Iron-limiting Conditions Research Articles

The heterologous expression of a plastocyanin in the diatom Phaeodactylum tricornutum improves cell growth under iron-deficient conditions.

We have investigated if the heterologous expression of a functional green alga plastocyanin in the diatom Phaeodactylum tricornutum can improve photosynthetic activity and cell growth. Previous in vitro assays showed that a single-mutant of the plastocyanin from the green algae Chlamydomonas reinhardtii is effective in reducing P. tricornutum photosystem I. In this study, in vivo assays with P. tricornutum strains expressing this plastocyanin indicate that even the relatively low intracellular concentrations of holo-plastocyanin detected (≈4 μM) are enough to promote an increased growth (up to 60%) under iron-deficient conditions as compared with the WT strain, measured as higher cell densities, content in pigments and active photosystem I, global photosynthetic rates per cell, and even cell volume. In addition, the presence of plastocyanin as an additional photosynthetic electron carrier seems to decrease the over-reduction of the plastoquinone pool. Consequently, it promotes an improvement in the maximum quantum yield of both photosystem II and I, together with a decrease in the acceptor side photoinhibition of photosystem II-also associated to a reduced oxidative stress-a decrease in the peroxidation of membrane lipids in the choroplast, and a lower degree of limitation on the donor side of photosystem I. Thus the heterologous plastocyanin appears to act as a functional electron carrier, alternative to the native cytochrome c6 , under iron-limiting conditions.

Role of aspartate ammonia-lyase in Pasteurella multocida

BackgroundPasteurella multocida is responsible for a highly infectious and contagious disease in birds, leading to heavy economic losses in the chicken industry. However, the pathogenesis of this disease is poorly understood. We recently identified an aspartate ammonia-lyase (aspA) in P. multocida that was significantly upregulated under iron-restricted conditions, the protein of which could effectively protect chicken flocks against P. multocida. However, the functions of this gene remain unclear. In the present study, we constructed aspA mutant strain △aspA::kan and complementary strain C△aspA::kan to investigate the function of aspA in detail.ResultDeletion of the aspA gene in P. multocida resulted in a significant reduction in bacterial growth in LB (Luria-Bertani) and MH (Mueller-Hinton) media, which was rescued by supplementation with 20 mM fumarate. The mutant strain △aspA::kan showed significantly growth defects in anaerobic conditions and acid medium, compared with the wild-type strain. Moreover, growth of △aspA::kan was more seriously impaired than that of the wild-type strain under iron-restricted conditions, and this growth recovered after supplementation with iron ions. AspA transcription was negatively regulated by iron conditions, as demonstrated by quantitative reverse transcription-polymerase chain reaction. Although competitive index assay showed the wild-type strain outcompetes the aspA mutant strain and △aspA::kan was significantly more efficient at producing biofilms than the wild-type strain, there was no significant difference in virulence between the mutant and the wild-type strains.ConclusionThese results demonstrate that aspA is required for bacterial growth in complex medium, and under anaerobic, acid, and iron-limited conditions.

Virulence-associated genes of avian pathogenic E. coli isolated from chickens with colibacillosis in Enugu State, Nigeria

Background: Strains of Escherichia coli that cause diseases in chickens are collectively known as avian pathogenic E. coli (APEC) and are one of the most reported poultry diseases associated with morbidity, mortality, reduced egg hatchability and condemnation of carcasses at processing. Their pathogenesis and virulence properties are mediated by several genes responsible for adherence to the respiratory tract, resistance to the host immunological defenses, multiplication under iron-restricted conditions and production of cytotoxic effects. The objective of this study was to ascertain the virulence associated genes in the APEC strains from chickens in Enugu State, Nigeria. Methods & Materials: Dead and moribund chickens submitted to the Necropsy Unit of the Department of Veterinary Pathology and Microbiology, University of Nigeria, Nsukka from various farms across Enugu State between January 2011 and December 2013 were sampled for the study. Liver, heart, spleen and oviduct samples from 182 chickens diagnosed of colibacillosis were collected and processed for isolation and identification of APEC strains following standard bacteriological procedures. Virulence genes were detected using multiplex polymerase chain reaction on isolated APEC strains targeting 12 virulence genes (FimH, pap, felA, sfa, fac, csgA, tsh, cvaC, kpsII, iss, iutA, and cnf). Data generated were analyzed by Chi square and correlation coefficient tests using Graph Pad Prism 5. Significance was accepted at 5% probability level. Results: Avian pathogenic E. coli strains were isolated from 52 of the 182 chickens examined. Out of the 12 virulence genes investigated, 9 (FimH, pap, felA, csgA, tsh, cvaC, kpsII, iss and iutA) were detected, with FimH gene being the most prevalent. The virulence genes occurred in various combinations, with fimH-csgA-iss being the most predominant. No significant association (P > 0.05) was found between the virulence genes and the chicken type and source of isolation. Conclusion: The distributions of virulence genes were similar to other extraintestinal or commensal E. coli from humans and other animals suggesting a zoonotic potential. The different pattern of a subset of putative genes implies divergent mechanisms of infection, and control could be challenging.

