Nitrogen Fixation During Blooms of Nodularia in Coastal Waters and Backwaters of the Arkona Sea (Baltic Sea) in 1974

Nitrogen fixation was investigated by means of the acetylene reduction method during the development of a water bloom of Nodularia in coastal waters of the Baltic Sea west of the island of Hiddensee and in backwaters showing different degrees of eutrophication. Depending on plankton density, the values found varied greatly. The maximum of nitrogen fixation values found in extremely dense water blooms under special conditions (Baltic Sea, 2250 μg N₂/l · h; Kleiner Jasmunder Bodden, 374 μg N₂/l · h) are up to 10³ times higher than from other parts of the Baltic Sea or from inland waters. The average nitrogenase activity determined for coastal water populations of the Baltic Sea is 2.15 pg N₂/heterocyst · h and that of the inmost backwaters 0.77 pg N₂/heterocyst · h. The relationship between N₂-fixation and nutrient content in water is discussed.     

Seasonal oxygen,nitrogen and phosphorus benthic cycling along an impacted Baltic Sea estuary: regulation and spatial patterns

The regulatory roles of temperature, eutrophication and oxygen availability on benthic nitrogen (N) cycling and the stoichiometry of regenerated nitrogen and phosphorus (P) were explored along a Baltic Sea estuary affected by treated sewage discharge. Rates of sediment denitrification, anammox, dissimilatory nitrate reduction to ammonium (DNRA), nutrient exchange, oxygen (O₂) uptake and penetration were measured seasonally. Sediments not affected by the nutrient plume released by the sewage treatment plant (STP) showed a strong seasonality in rates of O₂ uptake and coupled nitrification–denitrification, with anammox never accounting for more than 20 % of the total dinitrogen (N₂) production. N cycling in sediments close to the STP was highly dependent on oxygen availability, which masked temperature-related effects. These sediments switched from low N loss and high ammonium (NH₄ ⁺) efflux under hypoxic conditions in the fall, to a major N loss system in the winter when the sediment surface was oxidized. In the fall DNRA outcompeted denitrification as the main nitrate (NO₃ ^?) reduction pathway, resulting in N recycling and potential spreading of eutrophication. A comparison with historical records of nutrient discharge and denitrification indicated that the total N loss in the estuary has been tightly coupled to the total amount of nutrient discharge from the STP. Changes in dissolved inorganic nitrogen (DIN) released from the STP agreed well with variations in sedimentary N₂ removal. This indicates that denitrification and anammox efficiently counterbalance N loading in the estuary across the range of historical and present-day anthropogenic nutrient discharge. Overall low N/P ratios of the regenerated nutrient fluxes impose strong N limitation for the pelagic system and generate a high potential for nuisance cyanobacterial blooms.     

Macroalgal blooms on southeast Florida coral reefs: I. Nutrient stoichiometry of the invasive green alga Codium isthmocladum in the wider Caribbean indicates nutrient enrichment

Invasive blooms of the siphonaceous green algae Codium spp. have been considered a symptom of coastal eutrophication but, to date, only limited biochemical evidence supports a linkage to land-based nutrient pollution. Beginning in the summer of 1990, spectacular blooms of unattached Codium isthmocladum developed on deep coral reef habitats in southern Palm Beach County and northern Broward County, and in subsequent years, attached populations formed on reefs in northern Palm Beach County.To better understand the nutrition of these HABs, we collected C. isthmocladum and other reef macroalgae from various locations in southeast Florida as well as the wider Caribbean region for tissue C:N:P analysis in order to gauge variability in the type and degree of N- and/or P-limited growth. Widespread nutrient enrichment in floridian C. isthmocladum populations was evidenced by significantly higher tissue P (0.06% versus 0.04% of dry weight) and lower C:N (12 versus 19), C:P (425 versus 980), and N:P (35 versus 50) ratios compared to more nutrient-depleted Caribbean populations. To determine nutrient availability on southeast Florida's reefs, we sampled near-bottom waters at a variety of locations for DIN (NH₄⁺ + NO₃⁻ + NO₂⁻) and SRP analysis. In general, concentrations of NH₄⁺, NO₃⁻and SRP were all high on southeast Florida's reefs compared to values reported for Caribbean coral reefs. Although summertime upwelling provides episodic NO₃⁻ and SRP enrichment to reefs in southeast Florida, these transient nutrient pulses have not historically supported C. isthmocladum blooms.We suggest that the widespread P enrichment of C. isthmocladum tissue and water column DIN:SRP ratios <16:1 in southeast Florida drive this system toward N limitation where low level NH₄⁺ enrichment becomes of paramount importance. Hence, the recent C. isthmocladum blooms appear to be supported by increasing land-based nutrient pollution, particularly, sewage that is enriched in NH₄⁺ and SRP at a low N:P ratio (<10:1) critical to sustaining balanced growth during bloom formation.     

