Dissimilatory Iodate Reduction by Marine Pseudomonas sp. Strain SCT

Bacterial iodate (IO₃⁻) reduction is poorly understood largely due to the limited number of available isolates as well as the paucity of information about key enzymes involved in the reaction. In this study, an iodate-reducing bacterium, designated strain SCT, was newly isolated from marine sediment slurry. SCT is phylogenetically closely related to the denitrifying bacterium Pseudomonas stutzeri and reduced 200 μM iodate to iodide (I⁻) within 12 h in an anaerobic culture containing 10 mM nitrate. The strain did not reduce iodate under the aerobic conditions. An anaerobic washed cell suspension of SCT reduced iodate when the cells were pregrown anaerobically with 10 mM nitrate and 200 μM iodate. However, cells pregrown without iodate did not reduce it. The cells in the former category showed methyl viologen-dependent iodate reductase activity (0.31 U mg⁻¹), which was located predominantly in the periplasmic space. Furthermore, SCT was capable of anaerobic growth with 3 mM iodate as the sole electron acceptor, and the cells showed enhanced activity with respect to iodate reductase (2.46 U mg⁻¹). These results suggest that SCT is a dissimilatory iodate-reducing bacterium and that its iodate reductase is induced by iodate under anaerobic growth conditions.     

Active Transport and Accumulation of Iodide by Newly Isolated Marine Bacteria

Iodide (I⁻)-accumulating bacteria were isolated from marine sediment by an autoradiographic method with radioactive ¹²⁵I⁻. When they were grown in a liquid medium containing 0.1 μM iodide, 79 to 89% of the iodide was removed from the medium, and a corresponding amount of iodide was detected in the cells. Phylogenetic analysis based on 16S rRNA gene sequences indicated that iodide-accumulating bacteria were closely related to Flexibacter aggregans NBRC15975 and Arenibacter troitsensis, members of the family Flavobacteriaceae. When one of the strains, strain C-21, was cultured with 0.1 μM iodide, the maximum iodide content and the maximum concentration factor for iodide were 220 ± 3.6 (mean ± standard deviation) pmol of iodide per mg of dry cells and 5.5 × 10³, respectively. In the presence of much higher concentrations of iodide (1 μM to 1 mM), increased iodide content but decreased concentration factor for iodide were observed. An iodide transport assay was carried out to monitor the uptake and accumulation of iodide in washed cell suspensions of iodide-accumulating bacteria. The uptake of iodide was observed only in the presence of glucose and showed substrate saturation kinetics, with an apparent affinity constant for transport and a maximum velocity of 0.073 μM and 0.55 pmol min⁻¹ mg of dry cells⁻¹, respectively. The other dominant species of iodine in terrestrial and marine environments, iodate (IO₃⁻), was not transported.     

Analysis of potassium iodate reduction in tissue homogenates using high performance liquid chromatography–inductively coupled plasma-mass spectrometry

Potassium iodate (KIO₃) and potassium iodide (KI) are the major salt iodization agents used worldwide. Unlike iodide (I⁻), iodate (IO₃⁻) should be reduced to I⁻ before it can be effectively used by the thyroid. In this study, we developed a new method for analyzing IO₃⁻ and I⁻ in tissue homogenates using high performance liquid chromatography coupled to inductively coupled plasma mass spectrometry (HPLC–ICP-MS). We further applied the method to demonstrate the KIO₃ reduction process by tissues in vitro. The effects of KIO₃ on the total antioxidative activity (TAA) and reduced nicotinamide adenine dinucleotide phosphate (NADPH) were also investigated here. Finally, we found that IO₃⁻ can be reduced to I⁻ by tissue homogenates and IO₃⁻ irreversibly decreases the antioxidant capability of tissues. Our studies suggest that KIO₃ might have a big effect on the redox balance of tissue and would further result in oxidative stress of organisms.     

