High Nutrient Conditions Research Articles

Substrate and nutrient limitation of ammonia-oxidizing bacteria and archaea in temperate forest soil

Ammonia-oxidizing microbes control the rate-limiting step of nitrification, a critical ecosystem process, which affects retention and mobility of nitrogen in soil ecosystems. This study investigated substrate (NH4+) and nutrient (K and P) limitation of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) in temperate forest soils at Coweeta Hydrologic Laboratory, a long-term ecological research site in western North Carolina, USA. We investigated substrate and nutrient limitation by amending soils with either ammonium or a nutrient solution containing P and K, then assessing the growth of these organisms during in situ soil incubations. We found substantial growth of both AOA and AOB during all incubations including unamended control incubations. Our results demonstrate that substrate availability limits nitrification by AOB and that high levels of substrate addition inhibit the growth of AOA in these soils. We found no evidence for nutrient limitation of AOB, though nutrient addition indirectly stimulated nitrification by AOB through increased nitrogen mineralization. Our data did suggest nutrient limitation by AOA, though it is unclear whether AOA significantly contribute to ammonia oxidation in this system. Furthermore, we show that AOB are responsible for the majority of ammonia oxidation in high substrate, high nutrient conditions.

Specific responses to nitrogen and phosphorus enrichment in cyanobacteria: Factors influencing changes in species dominance along eutrophic gradients

Anthropogenic eutrophication is a worldwide problem, causing proliferation of cyanobacterial masses, some of which may be toxic. However, little is known about whether the response to nutrient enrichment differs among cyanobacterial species. To address this issue, distinct patterns in growth and competitive response of benthic cyanobacteria under N and P nutrient regimes were studied. Nine cyanobacterial species, collected from Guadarrama river biofilms at several locations with different nutrient concentrations, were isolated and used for a series of N and P enrichment bioassays. In competition experiments with a mixture of all nine species, a great predominance of certain cyanobacteria over others was noted at high nutrient conditions, while under low nutrient conditions some others dominated. On the basis of these results four selected strains were subjected to a gradient of different concentrations of phosphate, nitrate and ammonium, in independent bioassays, both in monocultures and mixed cultures. Depending on the concentration of N and P, stimulation or inhibition of growth was observed. Some species grew better, dominating at high nutrient concentrations, while higher yields were recorded for others under low nutrient regimes, dominating in these conditions. Results from this study clarify previously published field observations, whereby a group of species occurred mostly in downstream nutrient-rich locations, while other was typical of upstream oligotrophic conditions. Our findings concerning differential growth in relation to nutrient concentrations may be useful for environmental management, because they help us predict which cyanobacteria may be expected to occur under certain conditions.

Development and Application of the 2010 Chesapeake Bay Watershed Total Maximum Daily Load Model

AbstractThe Phase 5.3 Watershed Model simulates the Chesapeake watershed land use, river flows, and the associated transport and fate of nutrient and sediment loads to the Chesapeake Bay. The Phase 5.3 Model is the most recent of a series of increasingly refined versions of a model that have been operational for more than two decades. The Phase 5.3 Model, in conjunction with models of the Chesapeake airshed and estuary, provides estimates of management actions needed to protect water quality, achieve Chesapeake water quality standards, and restore living resources. The Phase 5.3 Watershed Model tracks nutrient and sediment load estimates of the entire 166,000 km2 watershed, including loads from all six watershed states. The creation of software systems, input datasets, and calibration methods were important aspects of the model development process. A community model approach was taken with model development and application, and the model was developed by a broad coalition of model practitioners including environmental engineers, scientists, and environmental managers. Among the users of the Phase 5.3 Model are the Chesapeake watershed states and local governments, consultants, river basin commissions, and universities. Development and application of the model are described, as well as key scenarios ranging from high nutrient and sediment load conditions if no management actions were taken in the watershed, to low load estimates of an all‐forested condition.

Benthic algae stimulate leaf litter decomposition in detritus‐based headwater streams: a case of aquatic priming effect?

