Rare earth elements sequestration in phytoliths: Partitioning patterns and influencing mechanism.

Rare earth elements (REEs) are integral to numerous high-tech industries, yet their biogeochemical cycling within ecosystems remains inadequately characterized. Recently, phytoliths have been identified as potentially significant sinks for REEs; however, their role in the cycling of these elements has been underestimated. In this work, we investigate the accumulation of REEs in phytoliths (PhytREEs) within the Greater Khingan Mountains region, employing an optimized wet oxidation method combined with heavy liquid flotation to quantify PhytREEs contents in surface soils. The results revealed an elevation-dependent pattern of PhytREEs concentration, with heightened levels at higher altitudes and diminishing concentrations towards the eastern plains. The enrichment coefficient of PhytREEs (ECPhytREEs) was found to be approximately 2.7 %, indicative of a moderately selective sequestration process. The multivariate analysis indicated that terrain complexity, climatic patterns, soil texture, and organic matter significantly influence the uptake and storage of REEs in plants, subsequently affecting their partitioning in phytoliths. Among these factors, the complexation of REEs with organic matter emerged as a pivotal mechanism facilitating their immobilization within phytoliths. Soil characteristics also play a non-negligible role in modulating REEs dynamics. Our findings highlight the predominant influence of climate on PhytREE storage, suggesting that climatic variables are the primary drivers modulating the bioavailability and ultimate sequestration of REEs within phytoliths. This study enhances our understanding of the biotic-abiotic interplay in the sequestration of REEs and underscores the need to incorporate phytoliths into models of terrestrial REE cycling.

Past Fire and Vegetation Change in the Hyperdiverse Forests of the Ecuadorian Amazon.

The Ecuadorian Amazon holds more biodiversity than most other places on Earth. Palms are a particularly dominant component of the vegetation; however, it remains unknown to what degree the pattern has persisted through time. Here, we investigate the persistence of palm dominance through time and the degree to which past human activities (e.g., fire, cultivation, and forest opening) have affected changes in palm abundances across five regions of the Ecuadorian Amazon. We analyzed soil cores (40-80 cm depth) from each region for charcoal (evidence of past fire) and phytoliths (evidence of past vegetation change). The timings of fires (based on 14C radiocarbon dates), the occurrence, recurrence, and number of fires (based on charcoal presence and abundance in samples), and the amount of change in palm abundances (based on phytoliths) varied within and between the studied regions. The charcoal and phytolith results indicate the presence of low levels of past human activity at all sites. Our results show that patterns of modern palm hyperdominance found in Amazonian forests have not been persistent through time, and that even low levels of past human activities can affect palm abundance.

Morphology and composition of calcium oxalate monohydrate phytoliths in the bark of Betula ermanii (stone birch): Case study from Sakhalin Island.

The morphology of calcium oxalate monohydrate precipitates (COM, Ca(C2O4)·H2O, P21/c, whewellite) occurring as crystals or intergrowths, as well as distribution of crystal-bearing idioblasts, have been studied for the first time in the bark of stone birch Betula ermanii from Sakhalin Island sampled in an area affected by mud volcanism and an unaffected typical forest environment taken for reference. The study addresses several issues (i) number and size of phytoliths and their distribution in different cell types; (ii) density of calcification in specific cells; (iii) habits of single crystals, twins, and complex intergrowths, as well as frequency of different morphologies and their relations. The trends of time-dependent morphological changes in separately analyzed crystals and intergrowths record the evolution of COM morphology from nuclei to mature grains. Of special interest are the nucleation sites and features of organic and inorganic seeds and nuclei for COM phytoliths. The precipitation process and crystal habits are mainly controlled by supersaturation, and it is thus important to constrain the Ca distribution patterns in different bark tissues. The B. ermanii samples were analyzed by several methods: scanning electron microscopy (SEM) for the distribution patterns and micromorphology of COM precipitates and bulk Ca content in bark; electron probe microanalysis (EPMA) for the mineral chemistry of COM precipitates; inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) for trace elements in bulk bark and wood. RESEARCH HIGHLIGHTS: The distribution and morphology of whewellite precipitates in the analyzed B. ermanii bark samples indicate that the aqueous solution was most strongly supersaturated with respect to the Ca(C2O4)·H2O solid phase at the parenchyma-sclerenchyma boundary, where most of the COM spherulites are localized and often coexist with large single crystals and contact COM twins.

