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Organic sediments in aquatic ecosystems are well known sinks for nutrients, silicon, and metal(loid)s. Organic matter-consuming organisms like invertebrate shredders, grazers, and bioturbators significantly affect element fixation or remobilization by changing redox conditions or binding properties of organic sediments. Little is known about the effect of filter feeders, like the zebra mussel Dreissena polymorpha , an invasive organism in North American and European freshwater ecosystems. A laboratory batch experiment exposing D . polymorpha (∼1200 organisms per m 2 ) to organic sediment from a site contaminated with arsenic, copper, lead, and uranium revealed a significant uptake and accumulation of arsenic, copper, iron, and especially uranium both into the soft body tissues and the seashell. This is in line with previous observations of metal(loid) accumulation from biomonitoring studies. Regarding its environmental impact, D . polymorpha significantly contributed to mobilization of silicon, iron, phosphorus, arsenic, and copper and to immobilization of uranium ( p < 0.001), probably driven by redox conditions, microbial activity within the gut system, or active control of element homeostasis. No net mobilization or immobilization was observed for zinc and lead, because of their low mobility at the prevailing pH of 7.5–8.5. The present results suggest that D . polymorpha can both ameliorate (nutrient mobilization, immobilization of toxicants mobile under oxic conditions) or aggravate negative effects (mobilization of toxicants mobile under reducing conditions) in ecosystems. Relating the results of the present study to observed population densities in natural freshwater ecosystems suggests a significant influence of D . polymorpha on element cycling and needs to be considered in future studies. [ABSTRACT FROM AUTHOR]

Global changes in temperature and nitrogen supply can influence key aspects of the global nutrient cycles, such as leaf litter decomposition and associated nutrient release. The effect of temperature and nitrogen supply on nutrient release during decomposition may be influenced by initial nutrient concentrations of the litter, which for grasses depends partly on silicon availability during plant growth. We conducted a five-month field experiment to test whether Phragmites australis litter silicon concentration alters the effect of climate warming and excess nitrogen supply on nutrient concentrations and litter mass loss during decomposition. Warming enhanced concentration of Ca, Mn and Fe in decomposed leaf litter, whereas excess N-supply increased the concentration of Fe and Ca in leaves. Initial Si concentrations of leaf litter influenced the stoichiometric ratios of the elements (in relation to P), and element:P ratios mostly increased for litter with intermediate and high Si concentrations, but patterns were less clear when initial litter Si concentrations were low. We show that grass litter decomposition was most strongly associated with litter Si:P, whereas C:N and N:P ratios were less important. Our results provide first evidence that global warming, enhanced N supply and changes in Si availability affect element cycling during litter decomposition. [ABSTRACT FROM AUTHOR]

The decomposition of plant litter is an important mechanism in regard to energy and nutrient dynamics of ecosystems. Silicon concentration of plant tissue can affect these processes by changing litter quality, i.e. nutrient stoichiometry and cellulose and phenols content. To determine which group of microbial decomposers benefits from high Si content in plants and how this impacts on animal decomposers, a batch experiment was conducted with reed leaf litter ( Phragmites australis ) differing in Si content in the presence/absence of invertebrate shredders ( Gammarus pulex ). Lipid concentration of G. pulex , in reed litter and fine particulate matter (FPOM) were examined. High Si concentration in reed resulted in a decline of gram positive bacteria in the heterotrophic biofilm and of gram negative bacteria in FPOM. The lipid composition in the next trophic level, the decomposer G. pulex , changed too, indicating a diet shift in favor of bacteria and algae with increasing litter Si concentration. Thus, basal decomposers were affected by the Si availability in plant resources, and these effects likely persist along the food chain, as FPOM is a dominant food supply for other groups, e.g. collectors. This impact of Si content on plant substrate quality for decomposer food webs may have global relevance, due to related modifications in carbon and nutrient cycling during litter decomposition. [ABSTRACT FROM AUTHOR]

