Ion Uptake Mechanisms Research Articles

Molecular insights into ferric-siderophore transport by the putative TonB-dependent transporter in Mycobacterium tuberculosis

Iron acquisition is critical to the virulence of Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis. To acquire iron within the host, Mtb secretes siderophores that chelate iron with high affinity. Siderophores scavenge iron from host cells using TonB-dependent transporters like FecA. We investigated molecular mechanisms of FecA-mediated ferric-siderophore transport in Mtb. Molecular docking and molecular dynamics simulations revealed a series of interactions between ferric siderophores and FecA. The initial binding occurs at a pocket located on the extracellular surface of FecA. The ligand then migrates deeper through the transport tunnel to a subsequent binding site, aided by conformational changes in FecA that expand the tunnel diameter. We observed the key roles of precise positioning of extracellular loops in the outer membrane barrel and plug domains in the optimal ligand binding and transport. Transport of ferric–siderophore complex into Mtb follows an induced fit model, with ligand interaction eliciting 2–10 Å shifts in the barrel and plug regions. By revealing the conformational dynamics enabling iron import, these findings provide molecular-level insights into a metal ion uptake mechanism in Mtb. Iron acquisition is essential for Mtb pathogenesis, so this work may inform novel therapeutic strategies that disrupt siderophore uptake pathways. Communicated by Ramaswamy H. Sarma

Impact of Various Levels of Salinity Stress on Growth Attributes of Some Salt-Sensitive and Tolerant Varieties of Mung Bean

Salinity stress poses a significant threat to crop productivity, and understanding the mechanisms of ion uptake and distribution in different cultivars is crucial for developing salt-tolerant varieties. This study investigates the patterns of ion uptake and distribution in salt-sensitive and salt-tolerant cultivars of Vigna radiata, commonly known as mung bean. Hydroponic experiments were conducted under controlled conditions to assess the responses of two contrasting cultivars to varying salt concentrations. The study reveals distinct differences in the ion uptake and distribution between salt-sensitive and salt-tolerant cultivars of V. radiata. Salt-sensitive cultivars exhibited increased sodium (Na+) accumulation in both roots and shoots under saline conditions, leading to disrupted ion homeostasis. In contrast, salt-tolerant cultivars demonstrated efficient ion regulation mechanisms, with a higher capacity for potassium (K+) retention and a lower Na+ to K+ ratio. The study also shed light on the involvement of ion transporters and channels in facilitating differential ion accumulation, highlighting the molecular mechanisms contributing to salt tolerance in Vigna radiata. These findings provide valuable insights into the adaptive strategies of salt-tolerant cultivars, offering a basis for targeted breeding programs aimed at enhancing salinity resilience in this economically significant legume. In summary, this research deepens our understanding of the intricate relationship between ion uptake and distribution in Vigna radiata varieties with varying salt tolerance. It lays the groundwork for developing robust crop varieties capable of thriving in saline environments.

Anomalously enhanced ion transport and uptake in functionalized angstrom-scale two-dimensional channels.

Emulating angstrom-scale dynamics of the highly selective biological ion channels is a challenging task. Recent work on angstrom-scale artificial channels has expanded our understanding of ion transport and uptake mechanisms under confinement. However, the role of chemical environment in such channels is still not well understood. Here, we report the anomalously enhanced transport and uptake of ions under confined MoS2-based channels that are ~five angstroms in size. The ion uptake preference in the MoS2-based channels can be changed by the selection of surface functional groups and ion uptake sequence due to the interplay between kinetic and thermodynamic factors that depend on whether the ions are mixed or not prior to uptake. Our work offers a holistic picture of ion transport in 2D confinement and highlights ion interplay in this regime.

Mechanisms of Na+ uptake from freshwater habitats in animals.

