JM Mo and Prof. GY Zhou conceived study; Prof. M Yoh analyzed part of the data and contributed to the development of DON model.     

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 variability in nutrient concentration and biofilm nutrient limitation in an urban watershed

Nutrient enrichment threatens river ecosystem health in urban watersheds, but the influence of urbanization on spatial variation in nutrient concentrations and nutrient limitation of biofilm activity are infrequently measured simultaneously. In summer 2009, we used synoptic sampling to measure spatial patterns of nitrate (NO₃ ⁻), ammonium (NH₄ ⁺), and soluble reactive phosphorus (SRP) concentration, flux, and instantaneous yield throughout the Bronx River watershed within New York City and adjacent suburbs. We also quantified biofilm response to addition of NO₃ ⁻, phosphate (PO₄ ³⁻), and NO₃ ⁻ + PO₄ ³⁻ on organic and inorganic surfaces in the river mainstem and tributaries. Longitudinal variation in NO₃ ⁻ was low and related to impervious surface cover across sub-watersheds, but spatial variation in NH₄ ⁺ and SRP was higher and unrelated to sub-watershed land-use. Biofilm respiration on organic surfaces was frequently limited by PO₄ ³⁻ or NO₃ ⁻ + PO₄ ³⁻, while primary production on organic and inorganic surfaces was nutrient-limited at just one site. Infrequent NO₃ ⁻ limitation and low spatial variability of NO₃ ⁻ throughout the watershed suggested saturation of biological N demand. For P, both higher biological demand and point-sources contributed to greater spatial variability. Finally, a comparison of our data to synoptic studies of forested, temperate watersheds showed lower spatial variation of N and P in urban watersheds. Reduced spatial variation in nutrients as a result of biological saturation may represent an overlooked effect of urbanization on watershed ecology, and may influence urban stream biota and downstream environments.     

Dissolved Organic Nitrogen Release and Bacterial Activity in the Upper Layers of the Atlantic Ocean

The variability of the percentage of extracellular dissolved organic nitrogen (DON) release (PER), along with the relationship between DON release and bacterioplankton activity, was examined during five oceanographic cruises, carried out in the upwelling region of the NW Iberian Peninsula, the SW Bay of Biscay, and a latitudinal transect in the Atlantic Ocean (50°N–35°S). Rates of nitrogen uptake, DON release, and bacterial production were measured at 66 stations and sampled between August 1998 and October 2000. The percentage of DON release relative to the gross uptake of ammonium (PER^NH4+) ranged from 3 to 46%, whereas that relative to total nitrogen (NH₄ ⁺ + NO₃ ⁻ + urea) gross uptake (PER^total) varied between 21 and 82%. The highest values for both PER^NH4+ and PER^total were found in oligotrophic oceanic waters (<0.25 mg chlorophyll a m⁻³). In oceanic environments, a positive relationship was found between nitrogen uptake and DON release rates, with a log–log linear regression slope significantly lower than 1, suggesting an inverse relationship between PER and gross nitrogen uptake rates. In contrast, in areas with higher productivity levels (>6 mg chlorophyll a m⁻³), such as the continental shelf off the NW Iberian Peninsula, PER held constant as nitrogen uptake increased. These results suggest the dominance of different processes controlling DON release in oceanic and neritic zones. DON release rates accounted for less than 15% of the variability observed in bacterial production rates, suggesting a weak response of bacterioplankton to phytoplankton on short time scales (hours). Furthermore, nitrogen budgets showed an excess of DON release in relation to bacterial requirements.     

Porewater Stoichiometry of Terminal Metabolic Products, Sulfate, and Dissolved Organic Carbon and Nitrogen in Estuarine Intertidal Creek-bank Sediments

