Influence of sulfate enrichment on the carbon dioxide and phosphate fluxes across the sediment–water interface

Fluxes of CO2 and o-P across the sediment-water interfacewere measured adding different amounts of sulfatein order to quantify the influence in these processes againsta control, and using chloramphenicol as an inhibitor ofbacterial activity. These experiments were performed underoxic and anoxic conditions. Results show that the additionof sulfate stimulated the fluxes of CO2 and o-P, whilethe use of chloramphenicol decreased these fluxes. Theratio of o-P release to Org-C release ranged from 1 to 5 underoxic conditions and from 18 to 42 under anoxicconditions.     

Influence of water-level fluctuation duration and magnitude on sediment–water nutrient exchange in coastal wetlands

This study examined the influence of water-level fluctuation (WLF) on sediment–water nutrient exchange in the Laurentian Great Lakes. Water levels in the Laurentian Great Lakes have been below the long-term mean for the past 15 years, causing the exposure of sediments that previously have been either continuously inundated or periodically exposed. The magnitude, duration, and frequency of WLF, as well as land-use history, each can influence the amount and type of sediment–water nutrient exchange. We collected sediment cores from relatively pristine coastal wetlands located on Beaver and Garden Islands in northern Lake Michigan. Sediment cores were taken from different water depths to simulate WLF magnitude; desiccation time was experimentally manipulated to simulate WLF duration. At these relatively pristine wetlands, desiccation time and water depth significantly influenced flux. However, nutrient exchange did not behave in a consistent fashion; phosphorus, nitrate, ammonium, and sulfate flux varied based on sediment exposure history and desiccation time. Sediment–water nutrient exchange rates also were compared to prior measurements taken from more impacted coastal wetlands in southern Lake Michigan and Saginaw Bay in Lake Huron. This comparison revealed a stronger influence of anthropogenic stress than desiccation time, with impacted wetland sediments releasing more soluble reactive phosphorus, sulfate, and ammonium, and retaining more nitrate, than pristine wetlands. Our results indicate that WLFs have the potential to influence sediment–water nutrient exchange, which may influence system productivity, but environmental context can override this influence.     

Influence of different mineral nitrogen sources (NO 3 − -N vs. NH 4 + -N) on arbuscular mycorrhiza development and N transfer in a Glomus intraradices–cowpea symbiosis

Labeled nitrogen (¹⁵?N) was applied to a soil-based substrate in order to study the uptake of N by Glomus intraradices extraradical mycelium (ERM) from different mineral N (NO ₃ ^? vs. NH ₄ ⁺ ) sources and the subsequent transfer to cowpea plants. Fungal compartments (FCs) were placed within the plant growth substrate to simulate soil patches containing root-inaccessible, but mycorrhiza-accessible, N. The fungus was able to take up both N-forms, NO ₃ ^? and NH ₄ ⁺ . However, the amount of N transferred from the FC to the plant was higher when NO ₃ ^? was applied to the FC. In contrast, analysis of ERM harvested from the FC showed a higher ¹⁵?N enrichment when the FC was supplied with ¹⁵NH ₄ ⁺ compared with ¹⁵NO ₃ ^? . The ¹⁵?N shoot/root ratio of plants supplied with ¹⁵NO ₃ ^? was much higher than that of plants supplied with ¹⁵NH ₄ ⁺ , indicative of a faster transfer of ¹⁵NO ₃ ^? from the root to the shoot and a higher accumulation of ¹⁵NH ₄ ⁺ in the root and/or intraradical mycelium. It is concluded that hyphae of the arbuscular mycorrhizal fungus may absorb NH ₄ ⁺ preferentially over NO ₃ ^? but that export of N from the hyphae to the root and shoot may be greater following NO ₃ ^? uptake. The need for NH ₄ ⁺ to be assimilated into organically bound N prior to transport into the plant is discussed.     