Life in fresh water is osmotically and energetically challenging for living organisms, requiring increases in ion uptake from dilute environments. However, mechanisms of ion uptake from freshwater environments are still poorly understood and controversial, especially in arthropods, for which several hypothetical models have been proposed based on incomplete data. One compelling model involves the proton pump V-type H+ ATPase (VHA), which energizes the apical membrane, enabling the uptake of Na+ (and other cations) via an unknown Na+ transporter (referred to as the "Wieczorek Exchanger" in insects). What evidence exists for this model of ion uptake and what is this mystery exchanger or channel that cooperates with VHA? We present results from studies that explore this question in crustaceans, insects, and teleost fish. We argue that the Na+/H+ antiporter (NHA) is a likely candidate for the Wieczorek Exchanger in many crustaceans and insects; although, there is no evidence that this is the case for fish. NHA was discovered relatively recently in animals and its functions have not been well characterized. Teleost fish exhibit redundancy of Na+ uptake pathways at the gill level, performed by different ion transporter paralogs in diverse cell types, apparently enabling tolerance of low environmental salinity and various pH levels. We argue that much more research is needed on overall mechanisms of ion uptake from freshwater habitats, especially on NHA and other potential Wieczorek Exchangers. Such insights gained would contribute greatly to our general understanding of ionic regulation in diverse species across habitats.

Determination of trace metals in vegetables using ICP-MS

Determination of trace metals in vegetables using ICP-MS

Endophytic halotolerant Bacillus velezensis FMH2 alleviates salt stress on tomato plants by improving plant growth and altering physiological and antioxidant responses

Salinity stress has significant deleterious effects on agricultural lands and plant yields. Plants undergo a series of physiological and molecular changes to reduce salt-induced damage. However, these mechanisms remain insufficient. The inoculation of plant growth promoting bacteria to improve plant health under stress conditions offers promise. Bacillus velezensis FMH2 has been shown to protect tomato fruits against black mold disease and to improve seed tolerance to abiotic stresses. During this study, the major physiological and metabolic changes connected with FMH2 mitigation of abiotic stress tolerance in tomato plants were explored. In presence of different salt levels, FMH2 showed a high potentiality to colonize internal plant tissues and to produce several plant growth promoting metabolites such as siderophores, indole acetic acid, and hydrolytic enzymes. FMH2-treatment promoted plant growth (root structure, plant elongation, leaf emission, fresh and dry weights, water content, etc.) in absence as well as in presence of salt stress. FMH2 treatment decreased endogenous Na+ accumulation and increased K+ and Ca2+ uptake. Furthermore, B. velezensis FMH2-treatment improved chlorophyll contents, membrane integrity and phenol peroxidase concentrations, and reduced malondialdehyde and hydrogen peroxide levels under saline conditions with a significant salinity × strain interaction. The present study suggests the endophytic strain FMH2 involved different mechanisms and regulatory functions to enhance plant oxidative systems and regulate ion uptake mechanisms supporting both growth and stress management.

Temperature Effects During a Sublethal Chronic Metal Mixture Exposure on Common Carp (Cyprinus carpio).

The aquatic environment is the final sink of various pollutants including metals, which can pose a threat for aquatic organisms. Waterborne metal mixture toxicity might be influenced by environmental parameters such as the temperature. In the present study, common carp were exposed for 27 days to a ternary metal mixture of Cu, Zn, and Cd at two different temperatures, 10 and 20°C. The exposure concentrations represent 10% of the 96 h-LC50 (concentration lethal for the 50% of the population in 96 h) for each metal (nominal metal concentrations of Cu: 0.08 μM; Cd: 0.02 μM and Zn: 3 μM). Metal bioaccumulation and toxicity as well as changes in the gene expression of enzymes responsible for ionoregulation and induction of defensive responses were investigated. Furthermore the hepatosomatic index and condition factor were measured as crude indication of overall health and energy reserves. The obtained results showed a rapid Cu and Cd increase in the gills at both temperatures. Cadmium accumulation was higher at 20°C compared to 10°C, whereas Cu and Zn accumulation was not, suggesting that at 20°C, fish had more efficient depuration processes for Cu and Zn. Electrolyte (Ca, Mg, Na, and K) levels were analyzed in different tissues (gills, liver, brain, muscle) and in the remaining carcasses. However, no major electrolyte losses were observed. The toxic effect of the trace metal ion mixture on major ion uptake mechanisms may have been compensated by ion uptake from the food. Finally, the metal exposure triggered the upregulation of the metallothionein gene in the gills as defensive response for the organism. These results, show the ability of common carp to cope with these metal levels, at least under the condition used in this experiment.