Porewater equilibration samplers were used to obtain porewater inventories of inorganic nutrients (NH₄⁺, NO_x, PO₄³⁻), dissolved organic carbon (DOC) and nitrogen (DON), sulfate (SO₄²⁻), dissolved inorganic carbon (DIC), hydrogen sulfide (H₂S), chloride (Cl⁻), methane (CH₄) and reduced iron (Fe²⁺) in intertidal creek-bank sediments at eight sites in three estuarine systems over a range of salinities and seasons. Sulfate reduction (SR) rates and sediment particulate organic carbon (POC) and nitrogen (PON) were also determined at several of the sites. Four sites in the Okatee River estuary in South Carolina, two sites on Sapelo Island, Georgia and one site in White Oak Creek, Georgia appeared to be relatively pristine. The eighth site in Umbrella Creek, Georgia was directly adjacent to a small residential development employing septic systems to handle household waste. The large data set (>700 porewater profiles) offers an opportunity to assess system-scale patterns of porewater biogeochemical dynamics with an emphasis on DOC and DON distributions. SO₄²⁻ depletion (SO₄²⁻)_Dep was used as a proxy for SR, and (SO₄²⁻)_Dep patterns agreed with measured (³⁵S) patterns of SR. There were significant system-scale correlations between the inorganic products of terminal metabolism (DIC, NH₄⁺ and PO₄³⁻) and (SO₄²⁻)_Dep, and SR appeared to be the dominant terminal carbon oxidation pathway in these sediments. Porewater inventories of DIC and (SO₄²⁻)_Dep indicate a 2:1 stoichiometry across sites, and the C:N ratio of the organic matter undergoing mineralization was between 7.5 and 10. The data suggest that septic-derived dissolved organic matter with a C:N ratio below 6 fueled microbial metabolism and SR at a site with development in the upland. Seasonality was observed in the porewater inventories, but temperature alone did not adequately describe the patterns of (SO₄²⁻)_Dep, terminal metabolic products (DIC, NH₄⁺, PO₄³⁻), DOC and DON, and SR observed in this study. It appears that production and consumption of labile DOC are tightly coupled in these sediments, and that bulk DOC is likely a recalcitrant pool. Preferential hydrolysis of PON relative to POC when overall organic matter mineralization rates were high appears to drive the observed patterns in POC:PON, DOC:DON and DIC:DIN ratios. These data, along with the weak seasonal patterns of SR and organic and inorganic porewater inventories, suggest that the rate of hydrolysis limits organic matter mineralization in these intertidal creek-bank sediments.     

Biofiltering efficiency, uptake and assimilation rates of Ulva clathrata (Roth) J. Agardh (Clorophyceae) cultivated in shrimp aquaculture waste water

The growth, biofiltering efficiency and uptake rates of Ulva clathrata were studied in a series of outdoor tanks, receiving waste water directly from a shrimp (Litopenaeus vannamei) aquaculture pond, under constant aeration and two different water regimes: (1) continuous flow, with 1 volume exchange a day (VE day^-1) and (2) static regime, with 1 VE after 4 days. Water temperature, salinity, pH, dissolved inorganic nitrogen (DIN), phosphate (PO₄), chlorophyll-a (chl-a), total suspended solids (TSS), macroalgal biomass (fresh weight) and tissue nutrient assimilation were monitored over 12 days. Ulva clathrata was highly efficient in removing the main inorganic nutrients from effluent water, stripping 70–82% of the total ammonium nitrogen (TAN) and 50% PO₄ within 15 h. Reductions in control tanks were much lower (Tukey HSD, P < 0.05). After 3 days, the mean uptake rates by the seaweed biomass under continuous flow were 3.09 mg DIN g DW day⁻¹ (383 mg DIN m⁻² day⁻¹) and 0.13 mg PO₄ g DW day⁻¹ (99 mg PO₄ m⁻² day⁻¹), being significantly higher than in the static regime (Tukey HSD, P < 0.05). The chl-a decreased in seaweed tanks, suggesting that U. clathrata inhibited phytoplankton growth. Correlations between the cumulative values of DIN removed from the water and total nitrogen assimilated into the seaweed biomass (r = 0.7 and 0.8, P < 0.05), suggest that nutrient removal by U. clathrata dominated over other processes such as phytoplankton and bacterial assimilation, ammonia volatilization and nutrient precipitation.     

Phytoplankton production after the collapse of the Larsen A Ice Shelf,Antarctica

Part of the Larsen A Ice Shelf (64°15′S to 74°15′S) collapsed during January 1995. A first oceanographic and biological data set from the newly free waters was obtained during December 1996. Typical shelf waters with temperatures near and below the freezing point were found. A nutrient-rich water mass (max: PO₄ ³⁻ 1.80 μmol L⁻¹ and NO₃ ⁻ 27.64 μmol L⁻¹) was found between 70 and 200 m depth. Chlorophyll-a (Chl-a) values (max 14.24 μg L⁻¹) were high; surface oxygen saturation ranged between 86 and 148%. Diatoms of the genera Nitzschia and Navicula and the prymnesiophyte Phaeocystis sp. were the most abundant taxa found. Mean daily primary production (Pc) estimated from nutrient consumption was 14.80 ± 0.17 mgC m⁻³ day⁻¹. Pc was significantly correlated with total diatom abundance and Chl-a. Calculated ΔpCO₂ (difference of the CO₂ partial pressure between surface seawater and the atmosphere) was –30.5 μatm, which could have contributed to a net CO₂ flux from the atmosphere to the sea and suggests the area has been a CO₂ sink during the studied period. High phytoplankton biomass and production values were found in this freshly open area, suggesting its importance for biological CO₂ pumping.     

