Impacts of humic substances, elevated temperature, and UVB radiation on bacterial communities of the marine sponge Chondrilla sp

Sponges are abundant components of coral reefs known for their filtration capabilities and intricate interactions with microbes. They play a crucial role in maintaining the ecological balance of coral reefs. Humic substances (HS) affect bacterial communities across terrestrial, freshwater, and marine ecosystems. However, the specific effects of HS on sponge-associated microbial symbionts have largely been neglected. Here, we used a randomized-controlled microcosm setup to investigate the independent and interactive effects of HS, elevated temperature, and UVB radiation on bacterial communities associated with the sponge Chondrilla sp. Our results indicated the presence of a core bacterial community consisting of relatively abundant members, apparently resilient to the tested environmental perturbations, alongside a variable bacterial community. Elevated temperature positively affected the relative abundances of ASVs related to Planctomycetales and members of the families Pseudohongiellaceae and Hyphomonadaceae. HS increased the relative abundances of several ASVs potentially involved in recalcitrant organic matter degradation (e.g., the BD2-11 terrestrial group, Saccharimonadales, and SAR202 clade). There was no significant independent effect of UVB and there were no significant interactive effects of HS, heat, and UVB on bacterial diversity and composition. The significant, independent impact of HS on the composition of sponge bacterial communities suggests that alterations to HS inputs may have cascading effects on adjacent marine ecosystems.

Survivability and proliferation of microorganisms in bentonite with implication to radioactive waste geological disposal: strong effect of temperature and negligible effect of pressure

As bentonite hosts a diverse spectrum of indigenous microorganisms with the potential to influence the long-term stability of deep geological repositories, it is essential to understand the factors influencing microbial activity under repository conditions. Here, we focus on two factors, i.e., temperature and swelling pressure, using a suspension of Cerny Vrch bentonite to boost microbial activity and evaluate microbial response. Suspensions were exposed either to different pressures (10, 12 and 15 MPa; to simulate the effect of swelling pressure) or elevated temperatures (60, 70, 80 and 90 °C; to simulate the effect of cannister heating) for four weeks. Each treatment was followed by a period of anaerobic incubation at atmospheric pressure/laboratory temperature to assess microbial recovery after treatment. Microbial load and community structure were then estimated using molecular-genetic methods, with presence of living cells confirmed through microscopic analysis. Our study demonstrated that discrete application of pressure did not influence on overall microbial activity or proliferation, implying that pressure evolution during bentonite swelling is not the critical factor responsible for microbial suppression in saturated bentonites. However, pressure treatment caused significant shifts in microbial community structure. We also demonstrated that microbial activity decreased with increasing temperature, and that heat treatment strongly influenced bentonite microbial community structure, with several thermophilic taxa identified. A temperature of 90 °C proved to be limiting for microbial activity and proliferation in all bentonite suspensions. Our study emphasizes the crucial role of a deep understanding of microbial activity under repository-relevant conditions in identifying possible strategies to mitigate the microbial potential within the deep geological repository and increase its long-term stability and safety.

Mobility of antimony in contrasting surface environments of a mine site: influence of redox conditions and microbial communities

Microbial processes can influence the complex geochemical behaviour of the toxic metalloid antimony (Sb) in mining environments. The present study is aimed to evaluate the influence of microbial communities on the mobility of Sb from solid phases to water in different compartments and redox conditions of a mining site in southwest (SW) Spain. Samples of surface materials presenting high Sb concentrations, from two weathered mining waste dumps, and an aquatic sediment were incubated in slurries comparing oxic and anoxic conditions. The initial microbial communities of the three materials strongly differed. Incubations induced an increase of microbial biomass and an evolution of the microbial communities' structures and compositions, which diverged in different redox conditions. The presence of active bacteria always influenced the mobility of Sb, except in the neutral pH waste incubated in oxic conditions. The effect of active microbial activities in oxic conditions was dependent on the material: Sb oxic release was biologically amplified with the acidic waste, but attenuated with the sediment. Different bacterial genera involved in Sb, Fe and S oxidation or reduction were present and/or grew during incubation of each material. The results highlighted the wide diversity of microbial communities and metabolisms at the small geographic scale of a mining site and their strong implication in Sb mobility.