Pre-contact and post-colonial ecological legacies shape Surinamese rainforests.

Disturbances in tropical forests can have long-lasting ecological impacts, but their manifestations (ecological legacies) in modern forests are uncertain. Many Amazonian forests bear the mark of past soil modifications, species enrichments, and fire events, but the trajectories of ecological legacies from the pre-contact or post-colonial period remain relatively unexplored. We assessed the fire and vegetation history from 15 soil cores ranging from 0 to 10 km from a post-colonial Surinamese archaeological site. We show that (1) fires occurred from 96 bc to recent times and induced significant vegetation change, (2) persistent ecological legacies from pre-contact and post-colonial fire and deforestation practices were mainly within 1 km of the archaeological site, and (3) palm enrichment of Attalea, Oenocarpus and Astrocaryum occurred within 0, 1, and 8 km of the archaeological site, respectively. Our results challenge the notion of spatially extensive and persistent ecological legacies. Instead, our data indicate that the persistence and extent of ecological legacies are dependent on their timing, frequency, type, and intensity. Examining the mechanisms and manifestations of ecological legacies is crucial in assessing forest resilience and Indigenous and local land rights in the highly threatened Amazonian forests.

A rapid and simple method for the extraction of biogenic silica (BSi) in phytolith-poor sediments and soils.

Phytoliths can be used to reconstruct human-nature dynamics over the long term (from decennial to centennial and millennial time scales) and may capture activities that cannot be reconstructed through other proxies. Phytoliths consist of fossil biogenic silica (BSi), formed in plant organs and then released into the soil with plant decay. When working in environmental contexts where the phytolith signal is highly diluted, as is the case in environments with a long history of land use, animal-plant interactions and open woody environments, the extraction of phytoliths remains a challenge. To address this issue, we developed an efficient method for the extraction of biogenic silica (BSi) from sediments and soils of contexts characterised by the long-term human and animal presence and disturbance, such as remnants of old agroforestry systems. The method we developed has a number of advantages, including: •An easy and time-efficient methodology to perform (with an overall processing time of 1.5/2 days for a batch of 16 samples)•An extraction method free from dangerous chemicals•A method amenable to non-experts without a prior background in lab extraction procedures.

Effects of leaf silicon on drought performance of tropical tree seedlings.

Elevated leaf silicon (Si) concentrations improve drought resistance in cultivated plants, suggesting Si might also improve drought performance of wild species. Tropical tree species, for instance, take up substantial amounts of Si, and leaf Si varies markedly at local and regional scales, suggesting consequences for seedling drought resistance. Yet, whether elevated leaf Si improves seedling drought performance in tropical forests is unknown. To manipulate leaf Si concentrations, seedlings of seven tropical tree species were grown in Si-rich and -poor soil, before exposing them to drought in the forest understorey. Survival, growth and wilting were monitored. Elevated leaf Si did not improve drought survival and growth in any of the species. In one species, drought survival was reduced in seedlings previously grown in Si-rich soil, contrary to our expectation. Our results suggest that elevated leaf Si does not improve drought resistance of wild tropical tree species. Elevated leaf Si may even reduce drought performance, suggesting differences in soil conditions influencing leaf Si may contribute to soil-related variation of tropical seedling performance. Furthermore, our results are at odds with most studies on cultivated species and show that alleviative effects of Si in crops cannot be generalized to wild plants in natural systems.

Phytolith assemblages reflect variability in human land use and the modern environment.