Composition of the xylem sap of tomato seedlings cultivated on media with HCO3 − and nitrogen source as NO3 − or NH4 +

The results of the experiments discussed here present changes in the chemical composition of xylem sap of tomato seedlings cultivated in hydroponics on media containing 5 mmol HCO₃ ^– and an N-source given as NO₃ ^–, NH₄ ⁺ or these two forms in different proportions. The occurrence of free NH₄ ⁺ in the xylem sap of NH₄ ⁺-seedlings and in NO₃ ^–-seedlings indicates that the process of N-assimilation was not only confined to roots. The application of HCO₃ ^– to the medium effected a decrease in the concentration of NH₄ ⁺ in the xylem sap of NH₄ ⁺-seedlings, having no effect on changes in the concentration of NO₃ ^– or NH₄ ⁺ in NO₃ ^–-seedlings. Malate, citrate, fumarate, and succinate were identified in the xylem sap. The concentration of carboxylates in NO₃ ^–-seedlings exceeded by about 50% that recorded in NH₄ ⁺-seedlings. The highest concentration of malate constituting from 80% to 93.5% of this fraction, was determined in this group of compounds. The enrichment of the medium with HCO₃ ^– ions induced an increase in the content of carboxylates, chiefly of malate. In these experimental conditions an increase in the malate concentration in the xylem sap of NO₃ ^– and NH₄ ⁺-seedlings reached relative values of 100% and 36%, respectively. The total concentration of amides and amino acids was about 2.6 times higher in the xylem sap of NH₄ ⁺-seedlings than in NO₃ ^–-seedlings. Amide glutamine was the main component of this fraction in xylem sap and its total concentration was about 3.3 times higher in NH₄ ⁺-seedlings than that determined in NO₃ ^–-seedlings. Glutamine, glutamate, aspargine, and aspartate constituted from 69% to 77% of this fraction. The concentration of the remaining amino acids varied from 0.6% to 7%. The enrichment of the medium with HCO₃ ^– ions also effected an increase in the concentration of amides and amino acids in the xylem sap by about 17% and 56% in the case of NO₃ ^– and NH₄ ⁺-seedlings, respectively, in comparison with the respective controls (without HCO₃ ^–). Abbreviations: DAG – days after germination; DIC – dissolved inorganic carbon; GOGAT – glutamine:2-oxoglutarate aminotransferase; GS – glutamine synthetase; PAR – photosynthetically active radiation; PEPc – phosphoenolpyruvate carboxylase     

Effect of the nitrogen form used in the growth medium (NO3 −, NH4 +) on alkaloid production in Datura stramonium L.

Plants of Datura stramonium var. tatula L. Torr. were cultivated on vermiculite and received two different mineral solutions. In one treatment only NO₃ ^–-nitrogen was added, while in the other NO₃ ^–-nitrogen was partly (20%) replaced by NH₄ ⁺-nitrogen. Total dose of nitrogen as well as interionic ratios were kept constant in both treatments. With the combined treatment (NO₃ ^–-NH₄ ⁺) a significant higher hyoscyamine content was found at the time when highest biomass was reached. This was apparently the result of an increased alkaloid biosynthesis. Also scopolamine content was positively influenced, but only at a point past maximal biomass yield.No significant differences in amounts of nitrogen bound per plant were found between both treatments.The higher alkaloid content observed with the combined treatment was associated with a higher relative proportion of bound nitrogen present in the alkaloids. It seems that more nitrogen is available for secondary metabolism when NH₄ ⁺-nitrogen is present in the culture medium.     

Influence of nitrogen form and NH4 +-N/:NO3 −-N ratios on adventitious shoot formation from pear (Pyrus communis) leaf explants in vitro

The effect of various basal salts media, containing different nitrogen levels on in vitro adventitious shoot regeneration from leaf explants of Louise Bonne Panachee and Seckel pear (Pyrus communis L.) were investigated. Among the different basal salt formulae tested, Nitsch (1969) gave significantly better regeneration in most of the experiments. Shoot regeneration was altered with different NH₄ ⁺-N/NO₃ ^–-N ratios. The best regeneration was obtained when NH₄ ⁺:NO₃ ^– was either 1:2 or 1:3 regardless of overall N concentration. In addition, these data show that NH₄ ⁺ was essential for adventitious shoot regeneration from pear leaf explants on White's (1943) medium.     