Microbial Degradation of Free and Halogenated Estrogens in River Water-Sediment Microcosms

Halogenated estrogens are formed during chlorine-based wastewater disinfection and have been detected in wastewater treatment plant effluent; however, very little is known about their susceptibility to biodegradation in natural waters. To better understand the biodegradation of free and halogenated estrogens in a large river under environmentally relevant conditions, we measured estrogen kinetics in aerobic microcosms containing water and sediment from the Willamette River (OR, USA) at two concentrations (50 and 1250 ng L^-1). Control microcosms were used to characterize losses due to sorption and other abiotic processes, and microbial dynamics were monitored using 16S rRNA gene sequencing and ATP. We found that estrogen biodegradation occurred on timescales of hours to days and that in river water spiked at 50 ng L^-1 half-lives were significantly shorter for 17β-estradiol (t_1/2,bio = 42 ± 3 h) compared to its monobromo (t_1/2,bio = 49 ± 5 h), dibromo (t_1/2,bio = 88 ± 12 h), and dichloro (t_1/2,bio = 98 ± 16 h) forms. Biodegradation was also faster in microcosms with high initial estrogen concentrations as well as those containing sediment. Free and halogenated estrone were important transformation products in both abiotic and biotic microcosms. Taken together, our findings suggest that biodegradation is a key process for removing free estrogens from surface waters but likely plays a much smaller role for the more highly photolabile halogenated forms.

A New Colorimetric Test for Accurate Determination of Plastic Biodegradation

As plastic waste is accumulating in both controlled waste management settings and natural settings, much research is devoted to search for solutions, also in the field of biodegradation. However, determining the biodegradability of plastics in natural environments remains a big challenge due to the often very low biodegradation rates. Many standardised test methods for biodegradation in natural environments exist. These are often based on mineralisation rates in controlled conditions and are thus indirect measurements of biodegradation. It is of interest for both researchers and companies to have tests that are more rapid, easier, and more reliable to screen different ecosystems and/or niches for their plastic biodegradation potential. In this study, the goal is to validate a colorimetric test, based on carbon nanodots, to screen biodegradation of different types of plastics in natural environments. After introducing carbon nanodots into the matrix of the target plastic, a fluorescent signal is released upon plastic biodegradation. The in-house-made carbon nanodots were first confirmed regarding their biocompatibility and chemical and photostability. Subsequently, the effectivity of the developed method was evaluated positively by an enzymatic degradation test with polycaprolactone with Candida antarctica lipase B. Finally, validation experiments were performed with enriched microorganisms and real environmental samples (freshwater and seawater), of which the results were compared with parallel, frequently used biodegradation measures such as O₂ and CO₂, dissolved organic carbon, growth and pH, to assess the reliability of the test. Our results indicate that this colorimetric test is a good alternative to other methods, but a combination of different methods gives the most information. In conclusion, this colorimetric test is a good fit to screen, in high throughput, the depolymerisation of plastics in natural environments and under different conditions in the lab.

Enhanced removal of warfare agent tri-nitro-toluene by a Methylophaga-dominated microbiome

Historical exposure of the marine environment to 2,4,6-trinitrotoluene (TNT) happened due to the dumping of left-over munitions. Despite significant research on TNT decontamination, the potential of marine microbiome for TNT degradation remains only little explored. In this study, TNT degradation experiments were conducted with sediment located near the World War I munition dumpsite - Paardenmarkt in the Belgian part of North Sea. A slow removal was observed using TNT as sole source of C and N, which could be enhanced by adding methanol. Degradation was reflected in nitro-reduced metabolites and microbial growth. 16S Illumina sequencing analysis revealed several enriched genera that used TNT as a sole source of C and N - Colwellia, Thalossospira, and Methylophaga. Addition of methanol resulted in increased abundance of Methylophaga, which corresponded to the rapid removal of TNT. Methanol enhanced the degradation by providing additional energy and establishing syntrophic association between methanol-utilizing and TNT-utilizing bacteria.