In detritus-based ecosystems, autochthonous primary production contributes very little to the detritus pool. Yet primary producers may still influence the functioning of these ecosystems through complex interactions with decomposers and detritivores. Recent studies have suggested that, in aquatic systems, small amounts of labile carbon (C) (e.g., producer exudates), could increase the mineralization of more recalcitrant organic-matter pools (e.g., leaf litter). This process, called priming effect, should be exacerbated under low-nutrient conditions and may alter the nature of interactions among microbial groups, from competition under low-nutrient conditions to indirect mutualism under high-nutrient conditions. Theoretical models further predict that primary producers may be competitively excluded when allochthonous C sources enter an ecosystem. In this study, the effects of a benthic diatom on aquatic hyphomycetes, bacteria, and leaf litter decomposition were investigated under two nutrient levels in a factorial microcosm experiment simulating detritus-based, headwater stream ecosystems. Contrary to theoretical expectations, diatoms and decomposers were able to coexist under both nutrient conditions. Under low-nutrient conditions, diatoms increased leaf litter decomposition rate by 20% compared to treatments where they were absent. No effect was observed under high-nutrient conditions. The increase in leaf litter mineralization rate induced a positive feedback on diatom densities. We attribute these results to the priming effect of labile C exudates from primary producers. The presence of diatoms in combination with fungal decomposers also promoted decomposer diversity and, under low-nutrient conditions, led to a significant decrease in leaf litter C:P ratio that could improve secondary production. Results from our microcosm experiment suggest new mechanisms by which primary producers may influence organic matter dynamics even in ecosystems where autochthonous primary production is low.

Nitrogen, phosphorus and silicon uptake kinetics by marine diatom Chaetoceros calcitrans under high nutrient concentrations

Nitrogen, phosphorus, and silicon uptake rates under very high nutrient concentration were investigated by culturing Chaetoceros calcitrans in varying nitrite and nitrate–nitrogen (NO2−+NO3−-N), phosphate–phosphorus (PO43−-P), and silicate–silicon (Si(OH)4-Si) concentrations to understand nutrient uptake kinetics. Uptake rates were evaluated by using the classic Michaelis–Menten equation. Maximum uptake rates (ρmax) of 0.0529, 0.0088, and 0.0150pmol/cell/h, and half-saturation constants (Ks) of 623, 133, and 71μm were determined for NO2−+NO3−-N, PO43−-P and Si(OH)4-Si, respectively. Results of this study indicate that under very high nutrient concentrations, C. calcitrans can effectively reduce nutrients from the surrounding waters. These nutrients are accumulated within intracellular vacuoles as nutrient pools and assimilated by the cell later. Assimilation efficiencies for nitrogen, silicon, and phosphorus were 55.29%, 19.23, and 15.87%, respectively. Uptake rates of NO2−+NO3−-N and Si(OH)4-Si may be applied for other marine phytoplankton under similar conditions of high nutrient conditions such as in shrimp farms or areas contaminated by wastewater. Results using the kinetic model can be applied to estimate nutrient uptake by phytoplankton in estuarine and coastal environments.

Archaeal distribution and abundance in water masses of the Arctic Ocean, Pacific sector

Marine planktonic Archaea have been recently recognized as an ecologically important component of marine prokaryotic biomass in the world’s oceans. Their abundance and meta bolism are closely connected with marine geochemical cycling. We evaluated the distribution of planktonic Archaea in the Pacific sector of the Arctic Ocean using fluorescence in situ hybridization (FISH) with catalyzed reporter deposition (CARD-FISH) and performed statistical analyses using data for archaeal abundance and geochemical variables. The relative abundance of Thaum archaeota generally increased with depth, and euryarchaeal abundance was the lowest of all planktonic prokaryotes. Multiple regression analysis showed that the thaumarchaeal relative abundance was negatively correlated with ammonium and dissolved oxygen concentrations and chlorophyll fluorescence. Canonical correspondence analysis showed that archaeal distributions differed with oceanographic water masses; in particular, Thaumarchaeota were abundant from the halocline layer to deep water, where salinity was higher and most nutrients were depleted. However, at several stations on the East Siberian Sea side of the study area and along the Northwind Ridge, Thaumarchaeota and Bacteria were proportionally very abundant at the bottom in association with higher nutrient conditions. The abundance of Euryarchaeota was high (>1.0 × 105 cells ml−1) at one of our stations in the Chukchi Sea and was positively correlated with temperature and ammonium concentration, suggesting that there could be ‘hot spots’ with increased euryarchaeal abundance in the Arctic Ocean.