Reduction of iodate in iodated salt to iodide during cooking with iodine as measured by an improved HPLC/ICP–MS method

Background: Iodate is a strong oxidant, and some animal studies indicate that iodate intake may cause adverse effects. A key focus of the safety assessment of potassium iodate as a salt additive is determining whether iodate is safely reduced to iodide in food. Objective: To study the reduction of iodate in table salt to iodide and molecular iodine during cooking. Materials and Methods: Fifteen food samples cooked with and without iodated salt were prepared in duplicate. The iodine in the cooked food was extracted with deionized water. The iodine species in the extracts were determined by using an improved high-performance liquid chromatography/inductively coupled plasma–mass spectrometry (HPLC/ICP–MS). The cooking temperature and the pH of the food were determined. Results: The conversion rate of iodate in iodated salt to iodide and molecular iodine was 96.4%±14.7% during cooking, with 86.8%±14.5% of the iodate converted to iodide ions and 9.6% ±6.2% converted to molecular iodine to lose. The limit of detection, limit of quantification, relative standard deviation and recovery rate of the method HPLC/ICP–MS were 0.70 μg/L for I⁻ (0.69 μg/L for IO₃⁻), 2.10 μg/L for I⁻ (2.06 μg/L for IO₃⁻), 2.6% and 101.6%±2.6%, respectively. Conclusion: Almost all iodate added to food was converted into iodide and molecular iodine during cooking. The improved HPLC/ICP–MS was reliable in the determination of iodine species in food extracts.     

Evaluation of Iodide and Iodate for Adsorption–Desorption Characteristics and Bioavailability in Three Types of Soil

Adsorption–desorption of iodine in two forms, viz., iodide (I⁻) and iodate (IO₃⁻), in three types of soil were investigated. The soils were: red soil developed on Quaternary red earths (REQ)— clayey, kaolintic thermic plinthite Aquult, Inceptisol soil (IS) and alluvial soil (AS)—Fluvio-marine yellow loamy soil. The isothermal curves of iodine adsorption on soils were described by Langmuir and Freundlich equation, and the maximum adsorption values (y _m) were obtained from the simple Langmuir model. As compared with the iodide, the iodate was adsorbed in higher amounts by the soils tested. Among three soils, the REQ soil adsorbed more iodine (I⁻ and IO₃⁻) than the IS and AS. The distribution coefficient (K _d) of iodine in the soils decreased exponentially with increasing iodine loading concentration. Desorption of iodine in soil was increased correspondingly with increasing adsorption values. The REQ soil had a greater affinity for iodine than the IS and AS at the same iodine loadings. In the pot experiment cultivated with pakchoi (Brassica chinensis L.) and added with two exogenous iodine sources, the iodide form was quickly taken up by pakchoi and caused more toxicity to the vegetable. The rate of iodine loss from soil was higher for iodide form as compared with the iodate. The iodine bioavailability was the highest but the persistence was the weakest in AS among the three soils tested, and the REQ soil showed just the opposite trend to that of the AS soil. This study is of theoretical importance to understand the relationship between iodine adsorption–desorption characteristics and their bioavailability in different soils and it also has practical implications for seeking effective alternatives of iodine biofortification to prevent iodine deficiency disorders.     

Photosynthesis and metabolism of sugars from lettuce plants (<Emphasis Type="Italic">Lactuca sativa</Emphasis> L. var. longifolia) subjected to biofortification with iodine

Iodine, essential to human life, is in part ingested through vegetable consumption, explaining the current application of this element in biofortification programs. Few data are available on the effects of iodine on main plant metabolisms such as carbon metabolism. The objective of this study was to determine the effect of the application of different doses (20, 40 and 80 μM) and forms of iodine (iodate [IO₃ ⁻] and iodide [I⁻]) on photosynthesis and carbohydrate metabolism in lettuce plants. None of these treatments exerted significant effects on the synthesis pathway or on sucrose degradation. Application of 80 μM of I⁻ reduced the photosynthesis rate, which may be associated with the reduction found in biomass and photosynthetic parameters (stomatic conductance and transpiration). This finding confirms that the application of high doses of I⁻ has a phytotoxic effect on plant physiology. In contrast, all IO₃ ⁻ treatments increased the biomass of the plants which showed an elevated photosynthetic rate, stomatic conductance, and transpiration (vs. controls). The differential crop behavior observed with the two forms of this trace element suggests that IO₃ ⁻ should be selected for future biofortification programs.     