Influence of submerged macrophytes,temperature, and nutrient loading on the development of redox potential around the sediment–water interface in lakes

Redox potential is a significant factor in aquatic systems to regulate the availability of nutrients and some metals. To assess the driving variables regulating redox potential, background parameters (dissolved oxygen, pH, temperature, chlorophyll-a, soluble reactive and total phosphorus content of water, coverage and height of submerged macrophytes) and redox potential profiles around the sediment–water interface (SWI) were measured in simulated shallow lake ecosystems. There were two nutrient regimes (enriched and non-enriched) and three temperature scenarios (unheated; +3.5°C; +5°C) installed in the experimental setups, which were constructed to study the effects of global climate change. Temperature did not have any detectable effect on redox potentials, and we presume that nutrient addition had only indirect positive effects through triggering phytoplankton dominance which causes macrophyte absence. When submerged macrophytes were present in high density (80–100% coverage), redox potentials at the SWI varied between 60–215 mV and the mean redox potential was 133 ± 34 mV (mean ± 1 SD). In contrast to this, when phytoplankton dominance was coupled to low macrophyte density (0–20% coverage), the range of redox potentials at the SWI was 160–290 mV and the mean redox potential was 218 ± 34 mV. The results revealed the primary importance of submersed macrophytes; macrophyte coverage determined alone the redox potential of the sediment–water interface by 81%. This study suggests that possible positive effects of macrophytes on redox potential can be suppressed by their negative effects in case of 80–100% coverage and total inhabitation of the water column.     

Effects of Al on nitrogen (NH 4 + and NO 3 − ) uptake,nitrate reductase activity and proton release in two sorghum cultivars differing in Al tolerance

After growth for 17 to 36 days on nutrient solutions with NH₄NO₃ as nitrogen source (pH 4.2) dry matter of sorghum genotype SC0283 was much less affected by Al (1.5 and 3.0 ppm) than that of genotype NB9040. In the absence of Al both cultivars released protons into the nutrient solution as a result of an excess of cationic nutrients taken up. When Al was present, this proton efflux per unit dry weight increased drastically, especially with the sensitive genotype NB9040. Chemical analysis of plant material and continuous analyses of NO ₃ ⁻ and NH ₄ ⁺ in the nutrient solution indicated, that the Al-induced shift in H⁺-balance of both genotypes could almost completely be attributed to a decreased NO ₃ ⁻ /NH ₄ ⁺ uptake ratio. In vivo nitrate reductase activity (NRA) was reduced in the shoot of NB9040 and to a lesser degree in SC0283. Al-induced decrease in NRA was accompanied by similar percentual decreases in NO ₃ ⁻ tissue concentrations. Therefore this decrease is interpreted as being indirect,i.e., the consequence of the reduced NO ₃ ⁻ uptake of the plants. A direct repression of NRA by Al seems also unlikely because nitrate reductase activity of the roots (where cellular Al-concentrations should be higher than in shoots) was not affected in Al-treated plants of either genotype.     

Role of cytokinin in enhanced productivity of maize supplied with NH4 + and NO3 −