The effects of physiological integration on biomass partitioning in plant modules: an experimental study with the stoloniferous herb Glechoma hederacea

Morphological and physiological plasticity are crucial attributes enabling plants to acquire resources from heterogeneous habitats. Although physiological integration can modify biomass partitioning in modules, especially when connected modules experience different conditions, its ecological importance has been largely overlooked. This experiment examined its effects on above- and belowground biomass partitioning by modules in the stoloniferous herb Glechoma hederacea. We studied how biomass allocation to roots by younger ramets was affected by connection to older ramets, and by nutrient conditions. A lower proportion of biomass was allocated to roots by younger ramets growing under low nutrient (LN) conditions when connected to older ramets in high nutrient (HN) conditions than when they were isolated, demonstrating localised modification of biomass partitioning due to physiological integration. The proportion of biomass allocated to roots by younger ramets was also lower when connected to older ramets in HN conditions than when connected to older ramets in LN conditions. Thus, the effect of integration on biomass partitioning depended on the nutrient conditions experienced by connected ramets. Such changes in biomass partitioning would result in more extensive stolon growth, and greater lateral displacement of new ramets. Understanding the ecological implications of phenotypic plasticity in plants will require further examination of the effects of physiological integration when connected modules experience contrasting growing conditions. This study demonstrates that such integration affects the biomass allocation strategy of connected ramets, enhancing resource acquisition in heterogeneous habitats. The widespread success of clonality in many communities is likely to be strongly promoted by this characteristic.

Inhibition effects of daughter ramets on parent of clonal plant Eichhornia crassipes

The aquatic invasive clonal Eichhornia crassipes was selected to study the effect of nutrients and stolon severing on clonal plants in less heterogenous aquatic environments. Under high nutrient conditions, parent ramets produced more daughter ramets and leaves with higher biomass, and, subsequent to severing daughter ramets, especially those in early stages; parents produced more daughter ramets and leaves with higher biomass. However, leaf number and biomass of individual daughter ramets did not differ from individuals with severed stolons and the control treatment. The biomass and leaves of the daughter ramets were lower than the control and other severing treatments that were carried out at later stages only when the nutrient level was low and the stolons of the daughter ramets were severed at early stages of growth. For this reason, the total number of ramets and leaves were lower in early stage severing treatments than severing treatments at later stages. After severing the connection with the daughter ramets, the root/shoot biomass ratio of the parent ramets increased, indicating that water hyacinths display morphological plasticity after stolon severing by increasing root growth. This study indicates that in aquatic environments where the nutrient distribution is less heterogenous in the same layer of water, daughter ramets of the water hyacinth can inhibit the growth and clonal reproduction of the parent, and this inhibition was strongest at early stages of ramet growth.