Iodate Reduction by Shewanella oneidensis Does Not Involve Nitrate Reductase

Microbial iodate (IO₃^?) reduction is a major component of iodine biogeochemical cycling and is the basis of alternative strategies for remediation of iodine-contaminated environments. The molecular mechanism of microbial IO₃^? reduction, however, is not well understood. In several microorganisms displaying IO₃^? and nitrate (NO₃^?) reduction activities, NO₃^? reductase is postulated to reduce IO₃^? as alternate electron acceptor. In the present study, whole genome analyses of 25 NO₃^?-reducing Shewanella strains identified various combinations of genes encoding one assimilatory (cytoplasmic Nas) and three dissimilatory (membrane-associated Nar and periplasmic Napα and Napβ) NO₃^? reductases. Shewanella oneidensis was the only Shewanella strain whose genome encoded a single NO₃^? reductase (Napβ). Terminal electron acceptor competition experiments in S. oneidensis batch cultures amended with both NO₃^? and IO₃^? demonstrated that neither NO₃^? nor IO₃^? reduction activities were competitively inhibited by the presence of the competing electron acceptor. The lack of involvement of S. oneidensis Napβ in IO₃^? reduction was confirmed via phenotypic analysis of an in-frame gene deletion mutant lacking napβA (encoding the NO₃^?-reducing NapβA catalytic subunit). S. oneidensis ΔnapβA was unable to reduce NO₃^?, yet reduced IO₃^? at rates higher than the wild-type strain. Thus, NapβA is required for dissimilatory NO₃^? reduction by S. oneidensis, while neither the assimilatory (Nas) nor dissimilatory (Napα, Napβ, and Nar) NO₃^? reductases are required for IO₃^? reduction. These findings provide the first genetic evidence that IO₃^? reduction by S. oneidensis does not involve nitrate reductase and indicate that S. oneidensis reduces IO₃^? via an as yet undiscovered enzymatic mechanism.     

Mechanism of Ion Permeation in Skeletal Muscle Chloride Channels

Voltage-gated Cl⁻ channels belonging to the ClC family exhibit unique properties of ion permeation and gating. We functionally probed the conduction pathway of a recombinant human skeletal muscle Cl⁻ channel (hClC-1) expressed both in Xenopus oocytes and in a mammalian cell line by investigating block by extracellular or intracellular I⁻ and related anions. Extracellular and intracellular I⁻ exert blocking actions on hClC-1 currents that are both concentration and voltage dependent. Similar actions were observed for a variety of other halide (Br⁻) and polyatomic (SCN⁻, NO₃ ⁻, CH₃SO₃ ⁻) anions. In addition, I⁻ block is accompanied by gating alterations that differ depending on which side of the membrane the blocker is applied. External I⁻ causes a shift in the voltage-dependent probability that channels exist in three definable kinetic states (fast deactivating, slow deactivating, nondeactivating), while internal I⁻ slows deactivation. These different effects on gating properties can be used to distinguish two functional ion binding sites within the hClC-1 pore. We determined K _D values for I⁻ block in three distinct kinetic states and found that binding of I⁻ to hClC-1 is modulated by the gating state of the channel. Furthermore, estimates of electrical distance for I⁻ binding suggest that conformational changes affecting the two ion binding sites occur during gating transitions. These results have implications for understanding mechanisms of ion selectivity in hClC-1, and for defining the intimate relationship between gating and permeation in ClC channels.     

Identification and characterization of thermophilic amylase producing bacterial isolates from the brick kiln soil

