Coexistence of Microaerophilic,Nitrate-Reducing,and Phototrophic Fe(II) Oxidizers and Fe(III) Reducers in Coastal Marine Sediment

Iron is abundant in sediments, where it can be biogeochemically cycled between its divalent and trivalent redox states. The neutrophilic microbiological Fe cycle involves Fe(III)-reducing and three different physiological groups of Fe(II)-oxidizing microorganisms, i.e., microaerophilic, anoxygenic phototrophic, and nitrate-reducing Fe(II) oxidizers. However, it is unknown whether all three groups coexist in one habitat and how they are spatially distributed in relation to gradients of O₂, light, nitrate, and Fe(II). We examined two coastal marine sediments in Aarhus Bay, Denmark, by cultivation and most probable number (MPN) studies for Fe(II) oxidizers and Fe(III) reducers and by quantitative-PCR (qPCR) assays for microaerophilic Fe(II) oxidizers. Our results demonstrate the coexistence of all three metabolic types of Fe(II) oxidizers and Fe(III) reducers. In qPCR, microaerophilic Fe(II) oxidizers (Zetaproteobacteria) were present with up to 3.2 × 10⁶ cells g dry sediment⁻¹. In MPNs, nitrate-reducing Fe(II) oxidizers, anoxygenic phototrophic Fe(II) oxidizers, and Fe(III) reducers reached cell numbers of up to 3.5 × 10⁴, 3.1 × 10², and 4.4 × 10⁴ g dry sediment⁻¹, respectively. O₂ and light penetrated only a few millimeters, but the depth distribution of the different iron metabolizers did not correlate with the profile of O₂, Fe(II), or light. Instead, abundances were homogeneous within the upper 3 cm of the sediment, probably due to wave-induced sediment reworking and bioturbation. In microaerophilic Fe(II)-oxidizing enrichment cultures, strains belonging to the Zetaproteobacteria were identified. Photoferrotrophic enrichments contained strains related to Chlorobium and Rhodobacter; the nitrate-reducing Fe(II) enrichments contained strains related to Hoeflea and Denitromonas. This study shows the coexistence of all three types of Fe(II) oxidizers in two near-shore marine environments and the potential for competition and interrelationships between them.     

Oxidation of Fe(II)‐EDTA by nitrite and by two nitrate‐reducing Fe(II)‐oxidizing Acidovorax strains

The enzymatic oxidation of Fe(II) by nitrate‐reducing bacteria was first suggested about two decades ago. It has since been found that most strains are mixotrophic and need an additional organic co‐substrate for complete and prolonged Fe(II) oxidation. Research during the last few years has tried to determine to what extent the observed Fe(II) oxidation is driven enzymatically, or abiotically by nitrite produced during heterotrophic denitrification. A recent study reported that nitrite was not able to oxidize Fe(II)‐EDTA abiotically, but the addition of the mixotrophic nitrate‐reducing Fe(II)‐oxidizer, Acidovorax sp. strain 2AN, led to Fe(II) oxidation (Chakraborty & Picardal, 2013). This, along with other results of that study, was used to argue that Fe(II) oxidation in strain 2AN was enzymatically catalyzed. However, the absence of abiotic Fe(II)‐EDTA oxidation by nitrite reported in that study contrasts with previously published data. We have repeated the abiotic and biotic experiments and observed rapid abiotic oxidation of Fe(II)‐EDTA by nitrite, resulting in the formation of Fe(III)‐EDTA and the green Fe(II)‐EDTA‐NO complex. Additionally, we found that cultivating the Acidovorax strains BoFeN1 and 2AN with 10 mm nitrate, 5 mm acetate, and approximately 10 mm Fe(II)‐EDTA resulted only in incomplete Fe(II)‐EDTA oxidation of 47–71%. Cultures of strain BoFeN1 turned green (due to the presence of Fe(II)‐EDTA‐NO) and the green color persisted over the course of the experiments, whereas strain 2AN was able to further oxidize the Fe(II)‐EDTA‐NO complex. Our work shows that the two used Acidovorax strains behave very differently in their ability to deal with toxic effects of Fe‐EDTA species and the further reduction of the Fe(II)‐EDTA‐NO nitrosyl complex. Although the enzymatic oxidation of Fe(II) cannot be ruled out, this study underlines the importance of nitrite in nitrate‐reducing Fe(II)‐ and Fe(II)‐EDTA‐oxidizing cultures and demonstrates that Fe(II)‐EDTA cannot be used to demonstrate unequivocally the enzymatic oxidation of Fe(II) by mixotrophic Fe(II)‐oxidizers.     