Profile of Mary Lou Guerinot

On the first Earth Day, April 22, 1970, Mary Lou Guerinot decided that she would major in biology in college in the hope of promoting environmental sustainability. As the 50th anniversary of Earth Day approaches, Guerinot, now a professor of biological sciences at Dartmouth College and a member of the National Academy of Sciences, is well on her way toward achieving that goal through her work on the molecular mechanisms of metal ion uptake and its regulation. Guerinot’s work is laying the foundation for environmentally sustainable, nutrient-dense crops, as well as plant-based solutions for removal of toxic metals from soil. Her Inaugural Article (1) reports the identification of a transcription factor essential for plant growth under iron deficiency. The finding moves her team closer to understanding the iron homeostasis pathway in plants, holding promise for improving agricultural productivity and human health. Mary Lou Guerinot. Image courtesy of Olga Zhaxybayeva (Dartmouth College, Hanover, NH). Guerinot was raised in upstate New York, where she attended the Rochester-based Catholic all-girls St. Agnes High School. “Unlike the boys’ Catholic high school nearby, it was basically science-light,” she says. “We didn’t have physics or calculus, and very few students went on to college to study science.” She and a few other students urged their math teacher to let them study calculus on their own. The hard work on this subject and others paid off, and Guerinot was accepted at Cornell University’s College of Agriculture and Life Sciences. She adhered to her decision to major in biology and earned a Bachelor of Science degree with distinction at Cornell in 1975. Four years later, Guerinot earned a doctorate in biology from Dalhousie University under the direction of David Patriquin, studying the sea urchin–lobster–kelp ecosystem. Envisioning life as a marine microbiologist, Guerinot did a postdoctoral fellowship at the …

Potassium supply reduces cesium uptake in Konara oak not by an alteration of uptake mechanism, but by the uptake competition between the ions

Radiocesium contamination of forests has been a severe problem after the Fukushima Daiichi Nuclear Power Plant accident in 2011. Bed logs of Konara oak (Quercus serrata Murray), used for mushroom cultivation, were an economically important product from the forests prior to their contamination. One of the potential countermeasures to reduce radiocesium content in trees is potassium fertilization, but the evidence for the effect of K+ in reducing Cs+ uptake has not been obtained yet in the woody plant. Therefore, we investigated the ability of rhizospheric K+ to suppress uptake and translocation of Cs+ in Konara oak seedlings through hydroponic experiments in order to clarify the effect of K+. Elemental analysis showed that the seedlings cultivated for 4 weeks under low-K (K+ = 50 μM) contained higher amount of Cs comparing to the seedlings cultivated under high-K (K+ = 3 mM). Then, the uptake rate of Cs+ and K+ in the seedlings from the solution having 50 μM K+ and 0.1 μM Cs+ was calculated using radioactive 137Cs+ and 42K+ to evaluate the effect of growth condition on the ion uptake mechanism. The interference between Cs+ and K+ at the site of root uptake was also evaluated based on the Cs+ and K+ uptake rates at K+ concentrations of 50 μM, 200 μM, and 3 mM in the seedlings grown under the medium-K (K+ = 200 μM) condition. As a result, the Cs+ uptake rate at 50 μM K+ was not influenced by the growth condition, whereas Cs+ uptake decreased when the uptake solution itself was supplemented with 3 mM K+. In addition, the Cs/K ratio in the seedlings was found to rise to exceed the Cs/K ratio in the culture solution as the rhizospheric K+ concentration increased, which was in contrast with previous findings in herbaceous plants. Our experiments demonstrated the first direct evidence for woody plants that a high K+ concentration can suppress Cs accumulation in Konara oak and that it was derived from competition for uptake between K+ and Cs+ in the rhizosphere, not from the growth K+ condition.