The present experiment was designed to isolate bacterial strains from the brick kiln soil and to check the activity and enzyme kinetics of amylase from these isolates. The bacterial colonies were isolated from soil samples through the serial dilution method. The bacterial isolates were identified through morphological, electron microscopic and molecular analysis. The 16S ribosomal RNA sequences of the isolates IR-1, IR-2, IR-3, IR-8, and IR-9 showed high similarities with Bacillus tequilensis, Bacillus paramycoides, Proteus alimentorum, Bacillus wiedmannii, and Pseudomonas aeruginosa, respectively. All of the bacterial isolates showed a positive catalase activity except IR-9. Furthermore, the isolates showed variable antagonistic effects against different bacterial pathogens. All of the strains produced indole acetic acid (IAA), and the concentrations increased in the presence of tryptophan application. The isolates showed the amylase enzyme activity and maximum activity of isolates was achieved in 4% starch concentration. The IR-9 isolate showed the highest amylase activity of 5.9 U/ml. The V_max values of the extracellular amylase from different bacterial isolates ranged between 12.90 and 50.00 IU ml⁻¹. The lowest K_m value of 6.33 mg starch was recorded for IR-8 and the maximum K_cat value of 2.50 min⁻¹ was observed for IR-3. The amylase activity of the isolates was significantly affected by a range of different incubation time, temperature, and pH values. Further tests are required before the potential utilization of these isolates for amylase production, and in the biopesticide and biofertilizer applications.     

Evidence for Microbial Fe(III) Reduction in Anoxic, Mining-Impacted Lake Sediments (Lake Coeur d'Alene, Idaho)

Mining-impacted sediments of Lake Coeur d'Alene, Idaho, contain more than 10% metals on a dry weight basis, approximately 80% of which is iron. Since iron (hydr)oxides adsorb toxic, ore-associated elements, such as arsenic, iron (hydr)oxide reduction may in part control the mobility and bioavailability of these elements. Geochemical and microbiological data were collected to examine the ecological role of dissimilatory Fe(III)-reducing bacteria in this habitat. The concentration of mild-acid-extractable Fe(II) increased with sediment depth up to 50 g kg⁻¹, suggesting that iron reduction has occurred recently. The maximum concentrations of dissolved Fe(II) in interstitial water (41 mg liter⁻¹) occurred 10 to 15 cm beneath the sediment-water interface, suggesting that sulfidogenesis may not be the predominant terminal electron-accepting process in this environment and that dissolved Fe(II) arises from biological reductive dissolution of iron (hydr)oxides. The concentration of sedimentary magnetite (Fe₃O₄), a common product of bacterial Fe(III) hydroxide reduction, was as much as 15.5 g kg⁻¹. Most-probable-number enrichment cultures revealed that the mean density of Fe(III)-reducing bacteria was 8.3 × 10⁵ cells g (dry weight) of sediment⁻¹. Two new strains of dissimilatory Fe(III)-reducing bacteria were isolated from surface sediments. Collectively, the results of this study support the hypothesis that dissimilatory reduction of iron has been and continues to be an important biogeochemical process in the environment examined.     

Reduced Sialylation Impacts Ventricular Repolarization by Modulating Specific K+ Channel Isoforms Distinctly

Voltage-gated K⁺ channels (K_v) are responsible for repolarizing excitable cells and can be heavily glycosylated. Cardiac K_v activity is indispensable where even minimal reductions in function can extend action potential duration, prolong QT intervals, and ultimately contribute to life-threatening arrhythmias. Diseases such as congenital disorders of glycosylation often cause significant cardiac phenotypes that can include arrhythmias. Here we investigated the impact of reduced sialylation on ventricular repolarization through gene deletion of the sialyltransferase ST3Gal4. ST3Gal4-deficient mice (ST3Gal4^−/−) had prolonged QT intervals with a concomitant increase in ventricular action potential duration. Ventricular apex myocytes isolated from ST3Gal4^−/− mice demonstrated depolarizing shifts in activation gating of the transient outward (I_to) and delayed rectifier (I_Kslow) components of K⁺ current with no change in maximum current densities. Consistently, similar protein expression levels of the three K_v isoforms responsible for I_to and I_Kslow were measured for ST3Gal4^−/− versus controls. However, novel non-enzymatic sialic acid labeling indicated a reduction in sialylation of ST3Gal4^−/− ventricular K_v4.2 and K_v1.5, which contribute to I_to and I_Kslow, respectively. Thus, we describe here a novel form of regulating cardiac function through the activities of a specific glycogene product. Namely, reduced ST3Gal4 activity leads to a loss of isoform-specific K_v sialylation and function, thereby limiting K_v activity during the action potential and decreasing repolarization rate, which likely contributes to prolonged ventricular repolarization. These studies elucidate a novel role for individual glycogene products in contributing to a complex network of cardiac regulation under normal and pathologic conditions.     