The invertebrate shredder Gammarus pulex L. is a key species for aquatic carbon turnover via litter decomposition and can thrive in high-arsenic (As) environments. To understand their strategies for coping with increased As concentrations while fulfilling their ecosystem functions, we analyzed the As concentration and speciation in their aquatic habitat and in leaves with heterotrophic biofilms as their natural food source. We also followed the As distribution and speciation on the cuticle and within the body of G. pulex by X-ray absorption spectroscopic imaging. Half of the total As on G. pulex was found to be associated with the cuticle but was not taken up. Removing this externally bound As yielded only arsenate in the wash solution which reflects the speciation of the surrounding aquatic phase and shows that this As does not undergo any biotransformation. The major pathway into the organism is suggested to be incorporation via food intake, but only very low amounts of As were taken up or translocated from the gut system to other tissues. In one of the main food sources, leaves, 68% arsenate and 29% monomethylarsenate were found. After ingestion into the gut system, up to 23% of the more toxic arsenite was seen, but a substantial share was methylated to dimethylarsenate (46–56%). Little arsenate and arsenite were found in the adjacent tissues. Besides 76–80% mono- and di-methylarsenate, 10–21% of the As was complexed as As(III)–S species. G. pulex plays an important role in As cycling and our results indicate that As translocation from the gut to other tissues is minimized, but a transformation to other As-species occurred. [ABSTRACT FROM AUTHOR]

Plant litter and organic sediments are main sinks for metals and metalloids in aquatic ecosystems. The effect of invertebrates as key species in aquatic litter decomposition on metal/metalloid fixation by organic matter is described only for shredders, but for grazers as another important animal group less is known. Consequently, a laboratory batch experiment was conducted to examine the effect of invertebrate grazers ( Lymnaea stagnalis L.) on metal/metalloid fixation/remobilization during aquatic litter decomposition. It could be shown that invertebrate grazers facilitate significantly the formation of smaller sizes of particulate organic matter (POM), as shown previously for invertebrate shredders. The metal/metalloid binding capacity of these smaller particles of POM is higher compared to leaf litter residuals. But element enrichment is not as high as shown previously for the effect by invertebrate shredders. Invertebrate grazers enhance also the mobilization of selected elements to the water, in the range also proven for invertebrate shredders but different for the different elements. Nonetheless invertebrate grazers activity during aquatic litter decomposition leads to a metal/metalloid fixation into leaf litter as part of sediment organic matter. Hence, the effect of invertebrate grazers on metal/metalloid fixation/remobilization contrasts partly with former assessments revealing the possibility of an enhanced metal/metalloid fixation. [ABSTRACT FROM AUTHOR]

Shortages of resources (chemical elements) used by growing industrial activities require new techniques for their acquisition. A suitable technique could be the use of wetlands for the enrichment of elements from produced water of the oil industry. Oil industries produce very high amounts of water in the course of oil mining. These waters may contain high amounts of rare elements. To our best knowledge nothing is known about the economic potential regarding rare element mining from produced water. Therefore, we estimated the amount of harvestable rare elements remaining in the effluent of a constructed wetland-pond system which is being used to treat and evaporate vast quantities of produced waters. The examined wetland system is located in the desert of the south-eastern Arabian Peninsula. This system manages 95,000 m³ per day within 350 ha of surface flow wetlands and 350 ha of evaporation ponds and is designed to be used for at least 20 years. We found a strong enrichment of some chemical elements in the water pathway of the system (e.g. lithium up to 896 µg L-1 and beryllium up to 139 µg L-1). For this wetland, lithium and beryllium are the elements with the highest economic potential resulting from a high price and load. It is calculated that after 20 years retention period 131 t of lithium and 57 t of beryllium could be harvested. This technique may also be useful for acquisition of rare earth elements. Other elements (e.g. strontium) with a high calculated load of 4500 tons in 20 years are not efficiently harvestable due to a relatively low market value. In conclusion, wetland treated waters from the oil industry offer a promising new acquisition technique for elements like lithium and beryllium. [ABSTRACT FROM AUTHOR]

Highlights: [•] Chironomus plumosus has a main impact on elemental mobilization from sediments. [•] DOC, N, Mg, Ca, Sr, Mo and U were highly influenced by bioturbation/bioirrigation. [•] Al, Fe, Co, Cu, Zn and Ce were affected only at start (larvae dig into sediments). [•] Mn, Ni, As, Cd and Cs mobilization were affected at start and slightly afterwards. [•] Invertebrate bioturbation/bioirrigation is a main process in element cycling. [Copyright &y& Elsevier]