Phytoliths preserved in soils and sediments can be used to provide unique insights into past vegetation dynamics in response to human and climate change. Phytoliths can reconstruct local vegetation in terrestrial soils where pollen grains typically decay, providing a range of markers (or lack thereof) that document past human activities. The ca. 6 million km2 of Amazonian forests have relatively few baseline datasets documenting changes in phytolith representation across gradients of human disturbances. Here we show that phytolith assemblages vary on local scales across a gradient of (modern) human disturbance in tropical rainforests of Suriname. Detrended correspondence analysis showed that the phytolith assemblages found in managed landscapes (shifting cultivation and a garden), unmanaged forests, and abandoned reforesting sites were clearly distinguishable from intact forests and from each other. Our results highlight the sensitivity and potential of phytoliths to be used in reconstructing successional trajectories after site usage and abandonment. Percentages of specific phytolith morphotypes were also positively correlated with local palm abundances derived from UAV data, and with biomass estimated from MODIS satellite imagery. This baseline dataset provides an index of likely changes that can be observed at other sites that indicate past human activities and long-term forest recovery in Amazonia. Supplementary Information: The online version contains supplementary material available at 10.1007/s00334-023-00932-2.

Elemental composition of grass phytoliths: Environmental control and effect on dissolution.

Plant phytoliths, which represent the main pool of silica (Si) in the form of hydrous Si oxide, are capable of providing valuable information on different aspect of environmental issues including paleo-environmental reconstruction and agricultural sustainability. Phytoliths may have different chemical composition, which, in turn, affects their preservation in soils ad impacts terrestrial cycle of the occluded elements including micro-nutrients and environmental toxicants. Yet, in contrast to sizable work devoted to phytoliths formation, dissolution and physico-chemical properties, the mechanisms that control total (major and trace) elemental composition and the impact that various elements exert on phytolith reactivity and preservation in soils remains poorly known. In order to fil this gap in knowledge, here we combined two different approaches - analytical trace element geochemistry and experimental physical chemistry. First, we assessed full elemental composition of phytoliths from different plants via measuring major and trace elements in 9 samples of grasses collected in northern Eurasia during different seasons, 18 grasses from Siberian regions, and 4 typical Si-concentrating plants (horsetail, larch, elm and tree fern). We further assessed the dissolution rates of phytoliths exhibiting drastically different concentrations of trace metals. In the European grasses, the variations of phytolith chemical composition among species were highly superior to the variations across vegetative season. Compared to European samples, Siberian grass phytoliths were impoverished in Ca and Sr, exhibited similar concentrations of Li, B, Na, Mg, K, V, Zn, Ni, Mo, As, Ba, and U, and were strongly enriched (x 100-1000) in lithogenic elements (trivalent and tetravalent hydrolysates), P, Mn, Fe and divalent metals. Overall, the variations in elemental composition between different species of the same region were lower compared to variations of the same species from distant regions. The main factors controlling phytoliths elemental composition are the far-range atmospheric (dust) transfer, climatic conditions (humidity), and, in a lesser degree, local lithology and anthropogenic pollution. Despite significant, up to 3 orders of magnitude, difference in TE composition of grass and other plant phytoliths, the dissolution rates of grass phytoliths measured in this study were similar, within the experimental uncertainty, to those of other plants studied in former works. Therefore, elemental composition of phytoliths has relatively minor impact on their preservation in soils.

Silicon-based anti-herbivore defense in tropical tree seedlings.