Eco-evolutionary litter feedback as a driver of exotic plant invasion

Many studies have examined positive feedbacks between invasive plant traits and nutrient cycling, but few have investigated whether feedbacks arise from introduction of pre-adapted species or from eco-evolutionary feedback that develops after introduction. Eco-evolutionary feedback could occur between an invader's leaf tissue C:N ratio and its response to litter accumulation. Previous modeling predicts that occurrence of this feedback would be reflected by: (1) field data showing higher litter:biomass ratios in the invasive range; (2) high C:N genotypes benefiting more from experimental litter additions than low C:N genotypes; (3) this beneficial effect on high C:N genotypes inducing a critical transition toward invader dominance when a critical amount of litter is added to a native species-dominated community experiencing low nutrient conditions. Here, we empirically tested these predictions for the invasive grass Phalaris arundinacea, which has undergone post-introduction evolutionary change toward attaining higher C:N ratios under high nutrient conditions. We performed a biogeographical comparison of litter:biomass ratios in the native (Europe) and invasive (USA) range, and an experiment with mesocosms from the invasive range under low nutrient conditions. Low and high C:N Phalaris genotypes were introduced into native-dominated and bare mesocosms, to which varying litter amounts were added. The biogeographical comparison revealed that litter:biomass ratios were higher in the invasive range. The mesocosm experiment showed that when grown in isolation, only high C:N genotypes responded positively to litter. This effect, however, was not strong enough to stimulate Phalaris when exposed to competition with native species. Our results suggest that eco-evolutionary feedback between Phalaris’ C:N ratio and litter accumulation could occur, but only under high nutrient conditions. Our experiments suggest that eco-evolutionary feedback may select for specialist rather than superior genotypes. Hence, genotypic variation induced by post-introduction admixture may be subject to context-dependent selection due to eco-evolutionary feedback, increasing trait variation within invasive populations.

Increase of apatite dissolution rate by Scots pine roots associated or not with Burkholderia glathei PML1(12)Rp in open-system flow microcosms

The release of nutritive elements through apatite dissolution represents the main source of phosphorus, calcium, and several micronutrients (e.g., Zn, Cu) for organisms in non-fertilized forest ecosystems. The aim of this study was to quantify, for the first time, the dissolution rate of apatite grains by tree roots that were or were not associated with a mineral weathering bacterial strain, and by various acids known to be produced by tree roots and soil bacterial strains in open-system flow microcosms. In addition, we explored whether the mobilization of trace elements (including rare earth elements) upon apatite dissolution was affected by the presence of trees and associated microorganisms. The dissolution rate of apatite by Scots pine plants that were or were not inoculated with the strain Burkholderia glathei PML1(12)Rp, and by inorganic (nitric) and organic (citric, oxalic and gluconic) acids at pH 5.5, 4.8, 3.8, 3.5, 3.0, and 2.0 was monitored in two controlled experiments: “plant–bacteria interaction” and “inorganic and organic acids”. Analyses of the outlet solutions in the “plant–bacteria interaction” experiment showed that Scots pine roots and B. glathei PML1(12)Rp produced protons and organic acids such as gluconate, oxalate, acetate, and lactate. The weathering budget calculation revealed that Scots pines (with or without PML1(12)Rp) significantly increased (factor>10) the release of Ca, P, As, Sr, Zn, U, Y, and rare earth elements such as Ce, La, Nd from apatite, compared to control abiotic treatment. Scanning electron microscopy observation confirmed traces of apatite dissolution in contact of roots. Most dissolved elements were taken up by Scots pine roots, i.e., approximately 50% of Ca, 70% of P, 30% of As, 70% of Sr, 90% of Zn, and 100% of U, Y, and rare earth elements. Interestingly, no significant additional effect due to the bacterial strain PML1(12)Rp on apatite dissolution and Scots pine nutrition and growth was observed. The “inorganic and organic acids” experiment demonstrated that the apatite dissolution efficacy of organic acids was higher than for the inorganic acid and varied in function of the acids: oxalic acid>citric acid>gluconic acid>nitric acid for pH ⩽3.5. In addition, apatite dissolution increased with increasing acidity for each acid. Only oxalic acid generated non-stoichiometric release of calcium and phosphorus from apatite in the solution at pH ⩽3.5, due to the precipitation of Ca-oxalate crystals at apatite surfaces. Comparison of the experiments revealed that the apatite dissolution rate by Scots pines supplied with nutritive solution at pH 5.5 reached 2.0×10−13molcm−2s−1 and was equivalent to rates with nitric acid at pH 3.2, gluconic acid at pH 3.5, citric acid at pH 3.7, and oxalic acid at pH 3.8.Altogether our results highlight that, through the production of weathering agents, notably protons and organic acids, tree roots and root-associated microorganisms are able to significantly increase the release of macro- and micro-nutrients from apatite, thus maintaining high-nutrient conditions to support their growth.