Maize Yield Response to Water Supply and Fertilizer Input in a Semi-Arid Environment of Northeast China

Maize grain yield varies highly with water availability as well as with fertilization and relevant agricultural management practices. With a 311-A optimized saturation design, field experiments were conducted between 2006 and 2009 to examine the yield response of spring maize (Zhengdan 958, Zea mays L) to irrigation (I), nitrogen fertilization (total nitrogen, urea-46% nitrogen,) and phosphorus fertilization (P₂O₅, calcium superphosphate-13% P₂O₅) in a semi-arid area environment of Northeast China. According to our estimated yield function, the results showed that N is the dominant factor in determining maize grain yield followed by I, while P plays a relatively minor role. The strength of interaction effects among I, N and P on maize grain yield follows the sequence N+I >P+I>N+P. Individually, the interaction effects of N+I and N+P on maize grain yield are positive, whereas that of P+I is negative. To achieve maximum grain yield (10506.0 kg·ha⁻¹) for spring maize in the study area, the optimum application rates of I, N and P are 930.4 m³·ha⁻¹, 304.9 kg·ha⁻¹ and 133.2 kg·ha⁻¹ respectively that leads to a possible economic profit (EP) of 10548.4 CNY·ha⁻¹ (CNY, Chinese Yuan). Alternately, to obtain the best EP (10827.3 CNY·ha⁻¹), the optimum application rates of I, N and P are 682.4 m³·ha⁻¹, 241.0 kg·ha⁻¹ and 111.7 kg·ha⁻¹ respectively that produces a potential grain yield of 10289.5 kg·ha⁻¹.     

Effects of high potassium iodate intake on iodine metabolism and antioxidant capacity in rats

BackgroundKIO₃ and KI are the most common salt iodization agents. Coincidentally, iodine exists naturally in high-iodine drinking water in the form of iodide (I⁻) or iodate (IO₃⁻). As an oxidizing substance, IO₃⁻ should be reduced to I⁻ before it can be effectively used by the thyroid. However, there is a lack of systematic studies on the metabolic process of high dose KIO₃ in vivo.MethodsThe iodine metabolism processes in the thyroid and serum of rats after high KIO₃ intake were determined using high-performance liquid chromatography-inductively coupled plasma-mass spectrometry (HPLC/ICP–MS) and arsenic cerium catalytic spectrophotometry. The changes of redox activity in the serum, thyroid, liver, and kidneys were observed by detecting total antioxidative activity (TAA).ResultsHigh doses of IO₃⁻ were completely reduced to I⁻ in vivo within 0.5 h. The level of organic bound iodine in the serum was stable, while the organic bound iodine in the thyroid increased to a plateau after intake of high-dose KIO₃. The levels of total iodine and I⁻ in serum and thyroid increased quickly, then all decreased after reaching the maximum absorption peak, and I⁻ had two absorption peaks in serum. The thyroid blocking dose of I⁻ was 0.5 mg/kg in rat. Additionally, high KIO₃ intake did not influence the TAA in serum and other tissues.ConclusionThe body is able to reduce and utilize high doses of KIO₃ ingested through the digestive tract. The metabolism of high KIO₃ in vivo is characterized by two absorption process of I⁻ in serum and the thyroid blocking effect. Moreover, a single intake of high-dose KIO₃ does not affect TAA in vivo. The results suggest that such excess IO₃⁻ may have be reduced in the digestive tract before I⁻ enters the blood.     