Ecophysiology and the energetic benefit of mixotrophic Fe(II) oxidation by various strains of nitrate-reducing bacteria

In order to assess the importance of nitrate-dependent Fe(II) oxidation and its impact on the growth physiology of dominant Fe oxidizers, we counted these bacteria in freshwater lake sediments and studied their growth physiology. Most probable number counts of nitrate-reducing Fe(II)-oxidizing bacteria in the sediment of Lake Constance, a freshwater lake in Southern Germany, yielded about 10⁵ cells mL⁻¹ of the total heterotrophic nitrate-reducing bacteria, with about 1% (10³ cells mL⁻¹) of nitrate-reducing Fe(II) oxidizers. We investigated the growth physiology of Acidovorax sp. strain BoFeN1, a dominant nitrate-reducing mixotrophic Fe(II) oxidizer isolated from this sediment. Strain BoFeN1 uses several organic compounds (but no sugars) as substrates for nitrate reduction. It also reduces nitrite, dinitrogen monoxide, and O₂, but cannot reduce Fe(III). Growth experiments with cultures amended either with acetate plus Fe(II) or with acetate alone demonstrated that the simultaneous oxidation of Fe(II) and acetate enhanced growth yields with acetate alone (12.5 g dry mass mol⁻¹ acetate) by about 1.4 g dry mass mol⁻¹ Fe(II). Also, pure cultures of Pseudomonas stutzeri and Paracoccus denitrificans strains can oxidize Fe(II) with nitrate, whereas Pseudomonas fluorescens and Thiobacillus denitrificans strains did not. Our study demonstrates that nitrate-dependent Fe(II) oxidation contributes to the energy metabolism of these bacteria, and that nitrate-dependent Fe(II) oxidation can essentially contribute to anaerobic iron cycling.     

Manganese inhibition of microbial iron reduction in anaerobic sediments

Potential mechanisms for the lack of Fe(II) accumulation in Mn(IV)‐con‐taining anaerobic sediments were investigated. The addition of Mn(IV) to sediments in which Fe(III) reduction was the terminal electron‐accepting process removed all the pore‐water Fe(II), completely inhibited net Fe(III) reduction, and stimulated Mn(IV) reduction. In a solution buffered at pH 7, Mn(IV) oxidized Fe(II) to amorphic Fe(III) oxide. Mn(IV) naturally present in oxic freshwater sediments also rapidly oxidized Fe(II). A pure culture of a dissimilatory FE(III)‐ and Mn(FV)‐reducing organism isolated from the sediments reduced Fe(III) to Fe(II) in the presence of Mn(IV) when ferrozine was present to trap Fe(II) before Mn(IV) oxidized it. Depth profiles of dissolved iron and manganese reported in previous studies suggest that Fe(II) diffusing up from the zone of Fe(III) reduction is consumed within the Mn(IV)‐reducing zone. These results demonstrate that preferential reduction of Mn(IV) by Fe(III)‐reducing bacteria cannot completely explain the lack of Fe(II) accumulation in anaerobic, Mn(IV)‐containing sedments, and indicate that Mn(IV) oxidation of Fe(II) is the mechanism that ultimately prevents Fe(II) accumulation.     