The Mechanism Controlling Plant Nutrient Concentrations in the Northern Adriatic Sea

A 20 year data set for the northern Adriatic was analyzed and the factors establishing the nutrient environment identified. Concentrations ranged widely (TIN 0.0–78, PO₂ 0.01–1.1, and SiO₄ 0.0–59 mmol m⁻³). In early winter remineralization increased concentrations. Characteristic winter, late spring and fall phytoplankton blooms alternately decreased and increased concentrations, as modified by river input. In summer nutrients were minimal under a semi-closed circulation pattern and high vertical stability, due to closely coupled nitrogen and phosphorus assimilation-regeneration processes and biogenic silica sedimentation. “New” primary production supported mainly by river input of “new” nutrients approximated “regenerated” primary production supported by regenerated nutrients, making the ecosystem especially sensitive to eutrophication pressure from anthropogenic increases in the Po River nutrient load.     

Influence of Eutrophication on Bacteria in Two Fjords of the Western Baltic

Longitudinal transects in the Schlei Fjord (Kiel Bight) revealed strong increases of microbiological parameters from the entrance towards the landlocked end of the fjord. In the inner part up to 14 × 10⁶ bacteria ml⁻¹ (microscopical count), 77 × 10⁻¹ saprophytic bacteria ml⁻¹ (plate count) and a thymidine incorporation rate of 0.436 nmol 1⁻¹ h⁻¹ were observed. The data were compared with earlier results from the same system and those from the nearby located Kiel Fjord. The observations indicate a very high degree of eutrophication in the Schlei Fjord and a lesser one in the Kiel Fjord. The significance of total bacterial numbers and numbers of saprophytic bacteria as indicators for eutrophication and contamination is discussed.     

Nutrient uptake and primary productivity in an urban estuary: using rate measurements to evaluate phytoplankton response to different hydrological and nutrient conditions

The northern San Francisco Estuary (nSFE) is an urban estuary supplied with anthropogenic nutrient inputs, yet spring blooms are uncommon and phytoplankton biomass is low. The low levels of chlorophyll (<5 µg L^?1) have likely contributed to declines in several native fishes, and there is a need to evaluate the conditions that could allow for increased phytoplankton. Increased ammonium (NH₄) loads have been hypothesized to modulate the magnitude of blooms in nSFE (the “NH₄ hypothesis”) as a result of inhibition of phytoplankton NO₃ uptake that limits access to the greater nitrogen (N) pool of nitrate (NO₃). This hypothesis, tested in enclosures, but not in the field until now, is that lack of access to NO₃ limits primary production and consequently the accumulation of chlorophyll. Here, we test this in the field with the following aims: (1) to observe the uptake response of phytoplankton in different flow and N loading conditions, (2) determine whether the sequence of uptake rates suggested by the “NH₄ hypothesis” occurs and (3) obtain depth-integrated nutrient uptake rates to better constrain published criteria for bloom formation. Weekly measurements of NH₄ and NO₃ uptake, and primary production rates were made during spring 2011–2012, along with nutrient and chlorophyll concentrations during two contrasting hydrological conditions of high vs low freshwater flow. In conditions with high freshwater flow (maximum of 2405 m³ s^?1), there were lower nutrient concentrations than with low/normal flows (e.g., NO₃ of 10 µmol L^?1 compared to 30 µmol L^?1), with low N uptake and primary production rates. With low flow (maximum of 1304 m³ s^?1), there was elevated chlorophyll and blooms occurred, especially in shallow well-lit shoals where chlorophyll reached 60 µg L^?1. The higher levels of chlorophyll and primary productivity resulted from uptake of ambient NO₃ by phytoplankton, and f-ratios >0.5. This was enabled by phytoplankton uptake of NH₄ to below inhibitory levels, as proposed by the “NH₄ hypothesis.” The depth-integrated uptake rate data were used to refine a model that yields flow and nutrient concentration criteria necessary for bloom formation and confirmed that washout flows were the most useful predictor of blooms. Understanding the interaction of phytoplankton biomass with nutrient variability requires evaluating changes in C and N uptake rates and river flow. These dynamic changes are central to understanding why some urban estuaries have lower productivity than expected, and would be difficult to evaluate using biomass data alone. This study points to the importance of treating inorganic N separately as NH₄ and NO₃ rather than lumping together as DIN and to use rate process data as a mechanistic way to understand, predict and minimize cultural eutrophication impacts.     