Nitrogen release from surface sand of a high energy beach along the southeastern coast of North Carolina,USA

This study examined changes in dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN) in coastal seawater after exposure to sand along a high energy beach face over an annual cycle between April 2004 and July 2005. Dissolved organic nitrogen, NO₃ ⁻, and NH₄ ⁺ were released from sand to seawater in laboratory incubation experiments clearly demonstrating that they are a potential source of N to underlying groundwater or coastal seawater. DON increases in seawater, after exposure to surface sands in laboratory experiments, were positively correlated with in situ water column DON concentrations measured at the same time as sand collection. Increase in NO₃ ⁻ and NH₄ ⁺ were not correlated with their in situ concentrations. This suggests that DON released from beach sands is relatively more recalcitrant while NO₃ ⁻ and NH₄ ⁺ are utilized rapidly in the coastal ocean. The release of N was seasonal with carbon to nitrogen ratios indicating that recent primary productivity was responsible for the largest fluxes in summer while more degraded humic material contributed to lower fluxes in winter. Fluxes of total dissolved nitrogen (DON and DIN) from surface sand (2.1 × 10⁻⁴ mol m⁻² h⁻¹) were similar to that of groundwater and more than an order of magnitude larger than rain deposition indicating the potential importance of surface sand derived nitrogen to the coastal zone with a corresponding impact on primary productivity.     

In situ release of coral mucus by <Emphasis Type="Italic">Acropora</Emphasis> and its influence on the heterotrophic bacteria

In situ mucus release by Acropora nobilis and degradation of mucus from A. nobilis and Acropora formosa, by heterotrophic bacteria were investigated at Bidong and Tioman Island, Malaysia. Mucus release rate for A. nobilis was on average 38.7 ± 35.2 mg C m⁻² h⁻¹, of which ca. 70% consisted of dissolved organic carbon (DOC) and 30% particulate organic carbon (POC). In the mucus degradation experiment, seawater-mucus mixtures were incubated and compared with control runs for 24 h. Bacterial abundance in the seawater-mucus mixture increased significantly and coincided with a decline in DOC concentration. In controls, bacteria and DOC did not significantly change. The coral mucus had a high content of inorganic phosphate. It is suggested that the coral mucus rich in DOC and phosphate can induce the high bacterial growth.     

The Status and Characteristics of Eutrophication in the Yangtze River (Changjiang) Estuary and the Adjacent East China Sea, China

Eutrophication has become increasingly serious and noxious algal blooms have been of more frequent occurrence in the Yangtze River Estuary and in the adjacent East China Sea. In 2003 and 2004, four cruises were undertaken in three zones in the estuary and in the adjacent sea to investigate nitrate (NO₃–N), ammonium (NH₄–N), nitrite (NO₂–N), soluble reactive phosphorus (SRP), dissolved reactive silica (DRSi), dissolved oxygen (DO), phytoplankton chlorophyll a (Chl a) and suspended particulate matter (SPM). The highest concentrations of DIN (NO₃–N+NH₄–N+NO₂–N), SRP and DRSi were 131.6, 1.2 and 155.6 μM, respectively. The maximum Chl a concentration was 19.5 mg m⁻³ in spring. An analysis of historical and recent data revealed that in the last 40 years, nitrate and SRP concentrations increased from 11 to 97 μM and from 0.4 to 0.95 μM, respectively. From 1963 to 2004, N:P ratios also increased from 30–40 up to 150. In parallel with the N and P enrichment, a significant increase of Chl a was detected, Chl a maximum being 20 mg m⁻³, nearly four times higher than in the 1980s. In 2004, the mean DO concentration in bottom waters was 4.35 mg l⁻¹, much lower than in the 1980s. In comparison with other estuaries, the Yangtze River Estuary was characterized by high DIN and DRSi concentrations, with low SRP concentrations. Despite the higher nutrient concentrations, Chl a concentrations were lower in the inner estuary (Zones 1 and 2) than in the adjacent sea (Zone 3). Based on nutrient availability, SPM and hydrodynamics, we assumed that in Zones 1 and 2 phytoplankton growth was suppressed by high turbidity, large tidal amplitude and short residence time. Furthermore, in Zone 3 water stratification was also an important factor that resulted in a greater phytoplankton biomass and lower DO concentrations. Due to hydrodynamics and turbidity, the open sea was unexpectedly more sensitive to nutrient enrichment and related eutrophication processes.     