Nitrogen‐cycling bacteria and archaea in the carbonate sediment of a coral reef

In the coarse‐grained carbonate sediments of coral reefs, advective porewater flow and the respiration of organic matter establish redox zones that are the scene of microbially mediated transformations of N compounds. To investigate the geobiology of N cycling in reef sediments, the benthic microbiota of Checker Reef in Kaneohe Bay, Hawaii, were surveyed for candidate nitrate reducers, ammonifying nitrite reducers, aerobic and anaerobic ammonia oxidizers (anammox) by identifying phylotypes of their key metabolic genes (napA, narG, nrfA, amoA) and ribotypes (unique RNA sequences) of anammox‐like 16S rRNA. Putative proteobacteria with the catalytic potential for nitrate reduction were identified in oxic, interfacial and anoxic habitats. The estimated richness of napA (≥202 in anoxic sediment) and narG (≥373 and ≥441 in oxic and interfacial sediment, respectively) indicates a diverse guild of nitrate reducers. The guild of nrfA hosts in interfacial reef sediment was dominated by Vibrio species. The identified members of the aerobic ammonium oxidizing guild (amoA hosts) were Crenarchaeota or close relatives of Nitrosomonadales. Putative anammox bacteria were detected in the RNA pool of Checker Reef sediment. More than half of these ribotypes show ≥90% identity with homologous sequences of Scalindua spp., while no evidence was found for members of the genera Brocadia or Kuenenia. In addition to exploring the diversity of these four nitrogen‐cycling microbial guilds in coral reef sediments, the abundances of aerobic ammonium oxidizers (amoA), nitrite oxidizers (nxrAB), ammonifying nitrite reducers (nrfA) and denitrifiers (nosZ) were estimated using real‐time PCR. Representatives of all targeted guilds were detected, suggesting that most processes of the biogeochemical N cycle can be catalyzed by the benthic microbiota of tropical coral reefs.     

Planktonic marine iron oxidizers drive iron mineralization under low‐oxygen conditions

Observations of modern microbes have led to several hypotheses on how microbes precipitated the extensive iron formations in the geologic record, but we have yet to resolve the exact microbial contributions. An initial hypothesis was that cyanobacteria produced oxygen which oxidized iron abiotically; however, in modern environments such as microbial mats, where Fe(II) and O₂ coexist, we commonly find microaerophilic chemolithotrophic iron‐oxidizing bacteria producing Fe(III) oxyhydroxides. This suggests that such iron oxidizers could have inhabited niches in ancient coastal oceans where Fe(II) and O₂ coexisted, and therefore contributed to banded iron formations (BIFs) and other ferruginous deposits. However, there is currently little evidence for planktonic marine iron oxidizers in modern analogs. Here, we demonstrate successful cultivation of planktonic microaerophilic iron‐oxidizing Zetaproteobacteria from the Chesapeake Bay during seasonal stratification. Iron oxidizers were associated with low oxygen concentrations and active iron redox cycling in the oxic–anoxic transition zone (<3 μm O₂, <0.2 μm H₂S). While cyanobacteria were also detected in this transition zone, oxygen concentrations were too low to support significant rates of abiotic iron oxidation. Cyanobacteria may be providing oxygen for microaerophilic iron oxidation through a symbiotic relationship; at high Fe(II) levels, cyanobacteria would gain protection against Fe(II) toxicity. A Zetaproteobacteria isolate from this site oxidized iron at rates sufficient to account for deposition of geologic iron formations. In sum, our results suggest that once oxygenic photosynthesis evolved, microaerophilic chemolithotrophic iron oxidizers were likely important drivers of iron mineralization in ancient oceans.     

pH and Eh on Aldabra atoll 1. comparison of marine and reshwater environments

An account is given of the range of pH and Eh in marine and freshwater aquatic sites on one atoll (Aldabra, Indian Ocean). There is a very wide range in values of pH and Eh for both marine and freshwater environments, the range for pH in general being wider in freshwater ones and Eh in marine ones. Photosynthetic microbial communities on lagoon sediments may be arranged in an approximate order of their association with sediments of particular Eh values. These are, from high to low: Scytonema sp., pennate diatoms, Schizothrix sp., Microcoleus chthonoplastes, Hyella balani, purple phototrophic bacteria.     

Pacific Northwest Marine Sediments Contain Ammonia-Oxidizing Bacteria in the β Subdivision of the Proteobacteria

The diversity of ammonia-oxidizing bacteria in aquatic sediments was studied by retrieving ammonia monooxygenase and methane monooxygenase gene sequences. Methanotrophs dominated freshwater sediments, while β-proteobacterial ammonia oxidizers dominated marine sediments. These results suggest that γ-proteobacteria such as Nitrosococcus oceani are minor members of marine sediment ammonia-oxidizing communities.     