Silicon-based defenses deter insect herbivores in many cultivated and wild grass species. Furthermore, in some of these species, silicon (Si) uptake and defense can be induced by herbivory. Tropical trees also take up Si and leaf Si concentrations vary greatly across and within species. As herbivory is a major driver of seedling mortality and niche differentiation of tropical tree species, understanding anti-herbivore defenses is pivotal. Yet, whether silicon is a constitutive and inducible herbivory defense in tropical forest tree species remains unknown. We grew seedlings of eight tropical tree species in a full factorial experiment, including two levels of plant-available soil Si concentrations (-Si/+Si) and a simulated herbivory treatment (-H/+H). The simulated herbivory treatment was a combination of clipping and application of methyl jasmonate. We then carried out multiple-choice feeding trials, separately for each tree species, in which leaves of each treatment combination were offered to a generalist caterpillar (Spodoptera frugiperda). Leaf damage was assessed. Three species showed a significant decrease in leaf damage under high compared to low Si conditions (by up to 72%), consistent with our expectation of Si-based defenses acting in tropical tree species. In one species, leaf damage was increased by increasing soil Si and in four species, no effect of soil Si on leaf damage was observed. Opposite to our expectation of Si uptake and defense being inducible by herbivory damage, simulated herbivory increased leaf damage in two species. Furthermore, simulated herbivory reduced Si concentrations in one species. Our results showed that tropical tree seedlings can be better defended when growing in Si-rich compared to Si-poor soils, and that the effects of Si on plant defense vary strongly across species. Furthermore, Si-based defenses may not be inducible in tropical tree species. Overall, constitutive Si-based defense should be considered part of the vast array of anti-herbivore defenses of tropical tree species. Our finding that Si-based defenses are highly species-specific combined with the fact that herbivory is a major driver of mortality in tropical tree seedling, suggests that variation in soil Si concentrations may have pervasive consequences for regeneration and performance across tropical tree species.

Miocene phytolith and diatom dataset from 10.3Myo diatomite formation, Fernley, Nevada, USA.

Phytoliths are opal silica particles formed within plant tissues. Diatoms are aquatic, single-celled photosynthetic algae with silica skeletons. Phytolith and diatom morphotypes vary depending on local environmental and climatic conditions and because their silicate structures preserve well, the study of phytolith and diatom morphotypes can be used to better understand paleoclimatic and paleoenvironmental dynamics and changes. This article presents original data from an 820cm-deep stratigraphy excavated at the Hazen diatomite deposits, a high-elevation desert paleolake in the Fernley District, Northern Nevada, USA. The site has been studied for an assemblage of fossilized threespine stickleback, Gasterosteus doryssus, that reveal adaptive evolution. For this study, a total of 157 samples were extracted at 20 cm intervals covering approximately 24,500 years. After extraction, the samples were mounted on slides and viewed under 400-1000x light microscopy, enabling classification of 14 phytolith and 45 diatom morphotypes. Our data support paleoenvironmental reconstructions of the Hazen Miocene paleolake.

Phytoliths analysis in root, culm, leaf and synflorescence of Rostraria cristata (Poaceae).

Phytoliths (siliceous structures) present in the plants have been employed in the fields of taxonomy and archaeology for many decades. Rostraria cristata is an economically important grass species (Poaceae) which accumulates silica in its different organs in the form of phytoliths. In order to understand the pattern of phytolith production and biochemical architecture of silica in R. cristata, leaf epidermis (blade) using the clearing solution method and different organs using the dry ashing method, X-ray diffraction and Fourier-transform infrared spectroscopy techniques were analyzed. Both abaxial and adaxial leaf epidermis showed the presence of acute bulbosus, rectangular sinuate and stomata phytolith morphotypes. Leaf including sheath and blade had the highest silica content. Characteristic phytolith morphotypes were present in different organs. A total of 34 phytolith morphotypes were present among which nine (9) were articulated and 25 were isolated forms. The most abundant were elongate scrobiculate (48.20%) in root and rectangular sinuate (26.16%) in leaf part. Other common phytolith morphotypes present in different organs of R. cristata were articulated elongate irregular, articulated elongate scrobiculate, acute bulbosus, and polygonal rondel etc. Leaf and synflorescence had the highest similarity based on presence/absence of phytolith morphotypes (Jaccard's similarity index). XRD studies revealed the presence of cristobalite, quartz, tridymite, zeolite etc. forms of silica in different organs. FTIR spectra showed that inplane stretching vibration of Si-C was unique to root, anti-symmetric stretching vibration of C-H was unique to leaf and Al2O3.SiO2 was found in synflorescence only. Our results show the characteristic pattern of phytoliths production in R. cristata.