Effect of nitrogen forms on growth,cell composition and N<Subscript>2</Subscript> fixation of <Emphasis Type="Italic">Cylindrospermopsis raciborskii</Emphasis> in phosphorus-limited chemostat cultures

The aim of this research was to test whether NH₄ ⁺ and NO₃ ⁻ affect the growth, P demand, cell composition and N₂ fixation of Cylindrospermopsis raciborskii under P limitation. Experiments were carried out in P-limited (200 μg l⁻¹ PO₄-P) chemostat cultures of C. raciborskii using an inflowing medium containing either 4,000 μg l⁻¹ NH₄-N, 4,000 μg l⁻¹ NO₃-N or no combined N. The results showed the cellular N:P and C:P ratios of C. raciborskii decreased towards the Redfield ratio with increasing dilution rate (D) due to the alleviation of P limitation. The cellular C:N and carotenoids:chlorophyll-a ratios also decreased with D, predominantly as a result of an increase in the chlorophyll-a and N content. The NH₄ ⁺ and NO₃ ⁻ supply reduced the P maintenance cell quota of C. raciborskii. Consequently, the biomass yield of the N₂-grown culture was significantly lower. The maximum specific growth rate of N₂-grown culture was also the lowest observed. It is suggested that these differences in growth parameters were caused by the P and energy requirement for heterocyte formation, nitrogenase synthesis and N₂ fixation. N₂ fixation was partially inhibited by NO₃ ⁻ and completely inhibited by NH₄ ⁺. It was probably repressed through the high N content of cells at high dissolved N concentrations. These results indicate that C. raciborskii is able to grow faster and maintain a higher biomass under P limitation where a sufficient supply of NH₄ ⁺ or NO₃ ⁻ is maintained. Information gained about the species-specific nutrient and pigment stoichiometry of C. raciborskii could help to access the degree of nutrient limitation in water bodies. Handling editor: Luigi Naselli-Flores     

Effect of nitrite on growth and microcystins production of <Emphasis Type="Italic">Microcystis aeruginosa</Emphasis> PCC7806

As a common pollutant, nitrite concentrations can approach 15 mg NO₂⁻-N L⁻¹ in some aquatic systems. Microcystis aeruginosa blooms are common and widespread in eutrophic freshwater bodies. In this study, M. aeruginosa was exposed to nitrite concentrations ranging from 0 to 15 mg NO₂⁻-N L⁻¹, and the responses of M. aeruginosa were investigated. The specific growth rates, maximum cell densities, light-saturated photosynthetic rates (P_m ^chla ), dark respiration rates (R_d ^chla ), and apparent photosynthetic efficiencies (α^chla ) showed a significant decline with nitrite concentrations increasing. Electrical conductivity and malondialdehyde contents investigation revealed cell membrane damage and apparent leakage of intracellular contents under high nitrite level conditions due to oxidative stress enhancement. Intracellular microcystin (MC)-LR content reached the highest value at 10 mg NO₂⁻-N L⁻¹; however, extracellular MC-LR contents showed a continuous increase until 15 mg NO₂⁻-N L⁻¹ owing to the increasing leakage of intracellular contents. These results elucidated that the high-level nitrite inhibited M. aeruginosa growth by rising oxidative stress, damaging cell membrane, and reducing photosynthesis. However, the moderate increase in nitrite concentrations promoted toxin production and release of toxin.     

Metabolic Imbalance in Coastal Vegetated (Posidonia oceanica) and Unvegetated Benthic Ecosystems

Community metabolism and dissolved organic and inorganic nutrient fluxes were assessed in impacted from fish farm discharges and reference vegetated (Posidonia oceanica) and adjacent unvegetated communities in the Aegean Sea, Greece. Both metabolism and nutrient fluxes significantly differed between impacted and reference communities, but the effect depended on community type and time of year. Net community production (NCP) in the impacted vegetated community decreased by 60%, respiration (R) by 34%, and gross primary production (GPP) by 44%. The GPP:R ratio declined more (35%) in the impacted unvegetated than in the corresponding vegetated community (15%), implying that proximity to the fish farm has a severe impact on the unvegetated community, leading to imbalanced metabolism (GPP < R) and heterotrophic (GPP:R = 0.9) conditions. Higher release of dissolved organic and inorganic carbon, nitrogen, and phosphorous was observed in the impacted vegetated community compared to the corresponding unvegetated one, implying intensification of mineralization in the seagrass community. On an annual scale, the impacted vegetated community supported increased DOC efflux by 204%, DON by 1639%, NH₄ by 122%, and NO₃ by 26%, whereas it supported release of DOP and PO₄ compared to the reference community, which removed these dissolved nutrients from the water column. The impacted unvegetated community supported an annual increase of DOC efflux by 208% and PO₄ by 42% and it released DON, NH₄, NO₃, and DOP, whereas the reference community took up these nutrients. Proximity to the fish farm altered the ecosystem state by lowering the productivity and by enhancing the nutrient release.     