Ion uptake pathways in European sea bass Dicentrarchus labrax

Ion uptake mechanisms are diverse in fish species, certainly linked to duplication events that have led to the presence of a multitude of paralogous genes. In fish, Na+ uptake involves several ion transporters expressed in different ionocyte subtypes. In the European sea bass Dicentrarchus labrax, several key transporters potentially involved in Na+ uptake have been investigated in seawater (SW) and following a 2 weeks freshwater (FW) acclimation. Using gel electrophoresis, we have shown that the Na+/H+-exchanger 3 (nhe3, slc9a3) is expressed in gills and kidney at both salinities. Quantitative realtime PCR analysis showed a significantly higher nhe3 expression in fresh water (FW) compared to SW. Its apical localization in a subset of gill ionocytes in freshwater-acclimated fish supports the role of NHE3 in Na+ uptake. Interestingly, NHE3-immunopositive cells also express basolateral Na+/K+/2Cl− cotransporter 1 (NKCC1) and are mainly localized in gill lamella. Among the three nhe2 (slc9a2) paralogs, only nhe2c shows differential branchial expression levels with higher mRNA levels in SW than in FW. The increased branchial expression of the ammonia transporter rhcg1 (Rhesus protein), nhe3 and cytoplasmic carbonic anhydrase (cac) in FW could indicate the presence of a functional coupling between ion transporters to form a Na+/NH4+ exchange complex. Acid-sensing ion channel 4 (asic4) seems not to be expressed in sea bass gills. Na+/Cl- cotransporter (ncc2a or ncc-like) is about three times more expressed in FW compared to SW suggesting coupled Na+ and Cl− uptake in a subset of gill ionocytes. Besides the main pump Na+/K+-ATPase, branchial NCC2a and NHE3 may be key players in ion uptake in sea bass following a long-term freshwater challenge.

Simultaneous Adsorption and Incorporation of Sr2+ at the Barite (001)–Water Interface

Ionically bonded minerals are ubiquitous and play a determinative role in controlling the mobility of toxic metals in natural environments. However, little is known about the mechanism of ion uptake by these mineral surfaces. Here, the sorption of strontium ions (Sr2+) to the barite (001)–water interface was studied using a combination of synchrotron X-ray scattering and three types of computational simulations (density functional theory, classical molecular dynamics (CMD), and CMD-metadynamics). In situ resonant anomalous X-ray reflectivity (RAXR) revealed that Sr2+ adsorbed on the barite surface as inner-sphere surface complexes and was incorporated within the outermost barite atomic layers. Density functional theory combined with CMD simulations confirmed the thermodynamic stability of these species, demonstrating almost equal magnitudes in the free energy of sorption between these species. Metadynamics simulations showed a more detailed feature in the free energy landscape for metal adsorption where a...

Review on Biosorption of Arsenic From Contaminated Water

Arsenic contamination in water is a worldwide problem. It is spreading in the environment as a result of numerous natural and anthropogenic sources having a massive influence on health of human and ecological systems. In order to mitigate this problem, there are various conventional technologies available for the removal of arsenic like oxidation, coagulation and flocculation, ion exchange, and membrane processes. However, there are various limitations associated with these techniques such as high cost and generation of toxic byproducts due to which attention has been paid to biosorption in the past few years, as it is an eco‐friendly as well as cost‐effective technique. Arsenic biosorption by various low‐cost adsorbents and biosorbents such as activated carbons, algae, submerged plants, bacteria, fungi, plant, and fruit wastes have been surveyed and critically reviewed in this paper along with their sorption efficiencies. Furthermore, the nonlinear models of adsorption dynamics and thermodynamic feasibility of arsenic biosorption together with detailed analysis of biological mechanisms of arsenic ion uptake by bacteria, fungi and algae has been discussed in detail in the present work. Alternatives of disposal of arsenic loaded sludge obtained after the biosorption of arsenic has also been addressed.