Influence of bioturbation on denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in freshwater sediments

Intensive agriculture leads to increased nitrogen fluxes (mostly as nitrate, NO₃ ^?) to aquatic ecosystems, which in turn creates ecological problems, including eutrophication and associated harmful algal blooms. These problems have focused scientific attention on understanding the controls on nitrate reduction processes such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Our objective was to determine the effects of nutrient-tolerant bioturbating invertebrates (tubificid oligochaetes) on nitrogen cycling processes, specifically coupled nitrification–denitrification, net denitrification, DNRA, and biogeochemical fluxes (O₂, NO₃ ^?, NH₄ ⁺, CO₂, N₂O, and CH₄) in freshwater sediments. A mesocosm experiment determined how tubificid density and increasing NO₃ ^? concentrations (using N¹⁵ isotope tracing) interact to affect N cycling processes. At the lowest NO₃ ^? concentration and in the absence of bioturbation, the relative importance of denitrification to DNRA was similar (i.e., 49.6 and 50.4 ± 8.1 %, respectively). Increasing NO₃ ^? concentrations in the control cores (without fauna) stimulated denitrification, but did not enhance DNRA, which significantly altered the relative importance of denitrification compared to DNRA (94.6 vs. 5.4 ± 0.9 %, respectively). The presence of tubificid oligochaetes enhanced O₂, NO₃ ^?, NH₄ ⁺ fluxes, greenhouse gas production, and N cycling processes. The relative importance of denitrification to DNRA shifted towards favoring denitrification with both the increase in NO₃ ^? concentrations and the increase of bioturbation activity. Our study highlights that understanding the interactions between nutrient-tolerant bioturbating species and nitrate contamination is important for determining the nitrogen removal capacity of eutrophic freshwater ecosystems.     

Ventilation and oxygen consumption in the hagfish, Myxine glutinosa L.

Ventilation was measured directly in the hagfish, Myxine glutinosa L., by means of an electro-magnetic blood flowmeter. Ventilatory flow and frequency increased from 0.86 ± 0.27 ml·min^?, and 18.2 ± 5.1·min^?, respectively, at 7°C to 1.70 ± 0.20 ml·min^?, and 70.1 ± 9.5·min^? at 15 ·C.Standard oxygen consumption,V?O₂, was measured in non-buried hagfish. V?O₂ was 0.57 ± 0.17μl O₂·g^?1·min^?1 at 7°C, and 0.85 ± 0.12μl O₂·g^?1·min^?1 at 15°C.     

Photosynthesis of Marine Macroalgae from Antarctica: Light and Temperature Requirements

The photosynthetic performance of macroalgae isolated in Antarctica was studied in the laboratory. Species investigated were the brown algae Himantothallus grandifolius, Desmarestia anceps, Ascoseira mirabilis, the red algae Palmaria decipiens, Iridaea cordata, Gigartina skottsbergii, and the green algae Enteromorpha bulbosa, Acrosiphonia arcta, Ulothrix subflaccida and U. implexa. Unialgal cultures of the brown and red algae were maintained at 0°C, the green algae were cultivated at 10°C. I_K values were between 18 and 53 μmol m^?2 s^?1 characteristic or low light adapted algae. Only the two Ulothrix species showed higher I_K values between 70 and 74 μmol m^?2 s^?1. Photosynthesis compensated dark respiration at very low photon fluence rates between 1.6 and 10.6 μmol m^?2 s^?1. Values of α were high: between 0.4 and 1.1 μmol O₂ g^?1 FW h^?1 (μmol m^?2 s^?1)^?1 in the brown and red algae and between 2.1 and 4.9 μmol O₂ g^?1 FW h^?1 (μmol m^?2 s^?1)^?1 in the green algal species. At 0°C P_max values of the brown and red algae ranged from 6.8 to 19.1 μmol O₂ g^?1 FW h^?1 and were similarly high or higher than those of comparable Arctic-cold temperate species. Optimum temperatures for photosynthesis were 5 to 10°C in A. mirabilis, 10°C in H. grandifolius, 15°C in G. skottsbergii and 20°C or higher in D. anceps and I. cordata. P: R ratios strongly decreased in most brown and red algae with increasing temperatures due to different Q₁₀ values for photosynthesis (1.4 to 2.5) and dark respiration (2.5 to 4.1). These features indicate considerable physiological adaptation to the prevailing low light conditions and temperatures of Antarctic waters. In this respect the lower depth distribution limits and the northern distribution boundaries of these species partly depend on the physiological properties described here.     