Silicon in Plants: Alleviation of Metal(loid) Toxicity and Consequential Perspectives for Phytoremediation.

For the majority of higher plants, silicon (Si) is considered a beneficial element because of the various favorable effects of Si accumulation in plants that have been revealed, including the alleviation of metal(loid) toxicity. The accumulation of non-degradable metal(loid)s in the environment strongly increased in the last decades by intensified industrial and agricultural production with negative consequences for the environment and human health. Phytoremediation, i.e., the use of plants to extract and remove elemental pollutants from contaminated soils, has been commonly used for the restoration of metal(loid)-contaminated sites. In our viewpoint article, we briefly summarize the current knowledge of Si-mediated alleviation of metal(loid) toxicity in plants and the potential role of Si in the phytoremediation of soils contaminated with metal(loid)s. In this context, a special focus is on metal(loid) accumulation in (soil) phytoliths, i.e., relatively stable silica structures formed in plants. The accumulation of metal(loid)s in phytoliths might offer a promising pathway for the long-term sequestration of metal(loid)s in soils. As specific phytoliths might also represent an important carbon sink in soils, phytoliths might be a silver bullet in the mitigation of global change. Thus, the time is now to combine Si/phytolith and phytoremediation research. This will help us to merge the positive effects of Si accumulation in plants with the advantages of phytoremediation, which represents an economically feasible and environmentally friendly way to restore metal(loid)-contaminated sites.

On the verge of domestication: Early use of C4 plants in the Horn of Africa.

The earliest evidence of agriculture in the Horn of Africa dates to the Pre-Aksumite period (ca. 1600 BCE). Domesticated C3 cereals are considered to have been introduced from the Near East, whereas the origin (local or not) and time of domestication of various African C4 species such as sorghum, finger millet, or t'ef remain unknown. In this paper, we present the results of the analysis of microbotanical residues (starch and phytoliths) from grinding stones recovered from two archaeological sites in northeastern Tigrai (Ethiopia), namely Mezber and Ona Adi. Together, both sites cover a time period that encompasses the earliest evidence of agriculture in the region (ca. 1600 BCE) to the fall of the Kingdom of Aksum (ca. 700 CE). Our data indicate that these communities featured complex mixed economies which included the consumption of both domestic and wild plant products since the Initial Pre-Aksumite Phase (ca. 1600 to 900 BCE), including C3 crops and legumes, but also C4 cereals and geophytes. These new data expand the record of C4 plant use in the Horn of Africa to over 1,000 y. It also represents the first evidence for the consumption of starchy products in the region. These results have parallels in the wider northeastern African region where complex food systems have been documented. Altogether, our data represent a significant challenge to our current knowledge of Pre-Aksumite and Aksumite economies, forcing us to rethink the way we define these cultural horizons.

Macroscopic dental measures in guinea pigs (Cavia porcellus) fed natural and pelleted diets of different abrasiveness: implications for wear and compensatory growth in a hypselodont species.

The drivers of dental wear and compensatory hypselodont tooth growth are of current research interest. Expanding previous macroscopic dental wear measurements based on microtomographic scans of guinea pigs (Cavia porcellus) fed natural diets, we added diet groups with different predicted drivers of dental wear and analysed how measured variables relate to each other. The teeth of guinea pigs fed either pelleted diets containing external abrasives of various shapes, sizes and percentages (n = 66) or natural whole-leaf diets (n = 36, low-phytolith lucerne or grass or high-phytolith bamboo) were evaluated. The bamboo-fed animals showed the lowest tooth height with deep dentine basins, similar to the pellet-fed animals. Deeper dentine basins generally correlated with higher occlusal surfaces, allowing the hypothesis that changes in the pressure signal due to lower basins could initiate compensatory growth and broadening of the whole tooth surface in hypselodont teeth. Macroscopic dental wear did not categorically differ between whole-leaf or pelleted diets or between diets with internal phytoliths or with external silicate abrasives. Supporting interpretations that tooth wear should be viewed as a response to the biomechanical properties of ingested feed which may or may not be aptly summarized by broad descriptors such as 'whole/pelleted' or 'natural/artificial'.