Denitrification and nitrous oxide emissions from riparian forests soils exposed to prolonged nitrogen runoff

Compared to upland forests, riparian forest soils have greater potential to remove nitrate (NO₃) from agricultural runoff through denitrification. It is unclear, however, whether prolonged exposure of riparian soils to nitrogen (N) loading will affect the rate of denitrification and its end products. This research assesses the rate of denitrification and nitrous oxide (N₂O) emissions from riparian forest soils exposed to prolonged nutrient runoff from plant nurseries and compares these to similar forest soils not exposed to nutrient runoff. Nursery runoff also contains high levels of phosphate (PO₄). Since there are conflicting reports on the impact of PO₄ on the activity of denitrifying microbes, the impact of PO₄ on such activity was also investigated. Bulk and intact soil cores were collected from N-exposed and non-exposed forests to determine denitrification and N₂O emission rates, whereas denitrification potential was determined using soil slurries. Compared to the non-amended treatment, denitrification rate increased 2.7- and 3.4-fold when soil cores collected from both N-exposed and non-exposed sites were amended with 30 and 60 μg NO₃-N g⁻¹ soil, respectively. Net N₂O emissions were 1.5 and 1.7 times higher from the N-exposed sites compared to the non-exposed sites at 30 and 60 μg NO₃-N g⁻¹ soil amendment rates, respectively. Similarly, denitrification potential increased 17 times in response to addition of 15 μg NO₃-N g⁻¹ in soil slurries. The addition of PO₄ (5 μg PO₄-P g⁻¹) to soil slurries and intact cores did not affect denitrification rates. These observations suggest that prolonged N loading did not affect the denitrification potential of the riparian forest soils; however, it did result in higher N₂O emissions compared to emission rates from non-exposed forest soils.     

High rates of nitrogen fixation (acetylene reduction) on coral skeletons following bleaching mortality

Nubbins of the coral Acropora aspera were artificially bleached and nitrogen fixation (acetylene reduction) rates were measured on the developing epilithic communities. Seasonal comparisons were made between corals that died in summer of heat stress and corals that died in winter from natural cold stress. Rates of acetylene reduction from artificially bleached corals peaked at 26.66 nmol cm⁻² h⁻¹ 2 weeks after summer mortality, while rates from natural winter mortality peaked at 18.07 nmol cm⁻² h⁻¹ 12 days after coral death. Comparative rates of acetylene reduction taken from live corals and coral rubble ranged between 0.56 and 1.16 nmol cm⁻² h⁻¹, and 0.15 and 12.77 nmol cm⁻² h⁻¹, respectively. N₂-fixation rates from dead corals were up to 30 times greater than those measured on live corals. The observed increase in N₂-fixation from dead corals may increase the availability of nitrogen for use in trophic processes within the reef for an extended period following the initial mortality event. If the spatial scale over which coral mortality has occurred in past thermal bleaching events is considered the ramifications of such an increase may be substantial.     

Net release of dissolved organic matter by the scleractinian coral Acropora pulchra

The net production of dissolved organic matter (DOM) and dissolved combined and free amino acids (DCAA and DFAA, respectively) by the hermatypic coral Acropora pulchra was measured in the submerged condition, and the production rates were normalized to the coral surface area, tissue biomass, and net photosynthetic rates by zooxanthellae. When normalized to the unit surface area, the production rates of dissolved organic carbon and nitrogen (DOC and DON, respectively) were 37 and 4.4 nmol cm^− 2 h^− 1, respectively. Comparing with the photosynthetic rate by zooxanthellae, which was measured by ¹³C-tracer accumulation in the soft tissue of the coral colony, the release rate of DOC corresponded to 5.4% of the daily net photosynthetic production. The tissue biomass of the coral colony was 178 µmol C cm^− 2 and 23 µmol N cm^− 2, indicating that the release of DOC and DON accounted for 0.021% h^− 1 and 0.019% h^− 1 of the tissue C and N, respectively. The C:N ratios of the released DOM (average 8.4) were not significantly different from those of the soft tissue of the coral colonies (average 7.7). While DFAA did almost not accumulate in the incubated seawater, DCAA was considerably released by the coral colonies at the rate of 2.1 nmol cm^− 2 h^− 1 on average. Calculating C and N contents of the hydrolyzable DCAA, it was revealed that about 20% and 50%–60% of the released bulk DOC and DON, respectively, were composed of DCAA.