Degradation of chlorinated solvents with reactive iron minerals in subsurface sediments from redox transition zones

Reactive iron (Fe) mineral coatings found in subsurface reduction-oxidation transition zones (RTZs) contribute to the attenuation of contaminants. An 18.3-m anoxic core was collected from the site, where constituents of concern (COCs) in groundwater included chlorinated solvents. Reactive Fe mineral coatings were found to be abundant in the RTZs. This research focused on evaluating reaction kinetics with anoxic sediments bearing ferrous mineral nano-coatings spiked with either tetrachloroethylene (PCE), trichloroethylene (TCE), or 1,4-dichlorobenzene (1,4-DCB). Reaction kinetics with RTZ sediments followed pseudo-first-order reactions for the three contaminants with 90% degradation achieved in less than 39 days. The second-order rate constants for the three COCs ranged from 6.20 × 10^-4 to 1.73 × 10^-3 Lg^-1h^-1 with pyrite (FeS₂), 4.97 × 10^-5 to 1.24 × 10^-3 Lg^-1h^-1with mackinawite (FeS), 1.25 × 10^-4 to 1.89 × 10^-4 Lg^-1h^-1 with siderite (FeCO₃), and 1.79 × 10^-4 to 1.10 × 10^-3 Lg^-1h^-1 with magnetite (Fe₃O₄). For these three chlorinated solvents, the trend for the rate constants followed: Fe(II) sulfide minerals > magnetite > siderite. The high reactivity of Fe mineral coatings is hypothesized to be due to the large surface areas of the nano-mineral coatings. As a result, these surfaces are expected to play an important role in the attenuation of chlorinated solvents in contaminated subsurface environments.

Role of indigenous microorganisms and organics in the release of iron and trace elements from sediments impacted by iron mine tailings from failed Fundão dam

After Fundão Dam failure in 2015, most of Gualaxo do Norte River in Doce River Basin in Brazil became silted by iron mining tailings consisting mainly of fine-grained quartz, hematite, and goethite. Previous work pointed to the possibility of reductive dissolution of iron and manganese from tailings, leading to mobilization of iron, manganese and trace elements. Several microorganisms were shown to reduce Fe(III) to Fe(II) and Mn(III, IV) to Mn(II) "in vitro", but their roles in mobilization of Fe and trace elements from freshwater sediments are poorly understood. In this work, bottom sediments and water collected in Gualaxo do Norte River were used to build anoxic microcosms amended with acetate, glucose or yeast extract, in order to access if heterotrophic microorganisms, either fermenters or dissimilatory Fe reducers, could reduce Fe(III) from minerals in the sediments to soluble Fe(II), releasing trace elements. The Fe(II) concentrations were measured over time, and trace elements concentrations were evaluated at the end of the experiment. In addition, minerals and biopolymers in bottom sediments were quantified. Results showed that organic substrates, notably glucose, fuelled microbial reduction of iron minerals and release of Fe(II), Mn, Ba, Al and/or Zn from sediments. In general, higher concentrations of organic substrates elicited mobilization of larger amounts of Fe(II) and trace elements from sediments. The results point to the possibility of mobilization of huge amounts of iron and trace elements from sediments to water if excess biodegradable organic matter is released in rivers affected by iron mine tailings.

Hyperthermophilic endospores germinate and metabolize organic carbon in sediments heated to 80°C

Cold surface sediments host a seedbank of functionally diverse thermophilic bacteria. These thermophiles are present as endospores, which are widely dispersed in aquatic environments. Here, we investigated the functional potential of endospore populations in cold surface sediments heated to 80°C. Microbial production of acetate was observed at 80°C and could be enhanced by supplying additional organic carbon substrates. Comparison of 16S rRNA gene amplicon libraries from 80°C enrichments to sediments heated to lower temperatures (50-70°C) showed that temperature selects for distinct populations of endospore-forming bacteria. Whereas sulfate-reducing thermophiles were enriched in 50-70°C incubations, 80°C exceeds their thermal tolerance and selects for hyperthermophilic organotrophic bacteria that are similarly detected in amplicon libraries from sediments heated to 90°C. Genome-resolved metagenomics revealed novel carbon cycling members of Symbiobacteriales, Thermosediminibacteraceae, Thermanaeromonas and Calditerricola with the genomic potential for the degradation of carbohydrates, sugars, amino acids and nucleotides. Endospores of thermophilic bacteria are deposited on seabed sediments worldwide where they remain dormant as they are buried in the accumulating sediments. Our results suggest that endospore populations could be activated by temperature increases encountered during burial and show the potential for organotrophic metabolic activity contributing to acetate generation in deep hot sediments.