Hemeprotein Tpx1 interacts with cell-surface heme transporter Str3 in Schizosaccharomyces pombe.

Str3 is a transmembrane protein that mediates low-affinity heme uptake in Schizosaccharomyces pombe. Under iron-limiting conditions, Str3 remains at the cell surface in the presence of increasing hemin concentrations. Using a proximity-dependent biotinylation approach coupled to mass spectrometry and coimmunoprecipitation assays, we report that the peroxiredoxin Tpx1 is a binding partner of Str3. Under microaerobic conditions, cells deficient in heme biosynthesis and lacking the heme receptor Shu1 exhibit poor hemin-dependent growth in the absence of Tpx1. Analysis of membrane protein preparations from iron-starved hem1Δ shu1Δ str3Δ tpx1Δ cells coexpressing Str3-GFP and TAP-Tpx1 showed that TAP-Tpx1 is enriched in membrane protein fractions in response to hemin. Bimolecular fluorescence complementation assays brought additional evidence that an interaction between Tpx1 and Str3 occurs at the plasma membrane. Results showed that Tpx1 exhibits an equilibrium constant value of 0.26μM for hemin. The association of Tpx1 with hemin protects hemin from degradation by H2 O2 . The peroxidase activity of hemin is lowered when it is bound to Tpx1. Taken together, these results revealed that Tpx1 is a novel interacting partner of Str3. Our data are the first example of an interaction between a cytoplasmic heme-binding protein and a cell-surface heme transporter.

Reduction in intestinal colonization and invasion of internal organs after challenge by homologous and heterologous serovars of Salmonella enterica following vaccination of chickens with a novel trivalent inactivated Salmonella vaccine

ABSTRACT A novel inactivated vaccine, comprising three serovars of Salmonella enterica (Enteritidis, serogroup O:9; Typhimurium, serogroup O:4; Infantis, serogroup O:7) grown under conditions of iron restriction and adjuvanted with aluminium hydroxide, was evaluated for efficacy following challenge by homologous and heterologous serovars. Chickens were vaccinated at 6 and 10 weeks of age by the intramuscular route and challenged 4 to 9 weeks after the second vaccination with serovars belonging to serogroup O:9 (Enteritidis), O:4 (Typhimurium and Heidelberg), O:7 (Infantis and Virchow), and O:8 (Hadar). All vaccinated birds produced a marked systemic antibody response against each of the component vaccine antigens by the time of challenge. Significant reductions in both colonization of the intestinal tract and invasion of internal organs were observed in vaccinated birds compared with non-vaccinated controls, irrespective of the challenge serovar. The findings suggest that broad serovar protection within the constitutive serogroups of an inactivated multi-valent vaccine is possible and could, therefore, play an important role in future Salmonella control programmes. RESEARCH HIGHLIGHTS Novel inactivated trivalent Salmonella chicken vaccine was developed and tested. Vaccine induced marked systemic antibody response against all vaccine antigens. Significant reductions in intestinal tract colonization and internal organ invasion. Vaccine efficacy demonstrated against homologous and heterologous serovars.