Manganese scavenging and oxidative stress response mediated by type VI secretion system in Burkholderia thailandensis

Type VI secretion system (T6SS) is a versatile protein export machinery widely distributed in Gram-negative bacteria. Known to translocate protein substrates to eukaryotic and prokaryotic target cells to cause cellular damage, the T6SS has been primarily recognized as a contact-dependent bacterial weapon for microbe-host and microbial interspecies competition. Here we report contact-independent functions of the T6SS for metal acquisition, bacteria competition, and resistance to oxidative stress. We demonstrate that the T6SS-4 in Burkholderia thailandensis is critical for survival under oxidative stress and is regulated by OxyR, a conserved oxidative stress regulator. The T6SS-4 is important for intracellular accumulation of manganese (Mn2+) under oxidative stress. Next, we identified a T6SS-4-dependent Mn2+-binding effector TseM, and its interacting partner MnoT, a Mn2+-specific TonB-dependent outer membrane transporter. Similar to the T6SS-4 genes, expression of mnoT is regulated by OxyR and is induced under oxidative stress and low Mn2+ conditions. Both TseM and MnoT are required for efficient uptake of Mn2+ across the outer membrane under Mn2+-limited and -oxidative stress conditions. The TseM-MnoT-mediated active Mn2+ transport system is also involved in contact-independent bacteria-bacteria competition and bacterial virulence. This finding provides a perspective for understanding the mechanisms of metal ion uptake and the roles of T6SS in bacteria-bacteria competition.

Толерaнтные к тяжелым метaллaм рaстения кaк состaвляющaя экологически чистых технологий

Industrial centers are areas of greatest pollution of various media with heavy metals. Near large industrialcenters, large number of heavy metals enter the soil, wherein soils of agrocenoses and cultivatedcrops can be polluted. A long period of self-cleaning of soils and the high cost of their artificial cleaningmake mankind seek for new ways to solve the problem associated with the contamination of soils withheavy metals. The most promising direction in this area is the study of the genetic potential of plants andthe identification of plant objects characterized by a minimum accumulation of heavy metals.The review presents data on the genotypic specificity of the adaptive potential of plants. Differenttypes of plants have different biological characteristics that determine the availability of heavy metals.The flow of heavy metals into plants depends on the characteristics of the absorption of an element bydifferent plant species. Soils characterized by a high polymetallic content of metals are distinguished bythe formation of specific communities.Not only different species, but also varieties of one species differ in ability to accumulate ions ofheavy metals, even with the same concentration in the soil. It is known that the ability to absorb, accumulate,and use chemical elements in plants is genetically determined. The physiological causes of thegenotypic specificity of the absorption of elements of mineral nutrition by plants are ambiguous: theycan be caused by differences, both in the primary mechanisms of ion uptake, and in their subsequenttransport and metabolism.Researchers identify selectively important indicators of technologically resistant varieties: plasticity,duration of vegetation period, yield, size and selectivity of root systems, detoxification and localizationof ecotoxicants in certain parts of the plant. An effective strategy to reduce yield losses is the use of varietiesthat have genes of tolerance or resistance to biotic and abiotic influences.

Layer Number Dependence of Li+ Intercalation on Few-Layer Graphene and Electrochemical Imaging of Its Solid-Electrolyte Interphase Evolution