Cloning,purification, kinetic and anion inhibition studies of a recombinant β-carbonic anhydrase from the Atlantic salmon parasite platyhelminth Gyrodactylus salaris

A β-class carbonic anhydrase (CA, EC 4.2.1.1) was cloned from the genome of the Monogenean platyhelminth Gyrodactylus salaris, a parasite of Atlantic salmon. The new enzyme, GsaCAβ has a significant catalytic activity for the physiological reaction, CO₂ + H₂O ⇋ HCO₃⁻ + H⁺ with a k_cat of 1.1 × 10⁵ s⁻¹ and a k_cat/K_m of 7.58 × 10⁶ M⁻¹ × s⁻¹. This activity was inhibited by acetazolamide (K_I of 0.46 µM), a sulphonamide in clinical use, as well as by selected inorganic anions and small molecules. Most tested anions inhibited GsaCAβ at millimolar concentrations, but sulfamide (K_I of 81 µM), N,N-diethyldithiocarbamate (K_I of 67 µM) and sulphamic acid (K_I of 6.2 µM) showed a rather efficient inhibitory action. There are currently very few non-toxic agents effective in combating this parasite. GsaCAβ is subsequently proposed as a new drug target for which effective inhibitors can be designed.     

The G-protein–gated K+ channel,IKACh, is required for regulation of pacemaker activity and recovery of resting heart rate after sympathetic stimulation

Parasympathetic regulation of sinoatrial node (SAN) pacemaker activity modulates multiple ion channels to temper heart rate. The functional role of the G-protein–activated K⁺ current (I_KACh) in the control of SAN pacemaking and heart rate is not completely understood. We have investigated the functional consequences of loss of I_KACh in cholinergic regulation of pacemaker activity of SAN cells and in heart rate control under physiological situations mimicking the fight or flight response. We used knockout mice with loss of function of the Girk4 (Kir3.4) gene (Girk4^−/− mice), which codes for an integral subunit of the cardiac I_KACh channel. SAN pacemaker cells from Girk4^−/− mice completely lacked I_KACh. Loss of I_KACh strongly reduced cholinergic regulation of pacemaker activity of SAN cells and isolated intact hearts. Telemetric recordings of electrocardiograms of freely moving mice showed that heart rate measured over a 24-h recording period was moderately increased (10%) in Girk4^−/− animals. Although the relative extent of heart rate regulation of Girk4^−/− mice was similar to that of wild-type animals, recovery of resting heart rate after stress, physical exercise, or pharmacological β-adrenergic stimulation of SAN pacemaking was significantly delayed in Girk4^−/− animals. We conclude that I_KACh plays a critical role in the kinetics of heart rate recovery to resting levels after sympathetic stimulation or after direct β-adrenergic stimulation of pacemaker activity. Our study thus uncovers a novel role for I_KACh in SAN physiology and heart rate regulation.     

Impact of Bacterial NO3? Transport on Sediment Biogeochemistry

Experiments demonstrated that Beggiatoa could induce a H₂S-depleted suboxic zone of more than 10 mm in marine sediments and cause a divergence in sediment NO₃⁻ reduction from denitrification to dissimilatory NO₃⁻ reduction to ammonium. pH, O₂, and H₂S profiles indicated that the bacteria oxidized H₂S with NO₃⁻ and transported S⁰ to the sediment surface for aerobic oxidation.     

Dissimilatory Selenate Reduction Potentials in a Diversity of Sediment Types

We measured potential rates of bacterial dissimilatory reduction of ⁷⁵SeO₄²⁻ to ⁷⁵Se⁰ in a diversity of sediment types, with salinities ranging from freshwater (salinity = 1 g/liter) to hypersaline (salinity = 320 g/liter and with pH values ranging from 7.1 to 9.8. Significant biological selenate reduction occurred in all samples with salinities from 1 to 250 g/liter but not in samples with a salinity of 320 g/liter. Potential selenate reduction rates (25 nmol of SeO₄²⁻ per ml of sediment added with isotope) ranged from 0.07 to 22 μmol of SeO₄²⁻ reduced liter⁻¹ h⁻¹. Activity followed Michaelis-Menten kinetics in relation to SeO₄²⁻ concentration (K_m of selenate = 7.9 to 720 μM). There was no linear correlation between potential rates of SeO₄²⁻ reduction and salinity, pH, concentrations of total Se, porosity, or organic carbon in the sediments. However, potential selenate reduction was correlated with apparent K_m for selenate and with potential rates of denitrification (r = 0.92 and 0.81, respectively). NO₃⁻, NO₂⁻, MoO₄²⁻, and WO₄²⁻ inhibited selenate reduction activity to different extents in sediments from both Hunter Drain and Massie Slough, Nev. Sulfate partially inhibited activity in sediment from freshwater (salinity = 1 g/liter) Massie Slough samples but not from the saline (salinity = 60 g/liter) Hunter Drain samples. We conclude that dissimilatory selenate reduction in sediments is widespread in nature. In addition, in situ selenate reduction is a first-order reaction, because the ambient concentrations of selenium oxyanions in the sediments were orders of magnitude less than their K_ms.     