Protocol for using autoclaved intertidal sediment as a medium to enrich marine cable bacteria

Cable bacteria (CB) are non-isolated filamentous bacteria in the family of Desulfobulbaceae, known for fostering centimeter-long electron transfer in sediments with pronounced redox zonation. This protocol details steps to extract CB filaments from cultured natural sediment, inoculate autoclaved sediment with extracted filaments, and subsequently evaluate the growth and enrichment of CB. We also describe the approaches for collecting suitable sediment, preparing autoclaved sediment, and manufacturing glass needles and hooks for the extraction of CB.

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Prepare autoclaved sediment as an enrichment medium for cable bacteria

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Manufacture glass needles and hooks as tools to extract cable bacteria

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Video demonstration of cable bacteria extraction and autoclaved sediment inoculation

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Recover prolific cable bacteria biomass grown in autoclaved sediment

Publisher’s note: Undertaking any experimental protocol requires adherence to local institutional guidelines for laboratory safety and ethics.

Natural Asphalt Seeps Are Potential Sources for Recalcitrant Oceanic Dissolved Organic Sulfur and Dissolved Black Carbon

Natural oil seepages contribute about one-half of the annual petroleum input to marine systems. Yet, environmental implications and the persistence of water-soluble hydrocarbons from these seeps are vastly unknown. We investigated the release of oil-derived dissolved organic matter (DOM) from natural deep sea asphalt seeps using laboratory incubation experiments. Fresh asphalt samples collected at the Chapopote asphalt volcano in the Southern Gulf of Mexico were incubated aerobically in artificial seawater over 4 weeks. The compositional changes in the water-soluble fraction of asphalt-derived DOM were determined with ultrahigh-resolution mass spectrometry (Fourier-transform ion cyclotron resonance mass spectrometry, FT-ICR-MS) and by excitation-emission matrix spectroscopy to characterize fluorescent DOM (FDOM) applying parallel factor (PARAFAC) analysis. Highly reduced aliphatic asphalt-derived DOM was readily biodegraded, while aromatic and sulfur-enriched DOM appeared to be less bioavailable and accumulated in the aqueous phase. A quantitative molecular tracer approach revealed the abundance of highly condensed aromatic molecules of thermogenic origin. Our results indicate that natural asphalt and potentially other petroleum seepages can be sources of recalcitrant dissolved organic sulfur and dissolved black carbon to the ocean.

Microbiome-mediated response to pulse fire disturbance outweighs the effects of fire legacy on plant performance

Fire plays a major role in structuring plant communities across the globe. Interactions with soil microbes impact plant fitness, scaling up to influence plant populations and distributions. Here we present the first factorial manipulation of both fire and soil microbiome presence to investigate their interactive effects on plant performance across a suite of plant species with varying life history traits. We conducted fully factorial experiments on 11 species from the Florida scrub ecosystem to test plant performance responses to soils with varying fire histories (36 soil sources), the presence/absence of a microbiome, and exposure to an experimental burn. Results revealed interactive 'pulse' effects between fire and the soil microbiome on plant performance. On average, post-fire soil microbiomes strongly reduced plant productivity compared to unburned or sterilized soils. Interestingly, longer-term fire 'legacy' effects had minor impacts on plant performance and were unrelated to soil microbiomes. While pulse fire effects on plant-microbiome interactions are short-term, they could have long-term consequences for plant communities by establishing differential microbiome-mediated priority effects during post-disturbance succession. The prominence of pulse fire effects on plant-microbe interactions has even greater import due to expected increases in fire disturbances resulting from anthropogenic climate change.