Bulk and surface electrochemical reactivity converge on ultrathin electrochemical interfaces such as few-layer graphene (FLG). In spite of the many recent works regarding the use of exfoliated graphene materials for charge storage, we still lack a detailed mechanistic understanding of how Li+ intercalates in FLG. Li+ intercalation in graphitic carbon is known to proceed through a staging mechanism, where intercalation occurs at selected interlayers at a time, rather than randomly on any available empty interlayer galleries, until reaching the nominal composition LiC6. Thus, on graphite this process is strongly dependent on the number of neighboring empty galleries at any given time. FLG naturally displays a limited number of galleries, therefore, we hypothesize that this process will also necessarily be distributed unevenly. In this study, we applied FLG (1-10 layers) to explore the alkaline ion uptake mechanisms. Chemical vapor deposition methods with varies recipes were applied to grow double and multilayer graphene. By multicycle wet transfer of bilayer graphene, 2-8 layer graphene with well-defined area and low defect density. Direct growth of multilayer graphene (>10 layers) serves as control group. Photolithography method was used to generate micro patterns on graphene as “point of entry” for Li+intercalation. Lithium ion intercalation on FLG, as measured via cyclic voltammetry, shows a staging-type process, which has strong dependence on the number of layers of graphene sheets that compose the electrode. On a graphene electrode with less than five layers, the numbers of intercalation/de-intercalation peaks were limited due to the lack of graphene interlayer galleries to stabilize Li ions. Those peaks broaden and increase in number as the layer number increases to six. Despite these mechanistic differences on ion intercalation, the formation of a solid-electrolyte interphase (SEI) was observed on all electrodes. Scanning electrochemical microscopy (SECM) in the feedback mode was used to demonstrate spatial resolved changes in the surface conductivity of FLG during SEI evolution. The fabrication of “ionic channels” on the FLG electrode provided preferential positions for Li ion insertion. The role of these channels can be evidenced using a Li-ion sensitive SECM imaging technique through localized flux measurements. This work highlights the impact of nano-structure and micro-structure on macroscopic electrochemical behavior and provides guidance to the mechanistic control of ion intercalation using atomically thin graphene interface. Figure 1

Cooking-Induced Corrosion of Metals

Uptake of metal ions into rice occurs while it is being cooked in metal vessels, leading to corrosion of the utensils. This study deals with silver, copper, and aluminum uptake during cooking in respective vessels, with a special emphasis on silver. The metal uptake is routed through solution, enhanced in the presence of specific anions like carbonate, and attenuated when the rice is polished. The concentration of silver in rice increases with the time of cooking with a concomitant decrease in the concentrations of Fe and Zn, suggesting a substitution mechanism for metal ion uptake. The results for some common rice varieties of use across the Indian subcontinent are presented. Similar behavior was observed for cooking in copper and aluminum vessels. Among the three metals studied, aluminum showed reduced uptake. Studies have been done to probe the interaction of metal ions with glucose and sucrose, and efficient complex formation was detected with all these ions, implying that starch can also form complex...

Removal of toxic aqueous ions using amorphous AlPO4

ABSTRACTAluminium phosphate (AlPO4) was characterized using X-ray diffractometry (XRD), Fourier Transform infrared (FTIR), point of zero charge (PZC) and dissolution studies. XRD showed the sample to be amorphous, FTIR confirmed the presence of OH groups on the surface and PZC was determined at pH 3.45. The dissolution study illustrated a decrease in dissolution with an increase in the pH. Potentiometric titration data were fit to the Gaines–Thomas equation, which showed that AlPO4 is a weakly acidic ion exchanger. Sorption studies were carried out at pH 4–6 and temperatures 293–323 K. The uptake of metal ions was observed to increase with an increase in the pH and temperature. The surface selectivity towards metal ions was found in the order Pb2+> Cu2+> Cd2+. Sorption data were fit to the new equation derived from the proposed mechanism for metal ion uptake. Various parameters such as stoichiometry of the surface H+ ion release, equilibrium constant, standard enthalpy, entropy and free energy changes were evaluated from the plots. The values of all these parameters were found to be closely related to the values reported in the literature.

FLUID AND ION SECRETION BY MALPIGHIAN TUBULES OF LARVAL CHIRONOMIDS, Chironomus riparius: EFFECTS OF REARING SALINITY, TRANSPORT INHIBITORS, AND SEROTONIN.