Anaerobic nitrate-dependent iron(II) bio-oxidation by a novel lithoautotrophic betaproteobacterium, strain 2002

Microbial nitrate-dependent Fe(II) oxidation is known to contribute to iron biogeochemical cycling; however, the microorganisms responsible are virtually unknown. In an effort to elucidate this microbial metabolic process in the context of an environmental system, a 14-cm sediment core was collected from a freshwater lake and geochemically characterized concurrently with the enumeration of the nitrate-dependent Fe(II)-oxidizing microbial community and subsequent isolation of a nitrate-dependent Fe(II)-oxidizing microorganism. Throughout the sediment core, ambient concentrations of Fe(II) and nitrate were observed to coexist. Concomitant most probable number enumeration revealed the presence of an abundant nitrate-dependent Fe(II)-oxidizing microbial community (2.4 x 10(3) to 1.5 x 10(4) cells g(-1) wet sediment) from which a novel anaerobic, lithoautotrophic, Fe(II)-oxidizing bacterium, strain 2002, was isolated. Analysis of the complete 16S rRNA gene sequence revealed that strain 2002 was a member of the beta subclass of the proteobacteria with 94.8% similarity to Chromobacterium violaceum, a bacterium not previously recognized for the ability to oxidize nitrate-dependent Fe(II). Under nongrowth conditions, both strain 2002 and C. violaceum incompletely reduced nitrate to nitrite with Fe(II) as the electron donor, while under growth conditions nitrate was reduced to gaseous end products (N2 and N2O). Lithoautotrophic metabolism under nitrate-dependent Fe(II)-oxidizing conditions was verified by the requirement of CO2 for growth as well as the assimilation of 14C-labeled CO2 into biomass. The isolation of strain 2002 represents the first example of an anaerobic, mesophilic, neutrophilic Fe(II)-oxidizing lithoautotroph isolated from freshwater samples. Our studies further demonstrate the abundance of nitrate-dependent Fe(II) oxidizers in freshwater lake sediments and provide further evidence for the potential of microbially mediated Fe(II) oxidation in anoxic environments.     

Abiotic oxidation of Fe(II) by reactive nitrogen species in cultures of the nitrate‐reducing Fe(II) oxidizer Acidovorax sp. BoFeN1 – questioning the existence of enzymatic Fe(II) oxidation

Nitrate‐reducing, Fe(II)‐oxidizing bacteria were suggested to couple with enzymatic Fe(II) oxidation to nitrate reduction. Denitrification proceeds via intermediates (, NO) that can oxidize Fe(II) abiotically at neutral and particularly at acidic pH. Here, we present a revised Fe(II) quantification protocol preventing artifacts during acidic Fe extraction and evaluate the contribution of abiotic vs. enzymatic Fe(II) oxidation in cultures of the nitrate‐reducing, Fe(II) oxidizer Acidovorax sp. BoFeN1. Sulfamic acid used instead of HCl reacts with nitrite and prevents abiotic Fe(II) oxidation during Fe extraction. Abiotic experiments without sulfamic acid showed that acidification of oxic Fe(II) nitrite samples leads to 5.6‐fold more Fe(II) oxidation than in anoxic samples because the formed NO becomes rapidly reoxidized by O₂, therefore leading to abiotic oxidation and underestimation of Fe(II). With our revised protocol using sulfamic acid, we quantified oxidation of approximately 7 mm of Fe(II) by BoFeN1 within 4 days. Without addition of sulfamic acid, the same oxidation was detected within only 2 days. Additionally, abiotic incubation of Fe(II) with nitrite in the presence of goethite as surface catalyst led to similar abiotic Fe(II) oxidation rates as observed in growing BoFeN1 cultures. BoFeN1 growth was observed on acetate with N₂O as electron acceptor. When adding Fe(II), no Fe(II) oxidation was observed, suggesting that the absence of reactive N intermediates (, NO) precludes Fe(II) oxidation. The addition of ferrihydrite [Fe(OH)₃] to acetate/nitrate BoFeN1 cultures led to growth stimulation equivalent to previously described effects on growth by adding Fe(II). This suggests that elevated iron concentrations might provide a nutritional effect rather than energy‐yielding Fe(II) oxidation. Our findings therefore suggest that although enzymatic Fe(II) oxidation by denitrifiers cannot be fully ruled out, its contribution to the observed Fe(II) oxidation in microbial cultures is probably lower than previously suggested and has to be questioned in general until the enzymatic machinery‐mediating Fe(II) oxidation is identified.     