Supplying both N forms (NH₄ ⁺+NO₃ ⁻) to the maize (Zea mays L.) plant can optimize productivity by enhancing reproductive development. However, the physiological factors responsible for this enhancement have not been elucidated, and may include the supply of cytokinin, a growth-regulating substance. Therefore, field and gravel hydroponic studies were conducted to examine the effect of N form (NH₄ ⁺+NO₃ ⁻ versus predominantly NO₃ ⁻) and exogenous cytokinin treatment (six foliar applications of 22 μM 6-benzylaminopurine (BAP) during vegetative growth versus untreated) on productivity and yield of maize. For untreated plants, NH₄ ⁺+NO₃ ⁻ nutrition increased grain yield by 11% and whole shoot N content by 6% compared with predominantly NO₃ ⁻. Cytokinin application to NO₃ ⁻-grown field plants increased grain yield to that of NH₄ ⁺+NO₃ ⁻-grown plants, which was the result of enhanced dry matter partitioning to the grain and decreased kernel abortion. Likewise, hydroponically grown maize supplied with NH₄ ⁺+NO₃ ⁻ doubled anthesis earshoot weight, and enhanced the partitioning of dry matter to the shoot. NH₄ ⁺+NO₃ ⁻ nutrition also increased earshoot N content by 200%, and whole shoot N accumulation by 25%. During vegetative growth, NH₄ ⁺+NO₃ ⁻ plants had higher concentrations of endogenous cytokinins zeatin and zeatin riboside in root tips than NO₃ ⁻-grown plants. Based on these data, we suggest that the enhanced earshoot and grain production of plants supplied with NH₄ ⁺+NO₃ ⁻ may be partly associated with an increased endogenous cytokinin supply.     

Effect of glycerol–water binary mixtures on the structure and dynamics of protein solutions

We have performed 20?ns of fully atomistic molecular dynamics simulations of Hen Egg-White Lysozyme in 0, 10, 20, 30, and 100% by weight of glycerol in water to better understand the microscopic physics behind the bioprotection offered by glycerol to naturally occuring biological systems. The solvent exposure of protein surface residues changes when glycerol is introduced. The dynamic behavior of the protein, as quantified by the incoherent intermediate scattering function, shows a nonmonotonic dependence on glycerol content. The fluctuations of the protein residues with respect to each other were found to be similar in all water-containing solvents, but different from the pure glycerol case. The increase in the number of protein–glycerol hydrogen bonds in glycerol–water binary mixtures explains the slowing down of protein dynamics as the glycerol content increases. We also explored the dynamic behavior of the hydration layer. We show that the short length scale dynamics of this layer are insensitive to glycerol concentration. However, the long length scale behavior shows a significant dependence on glycerol content. We also provide insights into the behavior of bound and mobile water molecules.     

Biochemical composition of particulate organic matter and bacterial dynamics at the sediment–water interface in a Mediterranean seagrass system

The seagrass Posidonia oceanica is the most productive system of theMediterranean Sea. In order to gather information on the temporal andspatial variability of the suspended particulate matter in relation tobacterial dynamics, water samples were collected at 10 cm above thesediments over a period of 13 months in the Prelo Bay (Ligurian Sea, NWMediterranean). Measurements of seston concentration, as well as theelemental (POC and PON) and biochemical composition (lipids, proteins,carbohydrates and nucleic acids) of particulate matter were carried out toassess the origin, composition and bacterial contribution to the foodpotentially available in the seagrass system to consumers. Lipids andproteins were the main biochemical classes of organic compounds, followed bycarbohydrates. Despite the highly refractory composition of the seagrassleaves, particulate organic matter was mostly composed of labile compounds(69.9% of POC). POM temporal patterns were controlled by currentspeed at the sediment–water interface that resuspended only smallparticles largely colonised by bacteria after an intensive process offractionation and aging. In the seagrass system, the POM appears to bedominated by bacteria (density ranging from 0.7 to 2.5×10⁹ cells l^™1, representing more than48.3% of POC and 68.7% of the biopolymeric carbon, as the sumof lipid, protein and carbohydrate carbon). This feature was characteristicof the seagrass system since much lower bacterial densities were foundoutside the Posidonia meadow. Bacteria were negatively correlated with theconcentration of nitrite and nitrate suggesting a selective utilisation ofinorganic nutrients to support their growth. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of NH4+/NO3− availability on nitrate reductase activity and nitrogen accumulation in wetland helophytes Phragmites australis and Glyceria maxima