Abstract: Increasing arsenic concentrations in freshwater ecosystems is of global concern. Processes affecting arsenic fluxes in catchments are known. These processes are in turn controlled by the underlying geology and air pollution history. In contrast to the knowledge on catchment processes less is known about the hydrochemical processes controlling the fixation/remobilization of arsenic within lakes and artificial reservoirs. Consequently, we examined a reservoir system in the Ore Mts. (Germany) regarding its sink and source potentials affecting arsenic fluxes. This area was faced with heavy deposition inputs from coal burning based acid rain until the beginning of the 1990s. Hereafter concentrations of sulfate and nitrate in runoff waters decreased, whereas dissolved organic carbon (DOC) concentrations are still increasing. Along with this, arsenic concentrations in the water discharge from the catchments increase. Our results reveal that the sediments of the investigated reservoir system contain high inventories of arsenic in association with ferric and organic phases. A nitrate deficit dependent arsenic release is suggested. It is indicated that arsenic release from the reservoir sediments may be controlled by water nitrate concentration, which in turn is dependent on the nitrate concentration in the runoff water from the catchment. [Copyright &y& Elsevier]

Abstract: Wetlands are well known for water treatment and as a sink for elements (e.g. metals/metalloids). The elements are mainly fixed by sediments, microbial biofilms and plants, whereas the organic part of the sediment has a potential important role. These organic sediments underlie the process of organic matter decomposition, which proceeds in the three distinct steps of leaching, microbial conditioning and fragmentation by aquatic fauna. The effect of organic matter fragmentation on elemental fixation was shown previously only under neutral pH conditions. Hence, a batch experiment was conducted to assess the effect of invertebrate shredder on elemental accumulation during litter decay under low acidic conditions. Here it is shown that this fragmentation of the organic material by aquatic fauna enhances the elemental fixation potential of the organic sediments significantly, also under low acidic conditions. During the process of fragmentation the surface area of the organic matter increases by increasing amount of smaller particles. This in turn results in higher adsorption capacity of a larger amount of growing biofilm enhancing the surface even more (heterotrophic microorganisms). The growing biofilm (periphyton) with its larger surface area and hence more functional groups result in higher elemental fixation. Therefore, invertebrate shredders as ecosystem engineers enhance the capability for elemental fixation by fragmentation of organic matter, which is an important process in wetlands ecosystems usable in a broad pH range. [Copyright &y& Elsevier]

Abstract: The rapid increase of shortages in resources of chemical elements (and ores) used for an increasing industrial production raises the question of alternative strategies for their acquisition. Simultaneously, the elemental load in aquatic ecosystems increases by anthropogenic activities. Polluted waters are purged actively by technical treatment plants or passively by wetlands. Wetlands are known to eliminate/fix pollutants with a potentially high efficiency. Regarding this elimination/fixation potential less is known about different types of wetlands for elemental recovery. This paucity of information prompted to us to assess the impact of main processes in different types of wetlands on the recovery potential of chemical elements showing advantages and disadvantages of autochthonous and allochthonous wetlands and possible solutions. We show that autochthonous as well as allochthonous wetlands are able to accumulate high amounts of elements, but it is suggested that combining autochthonous/allochthonous processes should result in a higher efficiency. [Copyright &y& Elsevier]

Abstract: Organic matter is known to accumulate high amounts of metals/metalloids, enhanced during the process of decomposition by heterotrophic biofilms (with high fixation capacity for metals/metalloids). The colonization by microbes and the decay rate of the organic matter depends on different litter properties. Main litter properties affecting the decomposition of organic matter such as the nutrient ratios and the content of cellulose, lignin and phenols are currently described to be changed by silicon availability. But less is known about the impact of silicon availability during plant growth on elemental fixation during decay. Hence, this research focuses on the impact of silicon availability during plant growth on fixation of 42 elements during litter decay, by controlling the litter properties. The results of this experiment are a significantly higher metal/metalloid accumulation during decomposition of plant litter grown under low silicon availability. This may be explained by the altered litter properties (mainly nutrient content) affecting the microbial decomposition of the litter, the microbial growth on the litter and possibly by the silicon double layer, which is evident in leaf litter with high silicon content and reduces the binding sites for metals/metalloids. Furthermore, this silicon double layer may also reduce the growing biofilm by reducing the availability of carbon compounds at the litter surface and has to be elucidated in further research. Hence, low silicon availability during plant growth enhances the metal/metalloid accumulation into plant litter during aquatic decomposition. [Copyright &y& Elsevier]