Degradation of the Adriatic medusa <Emphasis Type="Italic">Aurelia</Emphasis> sp. by ambient bacteria

The decomposition of jellyfish after major bloom events results in the release of large amounts of nutrients, which can significantly alter nutrient and oxygen dynamics in the surrounding environment. The response of the ambient bacterial community to decomposing jellyfish biomass was evaluated in two marine ecosystems, the Gulf of Trieste (northern Adriatic Sea) and Big Lake (Mljet Island, southern Adriatic Sea). The major difference between these two ecosystems is that Aurelia sp. medusae occur throughout the year in the oligotrophic Big Lake, whereas in the mesotrophic Gulf of Trieste, they occur only seasonally and often as blooms. Addition of homogenized jellyfish to enclosed bottles containing ambient water from each of these systems triggered considerable changes in the bacterial community dynamics and in the nutrient regime. The high concentrations of protein, dissolved organic phosphorous (DOP), and PO₄ ³⁻ immediately after homogenate addition stimulated increase in bacterial abundance and production rate, coupled with NH₄ ⁺ accumulation in both ecosystems. Our preliminary results of the bacterial community structure, as determined with denaturing gradient gel electrophoresis, indicated differences in the bacterial community response between the two ecosystems. Despite divergence in the bacterial community responses to jellyfish homogenate, increased bacterial biomass and growth rates in both distinctive marine systems indicate potentially significant effects of decaying jellyfish blooms on microbial plankton.     

121 Influence of Benthic vs. Pelagic Seeding on Coastal Diatom Bloom Development

Diatom blooms may be initiated by cells that have survived in the plankton or germinated resting stages from the sediments. However, it is not well understood how these different inocula contribute to bloom development. We followed diatom community development in twenty‐liter microcosms given different inocula. Surface sediment and phytoplankton were collected in Gullmar Fjord, Sweden. Replicate microcosms were then dosed with local sediment and/or plankton and incubated in situ in Gullmar Fjord. We also followed the concurrent development of the phytoplankton community in the fjord. Experiments run in both spring and fall 2002 showed that bloom development in the microcosms was significantly faster when seeded by planktonic cells. However, addition of sediment may have stimulated planktonic growth and also provided additional propagules. The type of inoculum used strongly influenced the diatom composition in the microcosms. Sediment additions, through germination of resting stages, resulted in communities dominated by Detonula confervacea and Thalassiosira minima in spring, and Skeletonema costatum in fall. Planktonic inocula resulted in blooms of T. nordenskioeldii and Chaetoceros debilis in spring, and S. costatum and several Chaetoceros spp. in fall. Microcosms dosed with both plankton and sediment showed a mixed species assemblage. Comparisons between the microcosms and fjord phytoplankton suggest an important role for benthic seeding of diatom blooms.     

Nitrogen uptake and regeneration (ammonium regeneration,nitrification and photoproduction) in waters of the West Florida Shelf prone to blooms of Karenia brevis