Fe(III) reduction during pyruvate fermentation by Desulfotomaculum reducens strain MI‐1

Desulfotomaculum reducens MI‐1 is a Gram‐positive, sulfate‐reducing bacterium also capable of reducing several metals, among which is Fe(III). Very limited knowledge is available on the potential mechanism(s) of metal reduction among Gram‐positive bacteria, despite their preponderance in the microbial communities that inhabit some inhospitable environments (e.g., thermal or hyperthermal ecosystems, extreme pH or salinity environments, heavy metal or radionuclide contaminated sediments). Here, we show that in the presence of pyruvate, this micro‐organism is capable of reducing both soluble Fe(III)‐citrate and solid‐phase hydrous ferric oxide, although growth is sustained by pyruvate fermentation rather than Fe(III) respiration. Despite the fact that Fe(III) reduction does not support direct energy conservation, D. reducens uses it as a complementary means of discarding excess reducing equivalent after H₂ accumulation in the culture headspace renders proton reduction unfavorable. Thus, Fe(III) reduction permits the oxidation of greater amounts of pyruvate than fermentation alone. Fe(III) reduction by D. reducens is mediated by a soluble electron carrier, most likely riboflavin. Additionally, an intracellular electron storage molecule acts as a capacitor and accumulates electrons during pyruvate oxidation for slow release to Fe(III). The reductase responsible for the transfer of electrons from the capacitor to the soluble carrier has not been identified, but data presented here argue against the involvement of c‐type cytochromes.     

Fermentation of trihydroxybenzenes by Pelobacter acidigallici gen. nov. sp. nov., a new strictly anaerobic,non-sporeforming bacterium

Five strains of rod-shaped, Gram-negative, non-sporing, strictly anaerobic bacteria were isolated from limnic and marine mud samples with gallic acid or phloroglucinol as sole substrate. All strains grew in defined mineral media without any growth factors; marine isolates required salt concentrations higher than 1% for growth, two freshwater strains only thrived in freshwater medium. Gallic acid, pyrogallol, 2,4,6-trihydroxybenzoic acid, and phloroglucinol were the only substrates utilized and were fermented stoichiometrically to 3 mol acetate (and 1 mol CO₂) per mol with a growth yield of 10g cell dry weight per mol of substrate. Neither sulfate, sulfur, nor nitrate were reduced. The DNA base ratio was 51.8% guanine plus cytosine. A marine isolate, Ma Gal 2, is described as type strain of a new genus and species, Pelobacter acidigallici gen. nov. sp. nov., in the family Bacteroidaceae. In coculture with Acetobacterium woodii, the new isolates converted also syringic acid completely to acetate. Cocultures with Methanosarcina barkeri converted the respective substrates completely to methane and carbon dioxide.     

Pyrite formation in marshes during early diagenesis

Pyrite was removed from peat cores by draining the sediments and allowing the pyrite to oxidize. Then the peat cores were placed back into intertidal salt marsh sediments to incubate. Pyrite accumulated rapidly in peat incubated in situ. A greater accumulation of pyrite was observed in peat that contained living grass than peat in which the grass had been killed.

Resin‐imbedded samples of peat from nearby sediments showed that small single crystals of pyrite were abundant, supporting the idea that pyrite in marshes forms rapidly through direct precipitation. Pyrite was also observed filling vascular channels in roots. It had been proposed that pyrite fills root channels in freshwater environments where the primary sulfur source used by sulfate‐reducing bacteria is organic sulfur rather than sulfate. The widespread occurrence of pyrite filling vascular channels in salt marsh peat makes it unlikely that pyrite morphology can be used to infer the salinity of the overlying water.