Abstract: Silicon is described as beneficial for grasses by enhancing yield and fitness via a considerable contribution to pathogen, drought, and pest resistance. Silicic acid is the predominant form for uptake and transport within the plant and will precipitate in leaves. But it is unknown whether polymeric nanosilicon compounds in its synthetic form, with an increasing concentration in aquatic environments, can be suitable for plant nutrition. Therefore, we investigated the uptake, transport, and deposition of silicic acid/silica within plants using synthetic nanosilica. Our results show a significant difference in silicon (Si) content within the different tissues of Phragmites australis. The nanosilica had been dissolved prior to the uptake by plants. The chemical form of Si during uptake was not traceable. A significant enhancement in the condensation state of the silica was found from root to leaves especially from culm to leaf tips visible by the increasing content of Q4-groups in the NMR spectra. We conclude that synthetic nanosilica has the same quality as source for the beneficial element Si like natural silica. Since the condensation state is described to control silica solubility, we suggest that different condensation states within the plant may result in different remobilization of silicon during decomposition of the plant material. [Copyright &y& Elsevier]

Abstract: Silicon is not an essential element in sensu stricto but affects the productivity of Poaceae. Recent field studies has shown the effect of silicon on cellulose and lignin content in bulk analyses of green compartments of different species, sampled before matter translocation in the course of senescence. Nevertheless, there is a gap in information about silicon availability and its effect on cellulose, lignin and phenol content in different plant tissues after nutrient resorption. In order to prove the effect of controlled silica supply on functional relevant carbon compounds Phragmites australis Trin. was grown in pot trials under three different levels of silicon surplus. After resorption of nutrients into rhizomes the content of silicon, lignin, cellulose and phenol were measured in different plant tissues. The results show that different levels of silicon surplus changed the plant cellulose, lignin and phenol content depending on plant tissue function. Cellulose content in tissues with stabilization function is reduced contrasting enhanced cellulose content in tissues without stabilization function. Furthermore, higher silicon surplus decreased the phenol content in photosynthetic active tissues and increased the phenol content in culm. Only weak silicon to lignin interaction was found. It is concluded that silicon affects the cellulose and phenol metabolism and the tradeoff between productivity and stabilization/defense. Considering these compounds as crucial factors in decay processes, silicon may be very important for the terrestrial and semi-terrestrial carbon turnover [Copyright &y& Elsevier]

Abstract: Soil aggregation is a principal ecosystem process mediated by soil biota. Collembola and arbuscular mycorrhizal (AM) fungi are important groups in the soil, and can interact in various ways. Few studies have examined collembola effects on soil aggregation, while many have quantified AM effects. Here, we asked if collembola have any effect on soil aggregation, and if they alter AM fungi-mediated effects on soil aggregation. We carried out a factorial greenhouse study, manipulating the presence of both collembola and AM fungi, using two different plant species, Sorghum vulgare and Daucus carota. We measured root length and biomass, AMF (and non-AMF) soil hyphal length, root colonization, and collembolan populations, and quantified water stable soil aggregates (WSA) in four size classes. Soil exposed to growth of AMF hyphae and collembola individually had higher WSA than control treatments. Moreover, the interaction effects between AMF and collembola were significant, with non-additive increases in the combined application compared to the single treatments. Our findings show that collembola can play a crucial role in maintaining ecological sustainability through promoting soil aggregation, and point to the importance of considering organism interactions in understanding formation of soil structure. [Copyright &y& Elsevier]