Competitive interaction between diatom Skeletonema costatum and dinoflagellate Prorocentrum donghaiense in laboratory culture

Skeletonema costatum and Prorocentrum donghaiense co-exist and appear alternately as dominant species in the natural phytoplankton community in the coastal area of the East China Sea. The mechanism that determines which of the two species dominates the succession remains unknown. We conducted a series of experiments to investigate the inter-specific relationship between S. costatum and P. donghaiense. The results of the P. donghaiense and S. costatum bialgal culture experiments indicated that S. costatum dominates P. donghaiense under high-nutrient conditions, and the interaction was independent of the initial chlorophyll a ratios. Adding S. costatum culture filtrates did not significantly influence the growth of P. donghaiense and S. costatum under high-nutrient conditions. Under low-nutrient conditions, the S. costatum filtrates showed a significant autoinhibitory effect, but they did not inhibit the growth of P. donghaiense because of the tolerance of P. donghaiense to allelochemicals. The results demonstrated that the autoallelopathy of S. costatum plays an important role in the competition and succession between S. costatum and P. donghaiense under low-nutrient concentrations, especially during phosphate deficiency.

Growth response of the submerged macrophyte Myriophyllum spicatum to sediment nutrient levels and water-level fluctuations

Sediment nutrient levels and water-level fluctuations are important factors that affect the development and growth of submerged macrophytes; however, little is known about the adaptive responses of macrophytes to these factors. We conducted an experiment using the submerged macrophyte Myriophyllum spicatum L. grown under 2 sediment nutrient levels (high: 2.7 mg g−1 total nitrogen, TN; 1.5 mg g−1 total phosphorus, TP; low: 1.45 mg g−1 TN and 0.70 mg g−1 TP) and 3 amplitudes of water-level fluctuation (static, ±50 cm, ±100 cm) in outdoor ponds. We hypothesized that increased nutrient supply would promote the growth of M. spicatum, which can acclimate to the negative effects of moderate water-level fluctuations. After 112 d of growth under high-nutrient conditions, the plants produced a greater shoot biomass (higher branch number and length), resulting in greater accumulation of total biomass. However, plant growth was inhibited by increasing the amplitude of the water-level fluctuations: at ±100 cm, the plants allocated more biomass to the roots and produced fewer and longer branches. Conversely, plant growth was promoted in the high-nutrient sediments at ±50 cm amplitude. The production of auto-fragments was increased in the high-nutrient sediment but was significantly decreased by high water-level fluctuations. Thus, sediment nutrient levels and water-level fluctuations have strong interactive effects on the growth and reproduction of M. spicatum, and increased sediment nutrients in combination with moderate water level fluctuations facilitate nutrient acquisition, plant growth, and reproduction. Our study implies that moderate water-level fluctuations benefit the restoration of submerged macrophyte communities, particularly in high-nutrient habitats.

Do soil nutrients mediate competition between grasses and Acacia saplings?