The West Florida Shelf (WFS) encompasses a range of environments from inshore estuarine to offshore oligotrophic waters, which are frequently the site of large and persistent blooms of the toxic dinoflagellate, Karenia brevis. The goals of this study were to characterize the nitrogen (N) nutrition of plankton across the range of environmental conditions on the WFS, to quantify the percentage of the plankton N demand met through in situ N regeneration, and to determine whether planktonic N nutrition changes when high concentrations of Karenia are present. In the fall of 2007, 2008, and 2009 we measured ambient nutrient concentrations and used stable isotope techniques to measure rates of primary production and uptake rates of inorganic N (ammonium, NH₄⁺, and nitrate, NO₃⁻), and organic N and carbon (C; urea and amino acids, AA) in estuarine, coastal, and offshore waters, as well as coastal sites with Karenia blooms present. In parallel, we also measured rates of in situ N regeneration – NH₄⁺ regeneration, nitrification, and photoproduction of NH₄⁺, nitrite and AA. Based on microscope observations, ancillary measurements, and previous monitoring history, Karenia blooms sampled represented three bloom stages – initiation in 2008, maintenance in 2007, and late maintenance/stationary phase in 2009. Nutrient concentrations were highest at estuarine sampling sites and lowest at offshore sites. Uptake of NH₄⁺ and NO₃⁻ provided the largest contribution to N nutrition at all sites. At the non-Karenia sites, in situ rates of NH₄⁺ regeneration and nitrification were generally sufficient to supply these substrates equal to the rates at which they were taken up. At Karenia sites, NO₃⁻ was the most important N substrate during the initiation phase, while NH₄⁺ was the most important N form used during bloom maintenance and stationary phases. Rates of NH₄⁺ regeneration were high but insufficient (85 ± 36% of uptake) to support the measured NH₄⁺ uptake at all the Karenia sites although nitrification rates far exceeded uptake rates of NO₃⁻. Taken together our results support the “no smoking gun” nutrient hypothesis that there is no single nutrient source or strategy that can explain Karenia's frequent dominance in the waters where it occurs. Consistent with other papers in this volume, our results indicate that Karenia can utilize an array of inorganic and organic N forms from a number of N sources.     

Spatial and seasonal variation in greenhouse gas and nutrient dynamics and their interactions in the sediments of a boreal eutrophic lake

Dynamics of greenhouse gases, CH₄, CO₂ and N₂O, and nutrients, NO ₂ ^– + NO ₃ ^– , NH ₄ ⁺ and P, were studied in the sediments of the eutrophic, boreal Lake Kevätön in Finland. Undisturbed sediment cores taken in the summer, autumn and winter from the deep and shallow profundal and from the littoral were incubated in laboratory microcosms under aerobic and anaerobic water flow conditions. An increase in the availability of oxygen in water overlying the sediments reduced the release of CH₄, NH ₄ ⁺ and P, increased the flux of N₂O and NO ₂ ^– + NO ₃ ^– , but did not affect CO₂ production. The littoral sediments produced CO₂ and CH₄ at high rates, but released only negligible amounts of nutrients. The deep profundal sediments, with highest carbon content, possessed the greatest release rates of CO₂, CH₄, NH ₄ ⁺ and P. The higher fluxes of these gases in summer and autumn than in winter were probably due to the supply of fresh organic matter from primary production. From the shallow profundal sediments fluxes of CH₄, NH₄ ⁺ and P were low, but, in contrast, production of N₂O was the highest among the different sampling sites. Due to the large areal extension, the littoral and shallow profundal zones had the greatest importance in the overall gas and nutrient budgets in the lake. Methane emissions, especially the ebullition of CH₄ (up to 84% of the total flux), were closely related to the sediment P and NH ₄ ⁺ release. The high production and ebullition of CH₄, enhances the internal loading of nutrients, lake eutrophication status and the impact of boreal lakes to trophospheric gas budgets.     

Herbicide management of invasive cattail (<Emphasis Type="Italic">Typha</Emphasis> × <Emphasis Type="Italic">glauca</Emphasis>) increases porewater nutrient concentrations

Invasive wetland plants are the primary targets of wetland management to promote native communities and wildlife habitat, but little is known about how commonly implemented restoration techniques influence nutrient cycling. We tested how experimental mowing, herbicide application, and biomass harvest (i.e., removal of aboveground biomass) treatments of Typha-invaded mesocosms altered porewater nutrient (NO₃ ^?, NH₄ ⁺, PO ₄ ^?3 ) concentration and supply rate, vegetation response, and light penetration to the soil surface. We found that while herbicide application eliminated the target species, it also reduced native plant density and biomass, as well as increased porewater nutrient concentration (PO ₄ ^?3 , NO₃ ^?) and supply rates (N, P, K) up to a year after treatments were implemented. Because herbicide application promotes nutrient enrichment, it may increase the likelihood of reinvasion by problematic wetland invaders, as well as cause eutrophication and deleterious algal blooms in adjacent aquatic systems. Our data suggest that biomass harvest should be considered by managers aiming to reduce Typha abundance without eradicating native diversity, avoid nutrient leaching, as well as possibly utilizing biomass for bioenergy.     