Abstract: The focus of this article is to combine two main areas of research activities in freshwater ecosystems: the effect of inorganic pollutants on freshwater ecosystems and litter decomposition as a fundamental ecological process in streams. The decomposition of plant litter in aquatic systems as a main energy source in running water ecosystems proceeds in three distinct temporal stages of leaching, conditioning and fragmentation. During these stages metals and metalloids may be fixed by litter, its decay products and the associated organisms. The global-scale problem of contaminated freshwater ecosystems by metals and metalloids has led to many investigations on the acute and chronic toxicity of these elements to plants and animals as well as the impact on animal activity under laboratory conditions. Where sorption properties and accumulation/remobilization potential of metals in sediments and attached microorganisms are quite well understood, the combination of both research areas concerning the impact of higher trophic levels on the modification of sediment sorption conditions and the influence of metal/metalloid pollution on decomposition of plant litter mediated by decomposer community, as well as the effect of high metal load during litter decay on organism health under field conditions, has still to be elucidated. So far it was found that microbes and invertebrate shredder (species of the genera Gammarus and Asellus) have a significant influence on metal fixation on litter. Not many studies focus on the impact of other functional groups affecting litter decay (e.g. grazer and collectors) or other main processes in freshwater ecosystems like bioturbation (e.g. Tubifex, Chironomus) on metal fixation/release. [Copyright &y& Elsevier]

Abstract: Organic-rich sediments are known to be effective accumulators for uranium and arsenic. Much is known about the capacity for metal or metalloid fixation by microbes and organic compounds as well as inorganic sediment particles. Experiments investigating the effect of microbes on the process of metal fixation in sediments require sterilized sediments as control treatment which is often realized by gamma-sterilization. Only few studies show that gamma-sterilization has an effect on the remobilization of metal and metalloids and on their physico-chemical properties. These studies deal with sediments with negligible organic content whereas almost nothing is known about organic-rich sediments including a probably high microbial activity. In view of this, we investigated the effect of gamma-sterilization of organic-rich sediments on uranium and arsenic fixation and release. After ten days within an exposure experiment we found a significant higher remobilization of uranium and arsenic in sterile compared to unsterile treatments. In line with these findings the content of dissolved organic carbon (DOC), manganese, and iron increased to even significantly higher concentration in the sterile compared to unsterile treatment. Gamma-sterilization seems to change the physico-chemical properties of organic-rich sediments. Microbial activity is effectively eliminated. From increased DOC concentrations in overlaying water it is concluded that microbes are eventually killed with leaching of cellular compounds in the overlaying water. This decreases the adsorption capacity of the sediment and leads to enhanced uranium and arsenic remobilization. [Copyright &y& Elsevier]

Abstract: Invertebrate shredders such as Gammarus pulex are key species in contaminated stream ecosystems. Although a number of previous studies examining differences in metal accumulation between the gut system and remaining tissues of invertebrates exist, few focus on wide range of metals and metalloids that are relevant to contaminated systems. This study compared accumulation of the commonest (at study site) 15 metals and metalloids between the gut system including feces and remaining tissues of G. pulex. All metals and metalloids measured were significantly higher (p <0.001, except Cu p <0.005) in the gut system including feces than remaining tissues of G. pulex. Metals and metalloids in body tissues without the gut system including feces were significantly lower (Al, Cr, Fe and Mn (p <0.005), Sr and U (p <0.01), Co (p <0.05)) in content for a number of elements when compared to washed, whole G. pulex specimens. As well, all elements measured were significantly higher (all elements (p <0.005) except Cu and Co (p <0.05)) in gut system including feces than washed, whole G. pulex specimens. These results indicate that in G. pulex the uptake of all 15 metals and metalloids examined across the gut epithelium is minimalized or that sequestration of these elements in gut epithelial cells may occur. [Copyright &y& Elsevier]

Abstract: Organic sediments are known to be a significant sink of inorganic elements in polluted freshwater ecosystems. Hence, we investigated the role of invertebrate shredders (the freshwater shrimp Gammarus pulex L.) in metal and arsenic enrichment into organic partitions of sediments in a wetland stream at former uranium mining site. Metal and metalloid content in leaf litter increased significantly during decomposition, while at the same time the carbon content decreased. During decomposition, G. pulex as a ecosystem engineer facilitated significantly the enrichment of magnesium (250%), manganese (560%), cobalt (310%), copper (200%), zinc (43%), arsenic (670%), cadmium (100%) and lead (1340%) into small particle sizes. The enrichments occur under very high concentrations of dissolved organic carbon. Small particles have high surface area that results in high biofilm development. Further, the highest amounts of elements were observed in biofilms. Therefore, invertebrate shredder like G. pulex can enhance retention of large amounts of metal and arsenic in wetlands. [Copyright &y& Elsevier]