AbstractThe structure and composition of savanna vegetation is influenced by resource availability and disturbance. Grasses, a major component of savanna systems, influence the tree‐grass balance by competing with trees for light, water and soil nutrients as well as providing fuel for fires. Overgrazing, and the ensuing decreased grass competition and fire effects resulting from diminished grass vigor and cover, are thought to be major drivers of bush encroachment. To investigate impacts of competition from grass on saplings of Acacia karroo Hayne. and Acacia nilotica (L.). Willd. ex. Del. under high and low soil fertility, six grass species, Aristida junciformis (Trin & Rupr.), Eragrostis capensis (Thunb.) Trin., Hyparrhenia hirta Stapf., Panicum maximum Jacq., Sporobolus africanus (Poir.) Robyns & Tournay and Themeda triandra Forssk., were planted in pots with a tree sapling as the central test species. The major competitive effects experienced by tree saplings were dependent on nutrient level and the identity of the competing grass species (P < 0.001). Aboveground A. karroo was more strongly affected by grass competition under high nutrient conditions, whereas an increase in nutrient level had little effect on the competition between A. nilotica and the grasses (P < 0.001). Acacia karroo was also strongly affected by grass competition belowground; however, the opposite trend was observed for A. nilotica (P < 0.001). Eragrostis capensis was one of the most competitive grass species, reducing sapling aboveground biomass by 85% and 65% under high and low nutrient conditions, respectively, and sapling belowground biomass by 71% on average. Increasing nutrient availability resulted in an increase in competition exerted on A. karroo by all grasses but little to no change for A. nilotica. Changes in savanna composition and structure are thus likely to be influenced by grass species composition and soil nutrient status.

E. coli 6S RNA release from RNA polymerase requires σ70 ejection by scrunching and is orchestrated by a conserved RNA hairpin

The 6S RNA in Escherichia coli suppresses housekeeping transcription by binding to RNA polymerase holoenzyme (core polymerase + σ⁷⁰) under low nutrient conditions and rescues σ⁷⁰-dependent transcription in high nutrient conditions by the synthesis of a short product RNA (pRNA) using itself as a template. Here we characterize a kinetic intermediate that arises during 6S RNA release. This state, consisting of 6S RNA and core polymerase, is related to the formation of a top-strand "release" hairpin that is conserved across the γ-proteobacteria. Deliberately slowing the intrinsic 6S RNA release rate by nucleotide feeding experiments reveals that σ⁷⁰ ejection occurs abruptly once a pRNA length of 9 nucleotides (nt) is reached. After σ⁷⁰ ejection, an additional 4 nt of pRNA synthesis is required before the 6S:pRNA complex is finally released from core polymerase. Changing the E. coli 6S RNA sequence to preclude formation of the release hairpin dramatically slows the speed of 6S RNA release but, surprisingly, does not alter the abruptness of σ⁷⁰ ejection. Rather, the pRNA size required to trigger σ⁷⁰ release increases from 9 nt to 14 nt. That a precise pRNA length is required to trigger σ⁷⁰ release either with or without a hairpin implicates an intrinsic "scrunching"-type release mechanism. We speculate that the release hairpin serves two primary functions in the γ-proteobacteria: First, its formation strips single-stranded "-10" 6S RNA interactions away from σ⁷⁰. Second, the formation of the hairpin accumulates RNA into a region of the polymerase complex previously associated with DNA scrunching, further destabilizing the 6S:pRNA:polymerase complex.

Seasonal phytoplankton dynamics in the coastal waters of the north-eastern Adriatic Sea

This study describes the dynamics of phytoplankton in relation to environmental factors in coastal waters of the north-eastern Adriatic Sea. The distant Po River's influence is rarely noted on the eastern coastal part of the northern Adriatic, but it does trigger the crucial alternations in the community, e.g. mass development of diatoms in July 2009. A highly variable ecosystem, like the northern Adriatic, sustains high species diversity. Even though quite a few species were present for more than several months, we identified the seminal species of the seasonal succession. During spring Prorocentrum micans, Leptocylindrus minimus, Chaetoceros throndsenii, Ceratium furca and Ceratium fusus were most characteristic. These species thrive in low salinity and mixed waters. Typical summer diatom species were Chaetoceros vixvisibilis and Proboscia alata, while Rhabdosphaera clavigera and Syracosphaera pulchra were identified as distinctive summer coccolithophorids. All the summer species preferred warmer waters with low nitrate and low silica content. The autumn community was characterised by species such as Calciosolenia murrayi, Chaetoceros socialis, Asterionellopsis glacialis, and Lioloma pacificum. These species were related not only to high nutrient conditions, but also to low Po River influence. In winter a prevalence of large diatoms Thalassiosira rotula, Neocalyptrella robusta and Pseudosolenia calcar-avis was recorded. The winter assemblage was characterised by species of cold water preference able to grow in well mixed conditions.