Phenotypic and proteomic approaches of the response to iron-limited condition in Staphylococcus lugdunensis

BackgroundStaphylococcus lugdunensis is a coagulase-negative Staphylococcus part of the commensal skin flora but emerge as an important opportunistic pathogen. Because iron limitation is a crucial stress during infectious process, we performed phenotypic study and compared proteomic profiles of this species incubated in absence and in presence of the iron chelator 2,2′-dipyridyl (DIP).ResultsNo modification of cell morphology nor cell wall thickness were observed in presence of DIP. However iron-limitation condition promoted biofilm formation and reduced the ability to cope with oxidative stress (1 mM H2O2). In addition, S. lugdunensis N920143 cultured with DIP was significantly less virulent in the larvae of Galleria mellonella model of infection than that grown under standard conditions. We verified that these phenotypes were due to an iron limitation by complementation experiments with FeSO4. By mass spectrometry after trypsin digestion, we characterized the first iron-limitation stress proteome in S. lugdunensis. Among 1426 proteins identified, 349 polypeptides were differentially expressed. 222 were more and 127 less abundant in S. lugdunensis incubated in iron-limitation condition, and by RT-qPCR, some of the corresponding genes have been shown to be transcriptionally regulated. Our data revealed that proteins involved in iron metabolism and carriers were over-expressed, as well as several ABC transporters and polypeptides linked to cell wall metabolism. Conversely, enzymes playing a role in the oxidative stress response (especially catalase) were repressed.ConclusionsThis phenotypic and global proteomic study allowed characterization of the response of S. lugdunensis to iron-limitation. We showed that iron-limitation promoted biofilm formation, but decrease the oxidative stress resistance that may, at least in part, explained the reduced virulence of S. lugdunensis observed under low iron condition.

Enterobactin-specific antibodies inhibit in vitro growth of different gram-negative bacterial pathogens

Enterobactin (Ent)-mediated high affinity iron acquisition is critically important for Gram-negative bacterial pathogens to survive and infect the host. Recently, we reported an efficient method to prepare novel Ent conjugate vaccines for inducing high level of Ent-specific antibodies, which displayed similar bacteriostatic feature as lipocalins, the host innate immune effectors with potent Ent-binding ability. The Ent-specific antibodies also showed a significant advantage over lipocalins by cross-reacting to various Ent derivatives including salmochelins, the glycosylated Ent that can help enteric pathogens evade the siderophore sequestration by host lipocalins. To demonstrate significant potential of the Ent conjugate vaccine for broader applications to prevent and control various Gram-negative infections in human and animal, in this study, we examined inhibitory effect of Ent-specific antibodies on the in vitro growth of three significant Gram-negative pathogens: Escherichia coli (n = 27), Salmonella enterica (n = 8), and Campylobacter spp. (n = 6). The tested strains were diverse with respect to hosts, geographical origins, serotypes, infection sites and siderophore productions. The Ent-specific antibodies significantly suppressed the growth of each tested strain under iron-restricted conditions. For example, the Ent-specific antibodies consistently exerted 2–5 log10 units of growth reduction on most tested avian pathogenic E. coli (9 of 10 strains) isolated in five countries. Despite various dynamic growth responses observed, notably, the Ent-specific antibodies displayed significantly higher magnitude of growth reduction than lipocalin-2 (up to 5 log10 units of difference) on majority of tested E. coli and S. enterica, which is likely due to sequestration of other siderophores (e.g., salmochelins) by the Ent-specific antibodies. Production of a variety of major siderophores by the tested E. coli and S. enterica strains was examined and confirmed by ultra high performance liquid chromatography-high resolution mass spectrometry analysis. Collectively, this study provides critical and compelling in vitro evidence supporting the feasibility of Ent-based immune interventions against several Gram-negative pathogens.

Functional characterization of the AGL1 aegerolysin in the mycoparasitic fungus Trichoderma atroviride reveals a role in conidiation and antagonism