Treatment of systemic infections due to mycobacteria and HIV infection can lead to paradoxical worsening, the immune reconstitution inflammatory syndrome, in a minority of patients. Herein we describe a patient with Whipple''s disease, a chronic systemic inflammatory disease caused by Tropheryma whipplei, who developed cutaneous and later ocular disease after initiation of antibiotic therapy. A 42-year-old man with a 12-year history of arthralgias presented with deteriorating health, including weight loss, diarrhea, fever, and acral hyperkeratosis. Whipple''s disease was suspected and subsequently confirmed by finding periodic acid–Schiff (PAS)–positive foamy macrophages and T whipplei DNA by polymerase chain reaction (PCR) assays in duodenal biopsy specimens. After 5 weeks of antibiotic treatment with ceftriaxone, erythema nodosum (EN)–like lesions developed on the legs and trunk. Notably, lesional and nonlesional skin harbored intracellular and extracellular degenerated bacteria that were associated with a neutrophilic and granulomatous inflammatory response in lesional skin. Continued antibiotic therapy was associated with recurring EN-like skin nodules, orbital swelling, and facial herpes simplex virus 1 infection. Corticosteroid therapy controlled the duration and severity of the EN-like nodules and orbital swelling. Apart from cutaneous hyperpigmentation, skin disease in Whipple''s disease is infrequent and can be categorized as disorders due to malnutrition from malabsorption or so-called reversal reactions consisting of reactive erythemas, and neutrophilic and granulomatous responses to T whipplei, the latter of which can represent an immune reconstitution inflammatory reaction after initiation of antibiotic therapy. Finally, based on the presence of T whipplei in normal skin, skin biopsy may serve as another site for diagnostic testing in patients suspected of having Whipple''s disease. [Copyright &y& Elsevier]

In melanoma patients, surgical excision of the first draining lymph node, the sentinel lymph node (SLN), is a routine procedure to evaluate lymphogenic metastases. Metastasis detection by histopathological analysis assesses multiple tissue levels with hematoxylin and eosin and immunohistochemically stained glass slides. Considering the amount of tissue to analyze, the detection of metastasis can be highly time-consuming for pathologists. The application of artificial intelligence in the clinical routine has constantly increased over the past few years. In this multi-center study, a deep learning method was established on histological tissue sections of sentinel lymph nodes collected from the clinical routine. The algorithm was trained to highlight potential melanoma metastases for further review by pathologists, without relying on supplementary immunohistochemical stainings (e.g. anti-S100, anti-MelanA). The established method was able to detect the existence of metastasis on individual tissue cuts with an area under the curve of 0.9630 and 0.9856 respectively on two test cohorts from different laboratories. The method was able to accurately identify tumour deposits>0.1 mm and, by automatic tumour diameter measurement, classify these into 0.1 mm to −1.0 mm and>1.0 mm groups, thus identifying and classifying metastasis currently relevant for assessing prognosis and stratifying treatment. Our results demonstrate that AI-based SLN melanoma metastasis detection has great potential and could become a routinely applied aid for pathologists. Our current study focused on assessing established parameters; however, larger future AI-based studies could identify novel biomarkers potentially further improving SLN-based prognostic and therapeutic predictions for affected patients. • Deep learning algorithm establishment on histological slides in a multi-center study. • The algorithm highlights melanoma metastases on H&E stained tissue of SLN. • The method could accurately identify melanoma deposits of>0.1 mm in diameter. • Metastases detected with an AUC of 0.96 and 0.99 in two cohorts, respectively. [ABSTRACT FROM AUTHOR]