The effect of NH₄⁺/NO₃⁻ availability on nitrate reductase (NR) activity in Phragmites australis and Glyceria maxima was studied in sand and water cultures with the goal to characterise the relationship between NR activity and NO₃⁻ availability in the rhizosphere and to describe the extent to which NH₄⁺ suppresses the utilization of NO₃⁻ in wetland plants.The NR activity data showed that both wetland helophytes are able to utilize NO₃⁻. This finding was further supported by sufficient growth observed under the strict NO₃⁻ nutrition. The distribution of NR activity within plant tissues differed between species. Phragmites was proved to be preferential leaf NO₃⁻ reducer with high NR activity in leaves (NR_max > 6.5 μmol NO₂⁻ g dry wt⁻¹ h⁻¹) under all N treatments, and therefore Phragmites seems to be good indicator of NO₃⁻ availability in flooded sediment. In the case of Glyceria the contribution of roots to plant NO₃⁻ reduction was higher, especially in sand culture. Glyceria also tended to accumulate NO₃⁻ in non-reduced form, showing generally lower leaf NR activity levels. Thus, the NR activity does not necessarily correspond with plant ability to take up NO₃⁻ and grow under NO₃⁻-N source. Moreover, the species differed significantly in the content of compounds interfering with NR activity estimation. Glyceria, but not Phragmites, contained cyanogenic glycosides releasing cyanide, the potent NR inhibitor. It clearly shows that the use of NR activity as a marker of NO₃⁻ utilization in individual plant species is impossible without the precise method optimisation.     

The kinetics of NH4 + and NO3 − uptake by Douglas fir from single N-solutions and from solutions containing both NH4 + and NO3 −

The kinetics of NH₄ ⁺ and NO₃ ⁻ uptake in young Douglas fir trees (Pseudotsuga menziesii [Mirb.] Franco) were studied in solutions, containing either one or both N species. Using solutions containing a single N species, the V_max of NH₄ ⁺ uptake was higher than that of NO₃ ⁻ uptake. The K_m of NH₄ ⁺ uptake and K_m of NO₃ ⁻ uptake differed not significantly. When both NH₄ ⁺ and NO₃ ⁻ were present, the V_max for NH₄ ⁺ uptake became slightly higher, and the K_m for NH₄ ⁺ uptake remained in the same order. Under these conditions the NO₃ ⁻ uptake was almost totally inhibited over the whole range of concentrations used (10–1000 μM total N). This inhibition by NH₄ ⁺ occurred during the first two hours after addition. ei]{gnA C}{fnBorstlap}     

Distribution of denitrifying bacterial communities in the stratified water column and sediment–water interface in two freshwater lakes and the Baltic Sea

We have studied the distribution and community composition of denitrifying bacteria in the stratified water column and at the sediment–water interface in lakes Plußsee and Schöhsee, and a near-shore site in the Baltic Sea in Germany. Although environmental changes induced by the stratification of the water column in marine environments are known to affect specific populations of denitrifying bacteria, little information is available for stratified freshwater lakes and brackish water. The aim of the present study was to fill this gap and to demonstrate specific distribution patterns of denitrifying bacteria in specific aquatic habitats using two functional markers for the nitrite reductase (nirK and nirS genes) as a proxy for the communities. The leading question to be answered was whether communities containing the genes nirK and nirS have similar, identical, or different distribution patterns, and occupy the same or different ecological niches. The genes nirK and nirS were analyzed by PCR amplification with specific primers followed by terminal restriction fragment length polymorphism (T-RFLP) and by cloning and sequence analysis. Overall, nirS-denitrifiers were more diverse than nirK-denitrifiers. Denitrifying communities in sediments were clearly different from those in the water column in all aquatic systems, regardless of the gene analyzed. A differential distribution of denitrifying assemblages was observed for each particular site. In the Baltic Sea and Lake Plußsee, nirK-denitrifiers were more diverse throughout the water column, while nirS-denitrifiers were more diverse in the sediment. In Lake Schöhsee, nirS-denitrifiers showed high diversity across the whole water body. Habitat-specific clusters of nirS sequences were observed for the freshwater lakes, while nirK sequences from both freshwater lakes and the Baltic Sea were found in common phylogenetic clusters. These results demonstrated differences in the distribution of bacteria containing nirS and those containing nirK indicating that both types of denitrifiers apparently occupy different ecological niches.     