Larvae of Chironomus riparius respond to ion-poor and brackish water (IPW, BW) conditions by activating ion uptake mechanisms in the anal papillae and reducing ion absorption at the rectum, respectively. The role that the Malpighian tubules play in ion and osmoregulation under these conditions is not known in this species. This study examines rates of fluid secretion and major cation composition of secreted fluid from tubules of C.riparius reared in IPW, freshwater (FW) and BW. Fluid secretion of tubules from FW and BW larvae was similar but tubules from IPW larvae secrete fluid at higher rates, are more sensitive to serotonin stimulation, and the secreted fluid contains less Na(+) . Therefore in IPW, tubules work in concert with anal papillae to eliminate excess water while conserving Na(+) in the hemolymph. Tubules do not appear to play a significant role in ion/osmoregulation under BW. Serotonin immunoreactivity in the nervous system and gastrointestinal tract of larval C. riparius was similar to that seen in mosquito larvae with the exception that the hindgut was devoid of staining. Hemolymph serotonin titer was similar in FW and IPW; hence, serotonin is not responsible for the observed high rates of fluid secretion in IPW. Instead, it is suggested that serotonin may work in a synergistic manner with an unidentified hormonal factor in IPW. Ion transport mechanisms in the tubules of C. riparius are pharmacologically similar to those of other insects.

Effects of salinity on upstream-migrating, spawning sea lamprey, Petromyzon marinus.

The sea lamprey, Petromyzon marinus, is an anadromous, semelparous species that is vulnerable to endangered in parts of its native range due in part to loss of spawning habitat because of man-made barriers. The ability of lampreys to return to the ocean or estuary and search out alternative spawning river systems would be limited by their osmoregulatory ability in seawater. A reduction in tolerance to salinity has been documented in migrants, although the underlying mechanisms have not been characterized. We examined the capacity for marine osmoregulation in upstream spawning migrants by characterizing the physiological effects of salinity challenge from a molecular perspective. Estuarine-captured migrants held in freshwater (FW) for ∼1 week (short-term acclimation) or 2 months (long-term acclimation) underwent an incremental salinity challenge until loss of equilibrium occurred and upper thresholds of 25 and 17.5, respectively, occurred. Regardless of salinity tolerance, all lamprey downregulated FW ion-uptake mechanisms [gill transcripts of Na(+):Cl(-) cotransporter (NCC/slc12a3) and epithelial Na(+) channel (ENaC/scnn1) and kidney Na(+)/K(+)-ATPase (NKA) protein and activity but not transcript]. At their respective salinity limits, lamprey displayed a clear osmoregulatory failure and were unable to regulate [Na(+)] and [Cl(-)] in plasma and intestinal fluid within physiological limits, becoming osmocompromised. A >90% drop in haematocrit indicated haemolysis, and higher plasma concentrations of the cytosolic enzymes alanine aminotransferase, aspartate aminotransferase and lactate dehydrogenase indicated damage to other tissues, including liver. However, >80% of short-term FW-acclimated fish were able to osmoregulate efficiently, with less haemolysis and tissue damage. This osmoregulatory ability was correlated with significant upregulation of the secretory form of Na(+):K(+):2Cl(-) cotransporter (NKCC1/slc12a2) transcript levels and the re-emergence of seawater-type ionocytes detected through immunohistochemical NKA immunoreactivity in the gill, the central ionoregulatory organ. This work sheds light on the molecular and physiological limits to the potential return to seawater for lampreys searching for alternative FW systems in which to spawn.

Kinetic Analysis of H(+)-Na(+) Selectivity in a Light-Driven Na(+)-Pumping Rhodopsin.

Krokinobacter eikastus rhodopsin 2 (KR2) is a recently identified light-driven Na(+) pump from a marine bacterium. KR2 pumps Na(+) in NaCl solution but pumps H(+) in the absence of Na(+) and Li(+). The Na(+) transport mechanism in KR2 has been extensively studied, whereas understanding of the H(+) transport mechanism is very limited. Here we studied ion uptake mechanisms and H(+)-Na(+) selectivity using flash photolysis. The results show that decay of the blue-shifted M intermediate is dependent on both [Na(+)] and [H(+)], indicating that KR2 competitively uptakes Na(+) or H(+) upon M decay. Comprehensive concentration dependence of Na(+) and H(+) revealed that the rate constant of H(+) uptake (kH) was much larger than that of Na(+) uptake (kNa) with a ratio (kH/kNa) of >10(3). Therefore, KR2 pumps only H(+) when Na(+) and H(+) concentrations are similar. On the contrary, KR2 pumps Na(+) exclusively under physiological conditions in which [Na(+)] is much greater than [H(+)].