Despite seminal papers that stress the significance of silicon (Si) in plant biology and ecology, most studies focus on manipulations of Si supply and mitigation of stresses. The ecological significance of Si varies with different levels of biological organization, and remains hard to capture. We show that the costs of Si accumulation are greater than is currently acknowledged, and discuss potential links between Si and fitness components (growth, survival, reproduction), environment, and ecosystem functioning. We suggest that Si is more important in trait-based ecology than is currently recognized. Si potentially plays a significant role in many aspects of plant ecology, but knowledge gaps prevent us from understanding its possible contribution to the success of some clades and the expansion of specific biomes. The significance of silicon (Si) in plant ecology remains elusive. Trait-based analyses show that Si accumulates most in leaves of small non-woody species and is linked to key functional traits, suggesting trade-offs and physiological constraints on the evolution of Si accumulation in plants. Plant Si-related functions affect fitness components (growth, survival, reproduction), but a sound understanding of Si–fitness relationships is missing. In grasses, there is evidence for a Si-mediated trade-off between growth and defense. Si accumulation is more expensive than is currently acknowledged, and cost–benefit analyses are pivotal to understand how variation is dependent on environment and on trade-offs with other traits. Si plays a significant role in many aspects of plant ecology, but major gaps prevent us from understanding why this trait has been selected for and has contributed to the success of specific clades. [ABSTRACT FROM AUTHOR]

Changing fire regimes in response to climate change are likely to have significant effects on terrestrial ecosystems and biogeochemical cycles. While effects of fire on some nutrient cycles have been quite well-studied, little attention has been paid to the silicon cycle. We used an alkaline continuous extraction to examine changes in the quantity and characteristics of alkaline extractable Si (AlkExSi) after applying two burning treatments (no heating, 350 °C and 550 °C) to three types of organic soil material (from spruce forest, beech forest and a commercial peat). The total AlkExSi measured was 25.1 ± 2.1 mg g − 1 and 15.4 ± 0.9 mg g − 1 for spruce and beech respectively, and 1.2 ± 0.5 mg g − 1 for peat. The alkaline extraction parameters confirm a purely biogenic AlkExSi source in untreated spruce and beech organic soil material samples. Organic soil material of beech forest had two biogenic silica pools, differing in reactivity during alkaline extraction. Burning severely alters the alkaline dissolution parameters suggesting a significant crystallization of biogenic Si (BSi) with increased burning severity. Additionally, dissolution experiments carried out in rain water showed that fire increased the solubility of BSi by a factor of 40 and 20 in the case of the spruce and beech organic soil material respectively. The extent of enhanced Si solubility appears to be a trade-off function between organic matter losses and degree of crystallization. The burned soils could provide a strong and immediate Si source for the environment. In situ ecosystem characteristics that affect the uptake-leaching balance will determine the fate of the dissolved Si. Ecosystems low in BSi, like Sphagnum peatland, will not show drastic alteration in the Si cycle due to fire. [ABSTRACT FROM AUTHOR]

Chemical weathering of silicate minerals releases elements into solution whereas the neoformation of secondary minerals works in the opposite direction, potentially confounding estimates of silicate weathering rates. Silicon isotopes (δ 30 Si) may be a useful tool to investigate these processes. Here, we present 82 δ 30 Si measurements from surface waters, pore waters, biogenic silica (BSi), clays, sand and vegetation from the Okavango Delta, Botswana, a freshwater sub-tropical, flood-pulse wetland. Hydrologically, the Okavango is dominated by evapotranspiration water losses to the atmosphere. It receives an annual pulse of water that inundates seasonal floodplains, while river baseflow is sufficient to maintain a permanent floodplain. δ 30 Si in dissolved silica (DSi) in surface waters along a 300 km transect at near-peak flood show a limited range (0.36–1.19‰), implying the Delta is well buffered by a balance of processes adding and removing DSi from the surface water. A key control on DSi concentrations is the uptake, production of BSi and recycling of Si by aquatic vegetation, although the net isotopic effect is necessarily small since all BSi re-dissolves on short timescales. In the sediments, BSi δ 30 Si ( n = 30) ranges from −1.49‰ to +0.31‰ and during dissolution, residual BSi tends towards higher δ 30 Si. The data permit a field-based estimate of the fractionation associated with BSi dissolution, ε 30 BSi-DSi = −0.26‰, though it is unclear if this is an artefact of the process of dissolution. Clay δ 30 Si ranges from −0.97‰ to +0.10‰, ( n = 15, mean = −0.31‰) and include the highest values yet published, which we speculate may be due to an equilibrium isotope effect during diagenetic transformation of BSi. Two key trends in surface water DSi δ 30 Si merit further examination: declining δ 30 Si in an area roughly corresponding to the permanent floodplains despite net DSi removal, and increasing δ 30 Si in the area corresponding to the seasonal floodplains. We infer that evaporative enrichment of surface waters creates two contrasting regimes. Chemical weathering of low δ 30 Si phases releases low δ 30 Si DSi in the relatively dilute waters of the permanent floodplains, whereas silicon removal via clay formation or vegetation uptake is the dominant process in the more enriched, seasonal floodplains. [ABSTRACT FROM AUTHOR]