Marsh sediments are characterized by higher carbon/sulfur ratios and pyritization (Fe‐pyritel(Fe‐pyrite + Fe‐HCl)) indices than marine subtidal sediments. Within wide ranges these indices do not seem to be very sensitive to salinity of flooding water or carbon concentrations in sediments. Oxidation and iron availability appear to be the major controls on pyrite accumulation in marshes. While pyrite concentrations in submerged sediments can be used as indicators of relative rates of sulfate reduction, sulfur storage in intertidal marsh sediments is not as tightly linked to this microbial process.     

Responses of Active Ammonia Oxidizers to Eutrophication and Oxygen Statuses in Taihu Freshwater Sediments

Abstract

Here, we employed DNA-based stable isotope probing (SIP) and molecular biology methods to investigate active ammonia oxidizer communities in suboxic sediments (0 to –2?cm) at the micromolar oxygen level and layers (–2 to –5?cm) at nanomolar oxygen concentrations from meso-eutrophic and light-eutrophic locations in Taihu Lake. The results revealed that ammonia-oxidizing archaea (AOA) were less active in the anoxic layer of meso-eutrophic sites, while ammonia-oxidizing bacteria (AOB) were less active in suboxic sediments of light-eutrophic sites after 8?weeks of incubation. The active AOA in the meso- and light-eutrophic sediments belonged to the Nitrosopumilus, Nitrosotalea, and Nitrososphaera clusters and the Nitrosopumilus and Nitrososphaera clusters, respectively, with Nitrosopumilus cluster as the predominant AOA, which took up a higher ratio in the light-eutrophic and suboxic layers than their counterparts. The advantageous active AOB were numerically predominated by the Nitrosomonas cluster in the suboxic layers, and the Nitrosospira cluster in the anoxic layers, respectively, both of which were distributed in diverse frequencies in different eutrophication statuses. The role and community composition diversities of active ammonia oxidizers in freshwater sediments were attributed to the different eutrophication (including nitrogen and organic carbon content) and oxygen statuses.     

Morphology of biogenic iron oxides records microbial physiology and environmental conditions: toward interpreting iron microfossils

Despite the abundance of Fe and its significance in Earth history, there are no established robust biosignatures for Fe(II)‐oxidizing micro‐organisms. This limits our ability to piece together the history of Fe biogeochemical cycling and, in particular, to determine whether Fe(II)‐oxidizers played a role in depositing ancient iron formations. A promising candidate for Fe(II)‐oxidizer biosignatures is the distinctive morphology and texture of extracellular Fe(III)‐oxyhydroxide stalks produced by mat‐forming microaerophilic Fe(II)‐oxidizing micro‐organisms. To establish the stalk morphology as a biosignature, morphologic parameters must be quantified and linked to the microaerophilic Fe(II)‐oxidizing metabolism and environmental conditions. Toward this end, we studied an extant model organism, the marine stalk‐forming Fe(II)‐oxidizing bacterium, Mariprofundus ferrooxydans PV‐1. We grew cultures in flat glass microslide chambers, with FeS substrate, creating opposing oxygen/Fe(II) concentration gradients. We used solid‐state voltammetric microelectrodes to measure chemical gradients in situ while using light microscopy to image microbial growth, motility, and mineral formation. In low‐oxygen (2.7–28 μm ) zones of redox gradients, the bacteria converge into a narrow (100 μm–1 mm) growth band. As cells oxidize Fe(II), they deposit Fe(III)‐oxyhydroxide stalks in this band; the stalks orient directionally, elongating toward higher oxygen concentrations. M. ferrooxydans stalks display a narrow range of widths and uniquely biogenic branching patterns, which result from cell division. Together with filament composition, these features (width, branching, and directional orientation) form a physical record unique to microaerophilic Fe(II)‐oxidizer physiology; therefore, stalk morphology is a biosignature, as well as an indicator of local oxygen concentration at the time of formation. Observations of filamentous Fe(III)‐oxyhydroxide microfossils from a ~170 Ma marine Fe‐Si hydrothermal deposit show that these morphological characteristics can be preserved in the microfossil record. This study demonstrates the potential of morphological biosignatures to reveal microbiology and environmental chemistry associated with geologic iron formation depositional processes.     