Abiotic reduction of nitrite by Fe(II): a comparison of rates and N2O production

Abiotic reduction of nitrite (NO₂^-) by Fe(II) species (i.e., chemodenitrification) has been demonstrated in a variety of natural environments and laboratory studies, and is a potentially significant source of atmospheric nitrous oxide (N₂O) emissions. It is, however, unclear how chemodenitrification rates and N₂O yields vary among heterogeneous Fe(II) species under similar conditions and whether abiotic reduction competes with biological NO₂^- reduction. Here, we measured rates of NO₂^- reduction and extents of N₂O production by several Fe(II) species under consistent, environmentally relevant conditions (i.e., pH 7.0, bicarbonate buffer, and 0.1 mM NO₂^-). Nitrite reduction rates varied significantly among the heterogeneous Fe(II) species with half-lives (t_1/2) ranging from as low as an hour to over two weeks following the trend of goethite/Fe(II) ∼ hematite/Fe(II) ∼ magnetites > maghemite/Fe(II) > sediment/Fe(II). Interestingly, we observed no clear trend of increasing NO₂^- reduction rates with higher magnetite stoichiometry (x = Fe²⁺/Fe³⁺). Nitrogen recovery as N₂O also varied significantly among the Fe species ranging from 21% to 100% recovery. We further probed both chemodenitrification and biological denitrification in the absence and presence of added aqueous Fe(II) with a sediment collected from the floodplain of an agricultural watershed. While abiotic NO₂^- reduction by the sediment + Fe(II) was much slower than the laboratory Fe(II) species, we found near complete mass N balance during chemodenitrification, as well as evidence for both abiotic and biological NO₂^- reduction potentially occurring in the sediment under anoxic conditions. Our results suggest that in redox active sediments and soils both chemodenitrification and biological denitrification are likely to occur simultaneously, and that agricultural watersheds may be significant sources of N₂O emissions.

Potential Activities and Long Lifetimes of Organic Carbon-Degrading Extracellular Enzymes in Deep Subsurface Sediments of the Baltic Sea

Heterotrophic microorganisms in marine sediments produce extracellular enzymes to hydrolyze organic macromolecules, so their products can be transported inside the cell and used for energy and growth. Therefore, extracellular enzymes may mediate the fate of organic carbon in sediments. The Baltic Sea Basin is a primarily depositional environment with high potential for organic matter preservation. The potential activities of multiple organic carbon-degrading enzymes were measured in samples obtained by the International Ocean Discovery Program Expedition 347 from the Little Belt Strait, Denmark, core M0059C. Potential maximum hydrolysis rates (V_max) were measured at depths down to 77.9mbsf for the following enzymes: alkaline phosphatase, β-d-xylosidase, β-d-cellobiohydrolase, N-acetyl-β-d-glucosaminidase, β-glucosidase, α-glucosidase, leucyl aminopeptidase, arginyl aminopeptidase, prolyl aminopeptidase, gingipain, and clostripain. Extracellular peptidase activities were detectable at depths shallower than 54.95mbsf, and alkaline phosphatase activity was detectable throughout the core, albeit against a relatively high activity in autoclaved sediments. β-glucosidase activities were detected above 30mbsf; however, activities of other glycosyl hydrolases (β-xylosidase, β-cellobiohydrolase, N-acetyl-β-glucosaminidase, and α-glucosidase) were generally indistinguishable from zero at all depths. These extracellular enzymes appear to be extremely stable: Among all enzymes, a median of 51.3% of enzyme activity was retained after autoclaving for an hour. We show that enzyme turnover times scale with the inverse of community metabolic rates, such that enzyme lifetimes in subsurface sediments, in which metabolic rates are very slow, are likely to be extraordinarily long. A back-of-the-envelope calculation suggests enzyme lifetimes are, at minimum, on the order of 230days, and may be substantially longer. These results lend empirical support to the hypothesis that a population of subsurface microbes persist by using extracellular enzymes to slowly metabolize old, highly degraded organic carbon.

How to grow your cable bacteria: Establishment of a stable single-strain culture in sediment and proposal of Candidatus Electronema aureum GS