Aegerolysins are small secreted pore-forming proteins that are found in both prokaryotes and eukaryotes. The role of aegerolysins in sporulation, fruit body formation, and in lysis of cellular membrane is suggested in fungi. The aim of the present study was to characterize the biological function of the aegerolysin gene agl1 in the mycoparasitic fungus Trichoderma atroviride, used for biological control of plant diseases. Gene expression analysis showed higher expression of agl1 during conidiation and during growth in medium supplemented with cell wall material from the plant pathogenic fungus Rhizoctonia solani as the sole carbon source. Expression of agl1 was supressed under iron-limiting condition, while agl1 transcript was not detected during T. atroviride interactions with the prey fungi Botrytis cinerea or R. solani. Phenotypic analysis of agl1 deletion strains (Δagl1) showed reduced conidiation compared to T. atroviride wild type, thus suggesting the involvement of AGL1 in conidiation. Furthermore, the Δagl1 strains display reduced antagonism towards B. cinerea and R. solani based on a secretion assay, although no difference was detected during direct interactions. These data demonstrate the role of AGL1 in conidiation and antagonism in the mycoparasitic fungus T. atroviride.

Extracellular heme recycling and sharing across species by novel mycomembrane vesicles of a Gram-positive bacterium

Microbes spontaneously release membrane vesicles (MVs), which play roles in nutrient acquisition and microbial interactions. Iron is indispensable for microbes, but is a difficult nutrient to acquire. However, whether MVs are also responsible for efficient iron uptake and therefore involved in microbial interaction remains to be elucidated. Here, we used a Gram-positive strain, Dietzia sp. DQ12-45-1b, to analyze the function of its MVs in heme-iron recycling and sharing between species. We determined the structure and constituent of MVs and showed that DQ12-45-1b releases MVs originating from the mycomembrane. When comparing proteomes of MVs between iron-limiting and iron-rich conditions, we found that under iron-limiting conditions, heme-binding proteins are enriched. Next, we proved that MVs participate in extracellular heme capture and transport, especially in heme recycling from environmental hemoproteins. Finally, we found that the heme carried in MVs is utilized by multiple species, and we further verified that membrane fusion efficiency and species evolutionary distance determine heme delivery. Together, our findings strongly suggest that MVs act as a newly identified pathway for heme recycling, and represent a public good shared between phylogenetically closely related species.

Siderophore piracy enhances Vibrio cholerae environmental survival and pathogenesis.

Vibrio cholerae, the aetiological agent of cholera, possesses multiple iron acquisition systems, including those for the transport of siderophores. How these systems benefit V. cholerae in low-iron, polymicrobial communities in environmental settings or during infection remains poorly understood. Here, we demonstrate that in iron-limiting conditions, co-culture of V. cholerae with a number of individual siderophore-producing microbes significantly promoted V. cholerae growth in vitro. We further show that in the host environment with low iron, V. cholerae colonizes better in adult mice in the presence of the siderophore-producing commensal Escherichia coli. Taken together, our results suggest that in aquatic reservoirs or during infection, V. cholerae may overcome environmental and host iron restriction by hijacking siderophores from other microbes.

Nocardamine-Dependent Iron Uptake in Pseudomonas aeruginosa: Exclusive Involvement of the FoxA Outer Membrane Transporter.

Iron is a key nutrient for almost all living organisms. Paradoxically, it is poorly soluble and consequently poorly bioavailable. Bacteria have thus developed multiple strategies to access this metal. One of the most common consists of the use of siderophores, small compounds that chelate ferric iron with very high affinity. Many bacteria are able to produce their own siderophores or use those produced by other microorganisms (exosiderophores) in a piracy strategy. Pseudomonas aeruginosa produces two siderophores, pyoverdine and pyochelin, and is also able to use a large panel of exosiderophores. We investigated the ability of P. aeruginosa to use nocardamine (NOCA) and ferrioxamine B (DFOB) as exosiderophores under iron-limited planktonic growth conditions. Proteomic and RT-qPCR approaches showed induction of the transcription and expression of the outer membrane transporter FoxA in the presence of NOCA or DFOB in the bacterial environment. Expression of the proteins of the heme- or pyoverdine- and pyochelin-dependent iron uptake pathways was not affected by the presence of these two tris-hydroxamate siderophores. 55Fe uptake assays using foxA mutants showed ferri-NOCA to be exclusively transported by FoxA, whereas ferri-DFOB was transported by FoxA and at least one other unidentified transporter. The crystal structure of FoxA complexed with NOCA-Fe revealed very similar siderophore binding sites between NOCA-Fe and DFOB-Fe. We discuss iron uptake by hydroxamate exosiderophores in P. aeruginosa cells in light of these results.