High concentrations of arsenic (As) in groundwater threaten the environment and public health. Geogenically, groundwater As contamination predominantly occurs via its mobilization from underground As-rich sediments. In an aquatic ecosystem, As is typically driven by several underlying processes, such as redox transitions, microbially driven reduction of iron (Fe) oxide minerals, and release of associated As. Notably, dissolved As mobilized from soils and sediments exhibits high affinity for dissolved organic matter (DOM). Thus, high DOM concentrations can increase As mobility. Therefore, it is crucial to understand the complex interactions and biogeochemical cycling of As, DOM, and Fe oxides. This review collates knowledge regarding the fate of As in multicomponent As–DOM–Fe systems, including ternary complexes involving both Fe and DOM. Additionally, the release mechanisms of As from sediments into groundwater in the presence of both Fe and DOM have been discussed. The mechanisms of As mobilization/sorption at the solid–water interface can be affected by negatively charged DOM competing for sorption sites with As on Fe (oxy)(hydr)oxides and may be further modified by other anionic ubiquitous species such as phosphate, silicic acid, or sulfur. This review emphasizes the need for a comprehensive understanding of the impact of DOM, Fe oxides, and related biogeochemical processes on As mobilization to aquifers. The review identifies important knowledge gaps that may aid in developing applicable practices for preventing the spread of As contamination in aquatic resources and traditional soil management practices. [Display omitted] • Arsenic (As)-iron (Fe)-dissolved organic matter (DOM) interaction in soil have been reviewed. • DOM compete with As for binding sites on Fe-oxides and enhance As desorption. • DOM can form complexes with As and increase its mobilization. • Reductive dissolutions of Fe-oxides in soils enhance As release to groundwater. [ABSTRACT FROM AUTHOR]

• Si influences biogeochemical cycling of nutrients such as P in minerotrophic peat. • Fe (II)–phosphate mobilization or dissolution occurs via competition for bindings sites. • Si can mobilize P and change DOC composition. • High Si concentration influences carbon quality & structure of iron phases (XAS) • Quinones & carboxylic groups are less abundant under high Si concentration. Prior studies have shown that silicon (Si) increases the dissolution of iron (Fe) phosphates, releasing phosphorus (P) into the soil solution of fen peat and thereby changing the composition of organic matter (OM). We formulated three questions on the occurring processes: [i] Does high Si concentration cause a mobilization of Fe phosphates? [ii] Does this mobilization result in a higher concentration of P, Fe, and dissolved organic carbon? [iii] Are there changes in the composition of organic matter? To this end, we conducted a laboratory experiment adding amorphous silica to fen peat and analyzed Fe mineralogy and carbon quality of the peat material by X-ray absorption spectroscopy (XAS). Under high Si concentration, significantly higher concentrations of P and Fe were found in soil solution. Moreover, XAS data indicated a dissolution of Fe (II) phosphates and a change in the composition of OM available to microbes with decreasing contributions of carboxylic groups and quinones. We propose to explain the effects as follows: under high Si concentration Fe (II) phosphates dissolve and OM rich in carboxylic groups are mobilized. As the nutrient concentration in the soil solution increased, the degradation of oxidized components of the OM, such as quinones and carboxylic groups is enhanced. [ABSTRACT FROM AUTHOR]