In situ nutrient monitoring: A tool for capturing nutrient variability and the antecedent conditions that support algal blooms

While increased nutrient concentrations and eutrophication are recognized to be among the causative factors contributing to algal blooms, including harmful algal blooms (HABs), relationships between nutrient fluxes and specific blooms are often not well defined or understood. In an attempt to better decipher these relationships, we employed in situ nutrient monitors to collect time-series data to document variability ranging from hourly to monthly, including rain events and the resulting response of phytoplankton biomass. Multiple deployments are reported here, all conducted in tributaries of Chesapeake Bay. The suite of nutrients that were monitored varied with deployment; examples given here include nitrate + nitrite (NO₃⁻ + NO₂⁻), ammonium (NH₄⁺), phosphate (PO₄³⁻), and urea. Common features in the data included highly varying concentrations on time-scales of a few hours related to tidal oscillations, and longer-term responses on the scale of days related to rainfall events. Increases following rainfall events for all nutrients generally tended to be many fold, up to an order of magnitude, higher than pre-rainfall concentrations. However, the time scale of response to rainfall for individual nutrients varied. Ephemeral increases in PO₄³⁻ and urea typically were contemporaneous with rain events, and were also followed by longer-term sustained increases relative to pre-rainfall levels. Increases in NO₃⁻ + NO₂⁻ and NH₄⁺ lagged rainfall events by a period of several days and generally lasted for several days. These dynamics generally would be missed by traditional manual sampling. Algal responses tended to follow the increases in nitrogen, underscoring nitrogen limitation in these systems even when ambient concentrations were not depleted.     

The effects of O2 on net photosynthesis at low temperature (5°C)

Abstract. It has been shown that atmospheric O₂ can either depress or stimulate the rate of apparent photosynthesis of white mustard depending on the environmental conditions: CO₂ concentration, light intensity and temperature. Stimulation by O₂ was observed only under high photon fluence rate and at high CO₂ concentrations. The critical CO₂ concentration below which O₂ was inhibiting and above which it was stimulating was dependent on the temperature of the assay: for plants grown at 12°C the critical CO₂ concentration was 13.35 mmol at 5° C and 21.92 mmol at 10° C. Stimulation by O₂ depended also on the growth temperature: for measurements at 26.31 mmol m^?3 CO₂, O₂ was stimulating at temperatures less than 12°C for plants grown at 12°C and less than 19°C for plants grown at 27°C. The efficiency of the O₂-dependent stimulation of net photosynthesis was maximum at 9.21 mol m^?3 O₂ at 26.31 mmol m^?3 CO₂. Oxygen-stimulation of net photosynthesis was detected in Nicotiana tabacum L. var Samsun, Lycopersicum esculentum L. and Chenopodium album L. At 5°C and under high photon fluence rate, O₂ increased the carboxylation capacity of the photosynthetic apparatus of mustard and decreased its affinity for CO₂. The O₂ inhibition of the net CO₂ uptake observed at low CO₂ concentrations was the result of a decrease in the affinity for carbon dioxide. The nature of the mechanism which causes the stimulation of photosynthesis is discussed.     

The Interaction of Chromium(VI) with Urease in Solution

The interaction between K₂Cr₂O₇ and urease was investigated using fluorescence, UV-vis absorption, and circular dichroism (CD) spectroscopy. The experimental results showed that the fluorescence quenching of urease by K₂Cr₂O₇ was a result of the formation of K₂Cr₂O₇–urease complex. The apparent binding constant K _A between K₂Cr₂O₇ and urease at 295, 302, and 309 K were obtained to be 2.14?×?10⁴, 1.96?×?10⁴, and 1.92?×?10⁴ L mol^?1, respectively. The thermodynamic parameters, ΔH° and ΔS° were estimated to be ?5.90 kJ mol^?1, 43.67 J mol^?1 K^?1 according to the Van’t Hoff equation. The electrostatic interaction played a major role in stabilizing the complex. The distance r between donor (urease) and acceptor (K₂Cr₂O₇) was 5.08 nm. The effect of K₂Cr₂O₇ on the conformation of urease was analyzed using UV-vis absorption, CD, synchronous fluorescence spectroscopy, and three-dimensional fluorescence spectra, the environment around Trp and Tyr residues were altered.     