Cable bacteria are multicellular filamentous bacteria within the Desulfobulbaceae that couple the oxidation of sulfide to the reduction of oxygen over centimeter distances via long distance electron transport (LDET). So far, none of the freshwater or marine cable bacteria species have been isolated into pure culture. Here we describe a method for establishing a stable single-strain cable bacterium culture in partially sterilized sediment. By repeated transfers of a single cable bacterium filament from freshwater pond sediment into autoclaved sediment, we obtained strain GS, identified by its 16S rRNA gene as a member of Ca. Electronema. This strain was further propagated by transferring sediment clumps, and has now been stable within its semi-natural microbial community for several years. Its metagenome-assembled genome was 93% complete, had a size of 2.76 Mbp, and a DNA G + C content of 52%. Average Nucleotide Identity (ANI) and Average Amino Acid Identity (AAI) suggest the affiliation of strain GS to Ca. Electronema as a novel species. Cell size, number of outer ridges, and detection of LDET in the GS culture are likewise consistent with Ca. Electronema. Based on these combined features, we therefore describe strain GS as a new cable bacterium species of the candidate genus Electronema, for which we propose the name Candidatus Electronema aureum sp.nov. Although not a pure culture, this stable single-strain culture will be useful for physiological and omics-based studies; similar approaches with single-cell or single-filament transfers into natural medium may also aid the characterization of other difficult-to-culture microbes.

Broad diversity of bacteria degrading 17ß-estradiol-3-sulfate isolated from river sediment and biofilm at a wastewater treatment plant discharge

Conjugated estrogens, such as 17β-estradiol-3-sulfate (E2-3S), can be released into aquatic environments through wastewater treatment plants (WWTP). There, they are microbiologically degraded into free estrogens, which can have harmful effects on aquatic wildlife. Here, the degradation of E2-3S in environmental samples taken upstream, downstream and at the effluent of a WWTP was assessed. Sediment and biofilm samples were enriched for E2-3S-degrading microorganisms, yielding a broad diversity of bacterial isolates, including known and novel degraders of estrogens. Since E2-3S-degrading bacteria were also isolated in the sample upstream of the WWTP, the WWTP does not influence the ability of the microbial community to degrade E2-3S.

Experimental Warming Reduces Survival, Cold Tolerance, and Gut Prokaryotic Diversity of the Eastern Subterranean Termite, Reticulitermes flavipes (Kollar)

Understanding the effects of environmental disturbances on insects is crucial in predicting the impact of climate change on their distribution, abundance, and ecology. As microbial symbionts are known to play an integral role in a diversity of functions within the insect host, research examining how organisms adapt to environmental fluctuations should include their associated microbiota. In this study, subterranean termites [Reticulitermes flavipes (Kollar)] were exposed to three different temperature treatments characterized as low (15°C), medium (27°C), and high (35°C). Results suggested that pre-exposure to cold allowed termites to stay active longer in decreasing temperatures but caused termites to freeze at higher temperatures. High temperature exposure had the most deleterious effects on termites with a significant reduction in termite survival as well as reduced ability to withstand cold stress. The microbial community of high temperature exposed termites also showed a reduction in bacterial richness and decreased relative abundance of Spirochaetes, Elusimicrobia, and methanogenic Euryarchaeota. Our results indicate a potential link between gut bacterial symbionts and termite's physiological response to environmental changes and highlight the need to consider microbial symbionts in studies relating to insect thermosensitivity.

Specific enrichment of hydrocarbonclastic bacteria from diesel-amended soil on biochar particles

Biochar has been proposed as a suitable biostimulant for the remediation of hydrocarbon contamination, and also has the potential to act as a carrier for hydrocarbonoclastic microorganisms which could bioaugment endogenous microbial communities. However, the evidence regarding the biostimulatory effects of biochars on hydrocarbon bioremediation is somewhat equivocal, possibly due to variability of the physicochemical properties of biochar and soil across studies. Here, we use standard biochars with defined properties produced from softwood pellets (SWP) and rice husk (RH) at pyrolysis temperatures of 550 °C or 700 °C to test the effects of biochar amendment on microbial community composition and hydrocarbon degradation in soil microcosms contaminated with diesel oil. Combining this approach for the first time with specific analysis of microbial community composition using amplicon sequence variants (ASVs), we find that oil contamination causes extreme short-term loss of soil microbial diversity, and highly-specific selection of a limited set of genera defined by 13 ASVs. Biochar ameliorates the short-term loss of diversity, and in the longer term (9 weeks), changes community composition in a type-specific manner. The majority of the 13 selected ASVs are further enriched on biochar particles, although SWP biochars perform better than RH biochar in enrichment of putative hydrocarbonoclastic Aquabacterium spp. However, complete degradation of normal (n) alkanes from the aliphatic hydrocarbon fraction is prevented in the presence of biochar amendment, possibly due to their adsorption onto the char surface. Furthermore, we show that putative hydrocarbon degraders released from diesel-amended soil can subsequently be enriched to high levels on SWP biochar particles in growth medium supplemented with diesel oil as the sole carbon source; these include selected ASVs representing the genera Rhodococcus, Aquabacterium, and Cavicella. This work suggests that use of biochar pre-enriched with endogenous, conditionally-rare hydrocarbon degrading bacteria is a promising strategy for bioaugmentation of diesel-contaminated soils.