Long-term fire and vegetation change in northwestern Amazonia.

Amazonian forest plots are used to quantify biodiversity and carbon sequestration, and provide the foundation for much of what is known about tropical ecology. Many plots are assumed to be undisturbed, but recent work suggests that past fire, forest openings, and cultivation created vegetation changes that have persisted for decades to centuries (ecological legacies). The Yasuní Forest Dynamics plot is one of the most biodiverse places on earth, yet its human history remains unknown. Here, we use charcoal and phytolith analysis to investigate the fire and vegetation history of the Yasuní forest plot, and compare results with nearby forest plots in Colombia (Amacayacu) and Peru (Medio Putumayo-Algodón [MPA]) to explore the spatial variability of past disturbances and ecological legacies in northwestern Amazonia. Three 14C dated charcoal fragments provided evidence for a modern (1956 CE) and a past fire event ca. 750 years ago at Yasuní, compared with fire ages of 1000-1600 years ago documented at Amacayacu and MPA. Small-scale disturbances and localized canopy openings also occurred in the Yasuní plot. Phytolith assemblages from Yasuní and Amacayacu showed more variability in past vegetation change than MPA. Low-intensity, non-continuous disturbances occurred at all three plots in the past, and our results highlight the variability of past human activities both in space and time in northwestern Amazonia. Our data also suggest that post-Columbian human disturbances from the Rubber Boom (AD 1850-1920) and subsequent oil exploration have likely left stronger ecological legacies than those left by pre-Columbian peoples in our studied regions.

Phytolith particulate matter and its potential human and environmental effects.

Particulate matter from both natural and anthropogenic sources is known to affect air quality and human health. However, the abundance and varied composition of the suspended particulate matter make it difficult to locate the precise precursors for some of these atmospheric pollutants. Plants deposit appreciable quantities of microscopic biogenic silica in and/or between their cells, known as phytoliths, which get released into the soil surface after the death and decomposition of plants. Dust storms from exposed terrains, forest fires, and stubble burning disperse these phytoliths into the atmosphere. Their durability, chemical composition, and diverse morphology prompt us to view phytoliths as a possible particulate matter that could impact air quality, climate, and human health. Estimating the phytolith particulate matter, its toxicity, and environmental impacts will help take effective and targeted policies for improving air quality and decreasing health risks.

Silica deposition in plants: scaffolding the mineralization.

BACKGROUND: Silicon and aluminium oxides make the bulk of agricultural soils. Plants absorb dissolved silicon as silicic acid into their bodies through their roots. The silicic acid moves with transpiration to target tissues in the plant body, where it polymerizes into biogenic silica. Mostly, the mineral forms on a matrix of cell wall polymers to create a composite material. Historically, silica deposition (silicification) was supposed to occur once water evaporated from the plant surface, leaving behind an increased concentration of silicic acid within plant tissues. However, recent publications indicate that certain cell wall polymers and proteins initiate and control the extent of plant silicification. SCOPE: Here we review recent publications on the polymers that scaffold the formation of biogenic plant silica, and propose a paradigm shift from spontaneous polymerization of silicic acid to dedicated active metabolic processes that control both the location and the extent of the mineralization. CONCLUSION: Protein activity concentrates silicic acid beyond its saturation level. Polymeric structures at the cell wall stabilize the supersaturated silicic acid and allow its flow with the transpiration stream, or bind it and allow its initial condensation. Silica nucleation and further polymerization are enabled on a polymeric scaffold, which is embedded within the mineral. Deposition is terminated once free silicic acid is consumed or the chemical moieties for its binding are saturated.

Climatic Drivers of Silicon Accumulation in a Model Grass Operate in Low- but Not High-Silicon Soils.