A preliminary comparison of the mariculture potential of Porphyra purpurea and Porphyra umbilicalis

Due to their rapid growth and nutrient assimilation,Porphyra spp. are good candidates for bioremediation and polyculture. The production potential of two strains of P. purpurea and P. umbilicalis from north-east USA was evaluated by measuring rates of photosynthesis (as O₂evolution) of material grown at 20 ^°C. Photosynthetic rates of P. umbilicalis were 80%higher than P. purpurea over the temperature range 5–20 ^°C, at both sub-saturating andsaturating irradiances (37 and 289 μmol photonm^-2 s^-1). Porphyra umbilicalis was more efficient at low irradiances (higher α) and had a higher P_max (23.0 vs 15.6 μmolO₂ g^-1 DW min^-1) than P.purpurea, suggesting that P. umbilicalis is a better choice for mass culture, where self-shading maybe severe. This revised version was published online in August 2006 with corrections to the Cover Date.     

The cause of PSI photoinhibition at low temperatures in leaves of Cucumis sativus, a chilling‐sensitive plant

In a chilling-sensitive plant, cucumber, chilling of leaves in the light results in irreversible damage to PSI. Recent in vitro studies suggested that hydroxyl radicals, which are formed in the presence of H₂O₂ and reduced Fe-S centers, are involved in the PSI inhibition. We therefore examined this possibility in vivo. Chilling of leaves at 5°C in the light caused a temporary increase in H₂O₂ concentration, which was probably due to the net H₂O₂ production in vivo. The activity, measured at 5°C, of the thylakoid ascorbate peroxidase (APX), a key enzyme of the H₂O₂-scavenging system, was about 20% of that measured at 25°C. The isolated thylakoids retaining high thylakoid APX activity did not show light-dependent net H₂O₂ production at 25°C. However, at 5°C, net production of H₂O₂ was observed. Since the rate of electron flow to molecular oxygen in the isolated thylakoids was ca 5 mmol e^? mol^?1 Chl s^?1 at 5°C, the H₂O₂-scavenging capacity was below this level. When intact leaves were illuminated at 5°C at an irradiance of 100 µmol m^?2 s^?1, the rate of electron transport through PSII was ca 20 mmol e^? mol^?1 Chl s^?1 and more than 80% of Q_A was in the reduced state. Since thylakoids are uncoupled in cucumber leaves at 5°C in the light. ATP is not formed and energy dissipation in the form of heat is suppressed. Therefore, the electron flow to molecular oxygen would be greater than 5 mmol e^? mol^?1 Chl s^?1. Moreover, under such conditions, components in the electron transport chain, including Fe-S centers in PSI, were probably reduced. These features indicate that, when cucumber leaves are chilled in the light, hydroxyl radicals can be produced by the Fenton reaction and cause damage to PSI.     

Accumulation of small heat shock proteins, including mitochondrial HSP22, induced by oxidative stress and adaptive response in tomato cells

Changes in gene expression, by application of H₂O₂, O₂°^–generating agents (methyl viologen, digitonin) and gamma irradiation to tomato suspension cultures, were investigated and compared to the well-described heat shock response. Two-dimensional gel protein mapping analyses gave the first indication that at least small heat shock proteins (smHSP) accumulated in response to application of H₂O₂ and gamma irradiation, but not to O₂°^– generating agents. While some proteins seemed to be induced specifically by each treatment, only part of the heat shock response was observed. On the basis of Northern hybridization experiments performed with four heterologous cDNA, corresponding to classes I–IV of pea smHSP, it could be concluded that significant amounts of class I and II smHSP mRNA are induced by H₂O₂ and by irradiation. Taken together, these results demonstrate that in plants some HSP genes are inducible by oxidative stresses, as in micro-organisms and other eukaryotic cells. HSP22, the main stress protein that accumulates following H₂O₂ action or gamma irradiation, was also purified. Sequence homology of amino terminal and internal sequences, and immunoreactivity with Chenopodium rubrum mitochondrial smHSP antibody, indicated that the protein belongs to the recently discovered class of plant mitochondrial smHSP. Heat shock or a mild H₂O₂ pretreatment was also shown to lead to plant cell protection against oxidative injury. Therefore, the synthesis of these stress proteins can be considered as an adaptive mechanism in which mitochondrial protection could be essential.