The role of Enterococcus faecalis during co-infection with avian pathogenic Escherichia coli in avian colibacillosis

ABSTRACT Enterococcus spp. (ENT) are frequently co-isolated with avian pathogenic E. coli (APEC) from poultry with colibacillosis, a leading cause of flock mortality. Although largely overlooked, ENT may play an active role in these infections. To assess the frequency of ENT co-isolation in colibacillosis, cultures were collected from birds with gross lesions of omphalitis, polyserositis, and septicaemia over a 3-year period from three turkey flocks and three broiler flocks. In birds diagnosed with colibacillosis based on gross findings and isolation of E. coli, ENT were co-isolated with APEC in 35.7% (n = 41/115) of colibacillosis mortality and 3.7% of total mortality (n = 41/1122). Co-isolated APEC and ENT pairs (n = 41) were further characterized using antimicrobial resistance phenotyping and in vitro co-culture assays. E. faecalis (EF) was the most commonly co-isolated species (68% n = 28/41) and tetracycline resistance was the resistance phenotype most commonly found among APEC (51% n = 21/41) and ENT (93% n = 38/41). Under iron-restricted conditions, EF enhanced APEC growth in a proximity-dependent manner and APEC grown in mixed culture with EF exhibited a significant growth and survival advantage (P ≤ 0.01). In an embryo lethality assay, APEC co-infection with EF resulted in decreased survival of broiler embryos compared to mono-infections (P ≤ 0.05). These data demonstrate that EF augmented APEC survival and growth under iron limiting conditions, possibly translating to the increased virulence of APEC in broiler embryos. Thus, ENT co-infections may be a previously unrecognized contributor to colibacillosis-related mortality. Further investigations into the mechanism of this interaction are warranted. RESEARCH HIGHLIGHTS Enterococcus is frequently co-isolated with avian pathogenic E. coli (APEC). Enterococcus faecalis (EF) enhances survival of APEC in iron restricted conditions. EF co-infection increases APEC virulence in broiler embryos.

Ferritinophagy is not required for colon cancer cell growth.

Ferritinophagy is a form of selective autophagy responsible for degrading intracellular ferritin, mediated by nuclear receptor coactivator 4 (NCOA4). NCOA4 plays significant roles in systemic iron homeostasis, and its disruption leads to simultaneous anemia and susceptibility to iron overload. The importance of iron colorectal cancer pathogenesis is well studied; however, the role of ferritinophagy in colon cancer cell growth has not been assessed. Disruption of ferritinophagy via NCOA4 knockout leads to only marginal differences in growth under basal and iron-restricted conditions. Moreover, NCOA4 played no significant role in cell death induced by 5-fluorouracil and erastin. Western blotting analysis for ferritin and transferrin receptor 1 found a dose-dependent effect on expression in both proteins in wild-type and NCOA4 knockout cell lines, but further investigation revealed no difference in growth response when treated at both high and low doses. Our data demonstrate a marginal role for ferritinophagy in growth both under normal and cytotoxic conditions in colon cancer cells, as well as a possible compensatory mechanism in colon cancer cells in response to ferroptosis induction.

A copper site is required for iron transport by the periplasmic proteins P19 and FetP.

Campylobacter jejuni is a leading cause of food-borne gastrointestinal disease in humans and uropathogenic Escherichia coli is a leading cause of urinary tract infections. Both human pathogens harbour a homologous iron uptake system (termed cjFetM-P19 in C. jejuni and ecFetM-FetP in E. coli). Although these systems are important for growth under iron limitation, the mechanisms by which these systems function during iron transport remain undefined. The copper ions bound to P19 and FetP, the homologous periplasmic proteins, are coordinated in an uncommon penta-dentate manner involving a Met-Glu-His3 motif and exhibit positional plasticity. Here we demonstrate the function of the Met and Glu residues in modulating copper binding and controlling copper positioning through site-directed variants, binding assays, and crystal structures. Growth of C. jejuni strains with these p19 variants is impaired under iron limited conditions as compared to the wild-type strain. Additionally, an acidic residue-rich secondary site is required for binding iron and function in vivo. Finally, western blot analyses demonstrate direct and specific interactions between periplasmic P19 and FetP with the large periplasmic domain of their respective inner membrane transporters cjFetM and ecFetM.