A review of changes in the fish assemblages of Levantine inland and marine ecosystems following the introduction of non-native fishes

The arrival of non‐native fishes in the Levant Basin began in the late 19th century. Whereas the presence of most of the 40 non‐native freshwater fishes stem from intentional introductions, either for aquaculture or pest control, the 62 species of non‐native marine fishes arrived by natural dispersal via the Suez Canal. Of the non‐native freshwater species, five have established successful breeding populations (mosquitofish Gambusia affinis, common carp Cyprinus carpio, crucian carp Carassius carassius, swordtail Xiphophorus hellerii and rainbow trout Oncorhynchus mykiss), and seven are regularly stocked in natural habitats (thinlip mullet Liza ramada, flathead mullet Mugil cephalus, European eel Anguilla anguilla, grass carp Ctenopharyngodon idella, Asian silver carp Hypophthalmichthys molitrix, bighead carp Aristichthys nobilis, black carp Mylopharyngodon piceus). Some non‐native species appear to have out‐competed native species. Gambusia affinis may have caused the extirpation of two native cyprinid fishes from the Qishon River basin (Levant silver carp Hemigrammocapoeta nana and common garra Garra rufa) and the southern Dead Sea (endemic Sodom's garra G. ghoerensis). The opening of the Suez Canal in 1869 allowed entry into the eastern Mediterranean of Indo‐Pacific and Erythrean biota, with the latter now dominating the community structure (50–90% of fish biomass) and function (altered native food web) of the Levantine littoral and infra‐littoral zones. The process has accelerated in recent years concurrent with a warming trend of the seawater. Record numbers of newly discovered non‐native species is leading to the creation of a human‐assisted Erythrean biotic province in the eastern Mediterranean.     

Errors in the determination of low levels of nitrate in lake sediments

Abstract

Error sources associated with the spectrophotometric determination of low levels (e.g. <2 μg g^?1) of nitrate in sediments have been examined and problems identified included incomplete nitrate recovery (attributable in part to anion resorption) and light scattering by colloidal (<0.45 μm) matter in extract solutions (minimised by using uncoloured extract in the reference beam). Optimum retrieval (>90%) of nitrate from the marine lake sediments studied was achieved with 15 min mixing with 0.1 M NH₄CI, using a sediment to extractant ratio of 1:30. The nitrate in the extracts was determined by reducing it to nitrite (using Cd powder), with subsequent colour development based on the addition of sulfanilic acid and N-1-naphthyl-ethylenediamine dihydrochloride. The reduction step was sensitive to the experimental conditions used, but was near quantitative using 0.1 M NH₄CI extracts. (Much lower transformation levels were observed when the nitrate solutions contained KCI or CaSO₄, or when Zn powder was used as the reductant). All the sediments tested sorbed nitrate ion from solution (some very avidly) and this sorbed ion was not readily retrieved by back extraction into NH₄CI solutions.     

Evidence for rapid microscale bacterial redox cycling of iron in circumneutral environments

The potential for microscale bacterial Fe redox cycling was investigated in microcosms containing ferrihydrite-coated sand and a coculture of a lithotrophic Fe(II)-oxidizing bacterium (strain TW2) and a dissimilatory Fe(III)-reducing bacterium (Shewanella alga strain BrY). The Fe(II)-oxidizing organism was isolated from freshwater wetland surface sediments which are characterized by steep gradients of dissolved O₂ and high concentrations of dissolved and solid-phase Fe(II) within mm of the sediment–water interface, and which support comparable numbers (10⁵–10⁶ mL⁻¹) of culturable Fe(II)-oxidizing and Fe(III)-reducing reducing. The coculture systems showed minimal Fe(III) oxide accumulation at the sand-water interface, despite intensive O₂ input from the atmosphere and measurable dissolved O₂ to a depth of 2 mm below the sand–water interface. In contrast, a distinct layer of oxide precipitates formed in systems containing Fe(III)-reducing bacteria alone. Examination of materials from the cocultures by fluorescence in situ hybridization indicated close physical juxtapositioning of Fe(II)-oxidizing and Fe(III)-reducing bacteria in the upper few mm of sand. Our results indicate that Fe(II)-oxidizing bacteria have the potential to enhance the coupling of Fe(II) oxidation and Fe(III) reduction at redox interfaces, thereby promoting rapid microscale cycling of Fe. This revised version was published online in August 2006 with corrections to the Cover Date.