Influence of hydrogen bonding on the structural transition of poly(methacrylic acid) chain in water–ethanol solution by molecular dynamics simulations

The conformational structure of dilute atactic-poly(methacrylic acid) (PMA) solution in binary water–ethanol mixture was investigated by molecular dynamics simulations, over 0–0.9 ethanol (co-solvent) fraction. The radius of gyration 〈R_g〉, torsion angle distribution, intra-chain hydrogen bonds (H-bonds), and H-bonds for PMA–water, PMA–ethanol and water–ethanol, atom–atom and atom–group pair radial distribution functions were analysed. An increase in the ethanol fraction leads to chain expansion. The non-monotonic variation of 〈R_g〉, commensurate with the experimentally observed behaviour of intrinsic viscosity [η], takes place by H-bonding effects between PMA, water and ethanol, as driven by the differences in the chemical structure of water and ethanol. The PMA repeat units are closer to the CH₂ groups as compared with CH₃ groups of ethanol, in the nearest coordination shell. Water as compared with ethanol is able to coordinate closer to the PMA repeat unit centre of mass. Intra-chain H-bonding depreciates with an increase in the ethanol content in solution. The changes, across the ethanol fraction range, in chain dimensions and of predicted intrinsic viscosity by the simulations, compare well with experimental results in the literature.     

Effects of benthic filamentous algae on the sediment–water interface in an acidic mining lake

Natural alkalinity generation by microbial sulphate reduction in acidic lakes is usually inhibited by the low pH and a low primary production which results in a lack of suitable organic carbon sources. In some acidic mining lakes mass developments of filamentous benthic algae occur. The effects of this periphyton layer on the biogeochemistry of the sediment–water interface were investigated by in situ microsensor measurements and laboratory incubations in Mining Lake Grünewalder Lauch (Germany). Microsensor measurements showed that the oxic–anoxic boundary was located in the periphyton layer and was moving up and down depending on light triggered photosynthesis. The sediment itself was permanently anoxic. The diurnal redox shift and the maintenance of neutral conditions in the periphyton layer lead to an effective precipitation of iron and phosphorus. Under the periphyton layer very high sulphate reduction rates up to 265 nmol cm⁻³ d⁻¹ were measured in the sediment. These are the highest rates reported for mining lakes so far. The microbial activity was high enough to keep the pH in the surface sediment neutral and contributed to natural alkalinity production. Handling editor: L. Naselli-Flores     

Unraveling the effects of water–sediment conditions and spatial patterns on Unionida assemblages in seasonally connected floodplain lakes

Floodplain lakes are good metacommunity systems to study the environmental and spatial processes structuring local assemblages. They are more connected during high-water periods and are more isolated during low-water periods. We evaluated the effects of lake spatial patterning and water and sediment conditions on Unionida species assemblages. Moran Eigenvector Maps were used to generate spatial variables representing spatial patterns at different scales. We sampled 35 lakes from the Pantanal floodplain, Brazil. To understand the effects of environmental and spatial variables, we performed Redundancy Analyses and variation partitioning to separate environmental and spatial pattern effects. Environmental variables explained almost twice the variation in the Pantanal mussel assemblages than did spatial variables. Unionida species presence was driven mainly by variations in sediment coarse sand and silt contents. The weak spatial patterns observed may be related to increased connectivity between lakes during floods, which facilitates mussel host fish dispersal. Mussel abundances were driven mainly by organic matter availability, but varied between species. Changes in lake connectivity can affect the regional sediment dynamics and affect mussel assemblages.