Iron acquisition system of Sphingobium sp. strain SYK-6, a degrader of lignin-derived aromatic compounds

Iron, an essential element for all organisms, acts as a cofactor of enzymes in bacterial degradation of recalcitrant aromatic compounds. The bacterial family, Sphingomonadaceae comprises various degraders of recalcitrant aromatic compounds; however, little is known about their iron acquisition system. Here, we investigated the iron acquisition system in a model bacterium capable of degrading lignin-derived aromatics, Sphingobium sp. strain SYK-6. Analyses of SYK-6 mutants revealed that FiuA (SLG_34550), a TonB-dependent receptor (TBDR), was the major outer membrane iron transporter. Three other TBDRs encoded by SLG_04340, SLG_04380, and SLG_10860 also participated in iron uptake, and tonB2 (SLG_34540), one of the six tonB comprising the Ton complex which enables TBDR-mediated transport was critical for iron uptake. The ferrous iron transporter FeoB (SLG_36840) played an important role in iron uptake across the inner membrane. The promoter activities of most of the iron uptake genes were induced under iron-limited conditions, and their regulation is controlled by SLG_29410 encoding the ferric uptake regulator, Fur. Although feoB, among all the iron uptake genes identified is highly conserved in Sphingomonad strains, the outer membrane transporters seem to be diversified. Elucidation of the iron acquisition system promises better understanding of the bacterial degradation mechanisms of aromatic compounds.

A Summary of the 2020 Gastric Cancer Summit at Stanford University

There exists no coherent national strategy for the early detection or prevention of gastric cancer in the United States (US), even among identified high-risk groups such as Asian Americans, African Americans, Hispanic Americans, and Alaska Native/American Indian peoples. As a result, patients with gastric cancer in the US are diagnosed at later stages and demonstrate worse overall survival compared to nations of East Asia with established screening programs (Table 1). The under-recognition of gastric cancer risk within minority communities is a significant unaddressed healthcare disparity.

Bioactivity of Serratiochelin A, a Siderophore Isolated from a Co-Culture of Serratia sp. and Shewanella sp.

Siderophores are compounds with high affinity for ferric iron. Bacteria produce these compounds to acquire iron in iron-limiting conditions. Iron is one of the most abundant metals on earth, and its presence is necessary for many vital life processes. Bacteria from the genus Serratia contribute to the iron respiration in their environments, and previously several siderophores have been isolated from this genus. As part of our ongoing search for medicinally relevant compounds produced by marine microbes, a co-culture of a Shewanella sp. isolate and a Serratia sp. isolate, grown in iron-limited conditions, was investigated, and the rare siderophore serratiochelin A (1) was isolated with high yields. Compound 1 has previously been isolated exclusively from Serratia sp., and to our knowledge, there is no bioactivity data available for this siderophore to date. During the isolation process, we observed the degradation product serratiochelin C (2) after exposure to formic acid. Both 1 and 2 were verified by 1-D and 2-D NMR and high-resolution MS/MS. Here, we present the isolation of 1 from an iron-depleted co-culture of Shewanella sp. and Serratia sp., its proposed mechanism of degradation into 2, and the chemical and biological characterization of both compounds. The effects of 1 and 2 on eukaryotic and prokaryotic cells were evaluated, as well as their effect on biofilm formation by Staphylococcus epidermidis. While 2 did not show bioactivity in the given assays, 1 inhibited the growth of the eukaryotic cells and Staphylococcus aureus.

Expression of SidD gene and physiological characterization of the rhizosphere plant growth-promoting yeasts