The effect of herbivory by the mite Orthogalumna terebrantis on the growth and photosynthetic performance of water hyacinth (Eichhornia crassipes)

Eutrophication of fresh water systems is one of the most important factors contributing to the invasion of fresh water bodies by water hyacinth, Eichhornia crassipes. The South American mite, Orthogalumna terebrantis, established on the weed in South Africa in the late 1980s, but the impact of mite herbivory on the weed has never been quantified. Water hyacinth was grown under low, medium and high nitrogen and phosphorus nutrient conditions and the effect of mite herbivory on the weed's growth was examined. Additionally, the impact of different mite herbivory intensities on the weed's photosynthetic performance was examined because herbivory may have more subtle effects on the plant than can be seen from changes in plant growth parameters. Water nutrient content had a great impact on plant growth, but growth was unaffected by mite herbivory in all levels of nutrients tested. Photosynthetic performance of water hyacinth leaves exposed to varying levels of mite herbivory was assessed by measuring net photosynthetic rate (A), leaf conductance (gl), transpiration rate (E) and intercellular CO2 concentration (Ci), and by measuring specific fluorescence parameters including maximal fluorescence (Fm), efficiency of photosystem II (Fv/Fm) and certain JIP-test parameters. Photosynthesis decreased as mite herbivory increased, but there was a positive correlation between gl, E and Ci, and the amount of leaf tissue damaged through mite feeding. The efficiency of photosystem II (PSII) decreased as mite herbivory increased, as seen in the altered fluorescence emission of mite-damaged plants, but this was not the consequence of decreased chlorophyll content. Feeding by O. terebrantis thus decreased water hyacinth photosynthetic rate and the light reaction performance, even at relatively low mite densities. These results show that the impact of a biological control agent on its host plant may not be obvious at a plant growth level, but may nonetheless affect the plant at a physiological level.

Assessment of Hydrogeochemical Characteristic in an Urbanized Estuary using Environmental Techniques

Assessment of spatial variat ion in hydrogeochemical characteristics was studied in a tropical estuary, Cochin India. Noticeable spatial variations was observed in transparency, turbidity, salinity, PO4 3- -P, NH4 + -N, d issolved Cd and Hg. Light limitat ion due to high turbidity reduced the primary production, even under high nutrient conditions. An enrichment of both dissolved and sediment Cd, Pb and Hg were noticed in the harbour region associated with high organic carbon and silty-clay fraction. The mu ltivariate statistical methods such as factor analysis (FA), cluster analysis (CA), canonical corre- spondence analysis (CCA) and analysis of variance (ANOVA) were emp loyed to appraise and interpret co mplex hydro- geochemical data, to categorize pollution sources for better understanding about various biogeochemical processes. CA and CCA showed a meaningful classificat ion of estuarine samples based on spatial criteria. Statistical analysis revealed that Secchi disk depth (SD), water salin ity, NO2 - -N, NO3 - -N, dissolved oxygen (DO), total phosphorus (TP), total nit rogen (TN), petroleum hydrocarbon (PHC), sediment pH, organic carbon (SOC) and Pb were the most crucial parameters influencing the hydrogeochemistry of the estuary.

Proposal for a method to estimate nutrient shock effects in bacteria.