The Impact of Gamma Radiation on Sediment Microbial Processes

Microbial communities have the potential to control the biogeochemical fate of some radionuclides in contaminated land scenarios or in the vicinity of a geological repository for radioactive waste. However, there have been few studies of ionizing radiation effects on microbial communities in sediment systems. Here, acetate and lactate amended sediment microcosms irradiated with gamma radiation at 0.5 or 30 Gy h⁻¹ for 8 weeks all displayed NO₃⁻ and Fe(III) reduction, although the rate of Fe(III) reduction was decreased in 30-Gy h⁻¹ treatments. These systems were dominated by fermentation processes. Pyrosequencing indicated that the 30-Gy h⁻¹ treatment resulted in a community dominated by two Clostridial species. In systems containing no added electron donor, irradiation at either dose rate did not restrict NO₃⁻, Fe(III), or SO₄²⁻ reduction. Rather, Fe(III) reduction was stimulated in the 0.5-Gy h⁻¹-treated systems. In irradiated systems, there was a relative increase in the proportion of bacteria capable of Fe(III) reduction, with Geothrix fermentans and Geobacter sp. identified in the 0.5-Gy h⁻¹ and 30-Gy h⁻¹ treatments, respectively. These results indicate that biogeochemical processes will likely not be restricted by dose rates in such environments, and electron accepting processes may even be stimulated by radiation.     

Permeation and Block of the Skeletal Muscle Chloride Channel,ClC-1, by Foreign Anions

A distinctive feature of the voltage-dependent chloride channels ClC-0 (the Torpedo electroplaque chloride channel) and ClC-1 (the major skeletal muscle chloride channel) is that chloride acts as a ligand to its own channel, regulating channel opening and so controlling the permeation of its own species. We have now studied the permeation of a number of foreign anions through ClC-1 using voltage-clamp techniques on Xenopus oocytes and Sf9 cells expressing human (hClC-1) or rat (rClC-1) isoforms, respectively. From their effect on channel gating, the anions presented in this paper can be divided into three groups: impermeant or poorly permeant anions that can not replace Cl⁻ as a channel opener and do not block the channel appreciably (glutamate, gluconate, HCO₃ ⁻, BrO₃ ⁻); impermeant anions that can open the channel and show significant block (methanesulfonate, cyclamate); and permeant anions that replace Cl⁻ at the regulatory binding site but impair Cl⁻ passage through the channel pore (Br⁻, NO₃ ⁻, ClO₃ ⁻, I⁻, ClO₄ ⁻, SCN⁻). The permeability sequence for rClC-1, SCN⁻ ∼ ClO₄ ⁻ > Cl⁻ > Br⁻ > NO₃ ⁻ ∼ ClO₃ ⁻ > I⁻ >> BrO₃ ⁻ > HCO₃ ⁻ >> methanesulfonate ∼ cyclamate ∼ glutamate, was different from the sequence determined for blocking potency and ability to shift the P _open curve, SCN⁻ ∼ ClO₄ ⁻ > I⁻ > NO₃ ⁻ ∼ ClO₃ ⁻ ∼ methanesulfonate > Br⁻ > cyclamate > BrO₃ ⁻ > HCO₃ ⁻ > glutamate, implying that the regulatory binding site that opens the channel is different from the selectivity center and situated closer to the external side. Channel block by foreign anions is voltage dependent and can be entirely accounted for by reduction in single channel conductance. Minimum pore diameter was estimated to be ∼4.5 Å. Anomalous mole-fraction effects found for permeability ratios and conductance in mixtures of Cl⁻ and SCN⁻ or ClO₄ ⁻ suggest a multi-ion pore. Hydrophobic interactions with the wall of the channel pore may explain discrepancies between the measured permeabilities of some anions and their size.