There is increasing evidence that rhizosphere microbes contribute to the stress mitigation process, but the mechanisms of this plant-microbe interaction are not yet understood. Siderophores-producing microorganisms have been considered important for enhancing metal tolerance in plants. In this study, rhizosphere yeasts were isolated from wheat (Triticum aestivum L.) and examined for siderophores production and heavy metal resistance. Out of thirty-five isolates, only eight yeast strains showed heavy metal-resistance and plant-growth promotion properties. The highest inorganic phosphate-solubilization was shown by Trichosporon ovoides IFM 63839 (2.98 mg ml−1) and Saccharomyces cerevisiae FI25-1F (2.54 mg ml−1). Two strains, namely YEAST-6 and YEAST-16 showed high siderophore production and heavy metal-resistance, were investigated for sidD gene expression under different levels of Cd2+ and Pb2+ toxicity stress. The heavy metal-resistant yeast strains were characterized and identified based on the phenotypic characteristics and their 18S rRNA genes sequence. SidD gene expression was induced by yeasts growing under iron-limiting conditions and excess of other heavy metal, suggesting that expression of sidD gene increases in the presence of 600–800 μM heavy metal but under iron limitation. Extensive studies of the microbe-plant micronutrient interactions will enrich our understanding of the rhizosphere role in the terms of plant growth promotion.

Deglacial Si remobilisation from the deep-ocean reveals biogeochemical and physical controls on glacial atmospheric CO2 levels

During the last glacial period, the sluggish deep Ocean circulation sequestered carbon into the abyss leading to the lowering of atmospheric CO2. The impact of this redistribution on biologically essential nutrients remains poorly constrained. Using sedimentary δ30Si of diatoms and biogenic accumulation rates in the Eastern Equatorial Pacific (EEP), we present evidences for the remobilisation of dissolved Silica (DSi) along with carbon from the deep ocean during the Last Deglaciation. Because DSi is essential for diatoms growing in the surface ocean, its concentration in the abyss during the glacial periods amounts to a negative feedback on the oceanic CO2 uptake. However, this effect can be muted by the increased Fe inputs during glacial periods which reduces diatom Si requirements in Fe limited regions such as the EEP. Our results from the EEP suggest that the efficiency of the biological CO2 pump and the size of the local CO2 source is tightly controlled by changes in DSi utilisation driven by Fe availability across the last glacial-interglacial transition.We use a modified PANDORA box model to illustrate that the inventory of DSi in the global ocean surface is controlled by Fe availability in HNLC areas rather than by straightforward Si supply though upwelling. The Holocene is characterised by a fast mode of Si cycling driven by high biological requirement for Si under conditions of iron limitation and efficient overturning, promoting CO2 outgassing and an inefficient biological C pump via the rapid exhaustion of DSi in the surface. The last glacial period saw slower marine Si cycling as a result of decreased DSi biological requirement under Fe-replete conditions in the sea surface and increased Si and CO2 sequestration in the abyssal ocean. The switch between the two modes of Si cycling happened at 15 ka BP, i.e. mid-deglaciation, and resulted in contrasting biological carbon drawdown responses in the EEP and globally between both phases of the deglacial CO2 rise. This illustrates that in addition to deep-sea CO2 storage and overturning, the efficiency of the biological pump also plays a crucial role in determining ocean-atmosphere CO2 exchange and shows the dual controls of ocean circulation and Fe-Si availability in this process.

The functional identification of Dps in oxidative stress resistance and virulence of Riemerella anatipestifer CH-1 using a new unmarked gene deletion strategy

Excessive iron in the bacterial cytoplasm can potentiate the production of harmful reactive oxygen species (ROS). Riemerella anatipestifer (R. anatipestifer, RA), a gram-negative bacterium, encodes an iron uptake system, but its iron detoxification mechanism is unknown. Here, the dps gene of R. anatipestifer CH-1 (RA-CH-1) was deleted using sacB as a counterselection marker. The dps mutant was more sensitive to H2O2 than the wild type in iron-rich conditions but not in iron-limited conditions, suggesting that Dps prevents H2O2-induced damage through iron binding. However, the dps mutant and wild type were identically sensitive to bactericidal antibiotics, and antibiotic treatment did not enhance RA-CH-1 ROS production. Furthermore, Dps prevents DNA damage by binding DNA. The RA-CH-1 dps transcript level was higher in the stationary phase than in the early and exponential phases and was increased by OxyR in the presence of H2O2. Finally, duckling colonization by the dps mutant was similar to that by the wild type at 48 h postinfection but significantly lower at 60 h postinfection, suggesting that RA-CH-1 Dps is not involved in host invasion but increases resistance to host clearance. Dps thus likely plays an important role in R. anatipestifer physiology and pathogenesis through protecting against oxidative stress.