Fe(III) Photoreduction Producing Feaq2+ in Oxic Freshwater Sediment

Iron(III) (Fe(III)) photoreduction plays an important role in Fe cycling and Fe(II) bioavailability in the upper ocean. Although well described for water columns, it is currently unknown to what extent light impacts the production of dissolved Fe(II) (Fe²⁺) in aquatic sediments. We performed high-resolution voltammetric microsensor measurements and demonstrated light-induced Fe²⁺ release in freshwater sediments from Lake Constance. Fe²⁺ concentrations increased up to 40 μM in the top 3 mm of the sediment during illumination in the visible range of light (400-700 nm), even in the presence of oxygen (100-280 μM). The Fe²⁺ release was strongly dependent on the organic matter content of the sediment. A lack of photoreduced Fe²⁺ was measured in sediments with concentrations of organic carbon <6 mg L^-1. The simultaneous presence of sedimentary Fe(III) photoreduction besides microbial and abiotic Fe²⁺ oxidation by oxygen suggests an active Fe redox cycling in the oxic and photic zone of aquatic sediments. Here, we provide evidence for a relevant contribution of Fe(III) photoreduction to the bio-geochemical Fe redox cycle in aquatic freshwater sediments.

N2O formation by nitrite-induced (chemo)denitrification in coastal marine sediment

Nitrous oxide (N2O) is a potent greenhouse gas that also contributes to stratospheric ozone depletion. Besides microbial denitrification, abiotic nitrite reduction by Fe(II) (chemodenitrification) has the potential to be an important source of N2O. Here, using microcosms, we quantified N2O formation in coastal marine sediments under typical summer temperatures. Comparison between gamma-radiated and microbially-active microcosm experiments revealed that at least 15-25% of total N2O formation was caused by chemodenitrification, whereas 75-85% of total N2O was potentially produced by microbial N-transformation processes. An increase in (chemo)denitrification-based N2O formation and associated Fe(II) oxidation caused an upregulation of N2O reductase (typical nosZ) genes and a distinct community shift to potential Fe(III)-reducers (Arcobacter), Fe(II)-oxidizers (Sulfurimonas), and nitrate/nitrite-reducing microorganisms (Marinobacter). Our study suggests that chemodenitrification contributes substantially to N2O formation from marine sediments and significantly influences the N- and Fe-cycling microbial community.

Impact of sterilization methods on dissolved trace metals concentrations in complex natural samples: Optimization of UV irradiation

Sterilization is essential for discriminating biotic responses from abiotic reactions in laboratory experiments investigating biogeochemical processes of complex natural samples. However, the conventional methods used to effectively sterilize materials or culture media do not allow sterilizing complex natural samples while maintaining biogeochemical balances. The aim of this study was to develop a low-cost and easy-to-use method to obtain geochemically unmodified and sterilized samples from complex lacustrine or coastal marine ecosystems. In preliminary assays, the impact of several sterilization methods (autoclaving, chemical poisoning, microwave, UV irradiation) on the trace metals balances was studied using borosilicate glass (BG), fluorinated ethylene-propylene (FEP) or polyethylene terephthalate (PET) bottles. Unlike other methods, UV sterilization had minor effects on the distribution of dissolved trace metals. Additional tests using complex lacustrine and coastal marine samples under 10 g/L sediments were performed using a homemade UV sterilization chamber designed to simultaneously irradiate a large number samples. Results showed: •very reproducible UV tests in BG and FEP bottles•faster sterilization using FEP bottles than using BG bottles•low variations of dissolved trace metals concentrations, except for Al, Cu, Fe and Zn.