BackgroundPlating methods are still the golden standard in microbiology; however, some studies have shown that these techniques can underestimate the microbial concentrations and diversity. A nutrient shock is one of the mechanisms proposed to explain this phenomenon. In this study, a tentative method to assess nutrient shock effects was tested.FindingsTo estimate the extent of nutrient shock effects, two strains isolated from tap water (Sphingomonas capsulata and Methylobacterium sp.) and two culture collection strains (E. coli CECT 434 and Pseudomonas fluorescens ATCC 13525) were exposed both to low and high nutrient conditions for different times and then placed in low nutrient medium (R2A) and rich nutrient medium (TSA).The average improvement (A.I.) of recovery between R2A and TSA for the different times was calculated to more simply assess the difference obtained in culturability between each medium. As expected, A.I. was higher when cells were plated after the exposition to water than when they were recovered from high-nutrient medium showing the existence of a nutrient shock for the diverse bacteria used. S. capsulata was the species most affected by this phenomenon.ConclusionsThis work provides a method to consistently determine the extent of nutrient shock effects on different microorganisms and hence quantify the ability of each species to deal with sudden increases in substrate concentration.

Photosynthetic response of Vallisneria natans (Lour.) Hara (Hydrocharitaceae) to increasing nutrient loadings

To explore the effects of water column nutrient loading on photosynthesis of the submerged macrophyte Vallisneria natans (Lour.) Hara during the growth season (June to October), we determined the diurnal and seasonal variation in rapid light curves of plants cultivated under 4 different nutrient concentrations (N-P [mg L−1]: (1) 0.5, 0.05; (2) 1.0, 0.1; (3) 5.0, 0.5; (4) 10.0, 1.0). Nutrient concentration significantly affected the magnitude of the rapid light curves of V. natans, but not the direction of their diurnal variations. At low nutrient conditions (N-P 1 [mg L−1]: 0.5, 0.05), the maximum relative electron transport rate (rETRmax) and minimum saturating irradiance (Ek) derived from rapid light curves were significantly lower than those of other treatments, and their seasonal variations were suppressed. These results indicated that photosynthesis of V. natans was inhibited by the lack of nutrients in water column. At high nutrient conditions (N-P 4, [mg L−1]: 10.0, 1.0), there was an increase in photosynthetic rate in the light-limited region of rapid light curve (α), and a decrease in rETRmax and Ek, relative to moderate nutrient conditions (N-P 2, [mg L−1]: 1.0, 0.1). In addition, at high nutrient concentrations, the rapid light curves of V. natans reached a plateau, and then markedly declined compared with those at the lower nutrient levels, especially in July and August. These results suggested that V. natans were adapted to low-light environments in the high-nutrient loading treatment.

Combined effects of seawater temperature and nutrient condition on growth and survival of juvenile sporophytes of the kelp Undaria pinnatifida (Laminariales; Phaeophyta) cultivated in northern Honshu, Japan

Recently, withering of farmed juvenile sporophytes of the kelp Undaria pinnatifida (Harvey) Suringar has led to reduced production of this species in northern Japan, possibly because of the high water temperature and low nutrient concentration in cultivation areas. This problem may be solved by introducing parental plants with greater tolerance to high temperature and low nutrient conditions. We examined the combined effects of various temperatures (15, 20, 24, and 27 °C) and nutrient availabilities (seawater enriched with 25 % PESI medium and non-enriched seawater) on the growth and survival of cultivated juvenile sporophytes (1–2 cm) collected from Matsushima Bay, Miyagi Prefecture in northern Japan and Naruto, Tokushima Prefecture in southern Japan. The relative growth rates of juvenile sporophytes from both locations were significantly greater at 15 and 20 °C than at 24 and 27 °C. The juvenile sporophytes cultured in enriched medium had significantly higher relative growth rates than those cultivated in non-enriched seawater. Dead juveniles were observed in non-enriched seawater at all temperatures and the survival percentage decreased with increasing seawater temperatures. Compared to the juvenile sporophytes from Matsushima Bay, those from Naruto showed greater tolerance to high temperature even under the low nutrient condition. These results suggest that the withering of juvenile sporophytes is caused by the combined effects of low nutrient availability and high temperature. A potential solution to this problem is to introduce ecotypes with greater tolerance to high temperature and low nutrient conditions from a warmer region of Japan.