Grasses are hyper-accumulators of silicon (Si), which is known to alleviate diverse environmental stresses, prompting speculation that Si accumulation evolved in response to unfavourable climatic conditions, including seasonally arid environments. We conducted a common garden experiment using 57 accessions of the model grass Brachypodium distachyon, sourced from different Mediterranean locations, to test relationships between Si accumulation and 19 bioclimatic variables. Plants were grown in soil with either low or high (Si supplemented) levels of bioavailable Si. Si accumulation was negatively correlated with temperature variables (annual mean diurnal temperature range, temperature seasonality, annual temperature range) and precipitation seasonality. Si accumulation was positively correlated with precipitation variables (annual precipitation, precipitation of the driest month and quarter, and precipitation of the warmest quarter). These relationships, however, were only observed in low-Si soils and not in Si-supplemented soils. Our hypothesis that accessions of B. distachyon from seasonally arid conditions have higher Si accumulation was not supported. On the contrary, higher temperatures and lower precipitation regimes were associated with lower Si accumulation. These relationships were decoupled in high-Si soils. These exploratory results suggest that geographical origin and prevailing climatic conditions may play a role in predicting patterns of Si accumulation in grasses.

New insight into the mechanisms of preferential encapsulation of metal(loid)s by wheat phytoliths under silicon nanoparticle amendment.

Silicon nanoparticles (SiNPs) have been widely used to immobilize toxic trace metal(loid)s (TTMs) in contaminated croplands. However, the effect and mechanisms of SiNP application on TTM transportation in response to phytolith formation and phytolith-encapsulated-TTM (PhytTTM) production in plants are unclear. This study demonstrates the promotion effect of SiNP amendment on phytolith development and explores the associated mechanisms of TTM encapsulation in wheat phytoliths grown on multi-TTM contaminated soil. The bioconcentration factors between organic tissues and phytoliths of As and Cr (> 1) were significantly higher than those of Cd, Pb, Zn and Cu, and about 10 % and 40 % of the total As and Cr that bioaccumulated in wheat organic tissues were encapsulated into the corresponding phytoliths under high-level SiNP treatment. These observations demonstrate that the potential interaction of plant silica with TTMs is highly variable among elements, with As and Cr being the two most strongly concentrated TTMs in the phytoliths of wheat treated with SiNPs. The qualitative and semi-quantitative analyses of the phytoliths extracted from wheat tissues suggest that the high pore space and surface area (≈ 200 m2 g-1) of phytolith particles could have contributed to the embedding of TTMs during silica gel polymerization and concentration to form PhytTTMs. The abundant SiO functional groups and high silicate-minerals in phytoliths are dominant chemical mechanisms for the preferential encapsulation of TTMs (i.e., As and Cr) by wheat phytoliths. Notably, the organic carbon and bioavailable Si of soils and the translocation of minerals from soil to plant aerial parts can impact TTM sequestration by phytoliths. Thus, this study has implications for the distribution or detoxification of TTMs in plants via preferential PhytTTM production and biogeochemical cycling of PhytTTMs in contaminated cropland following exogenous Si supplementation.

Changing perspectives on early hominin diets.

It is axiomatic that knowledge of the diets of extinct hominin species is central to any understanding of their ecology and our evolution. The importance of diet in the paleontological realm has led to the employment of multiple approaches in its elucidation. Some of these have deep historical roots, while others are dependent upon more recent technical and methodological advances. Historically, studies of tooth size, shape, and structure have been the gold standard for reconstructing diet. They focus on species-level adaptations, and as such, they can set theoretical brackets for dietary capabilities within the context of specific evolutionary moments. Other methods (e.g., analyses of dental calculus, biogeochemistry, and dental microwear) have only been developed within the past few decades, but are now beginning to yield evidence of the actual foods consumed by individuals represented by fossil remains. Here we begin by looking at these more "direct" forms of evidence of diet before showing that, when used in conjunction with other techniques, these "multi-proxy" approaches can raise questions about traditional interpretations of early hominin diets and change the nature of paleobiological interpretations.