Biogenic Silica Production Research Articles

Quantifying the biogenic silica production of picoplankton in the oligotrophic ocean: A case study in the South China Sea

Silicon (Si) utilization is not limited to eukaryotes. Recent research has suggested that the pattern of a large contribution of picocyanobacteria to biogenic silica (bSi) stocks might be widespread in the oligotrophic open ocean. We are the first to measure the size-fractionated bSi standing stocks and production rates in the oligotrophic South China Sea (SCS), which has obvious characteristics of oligotrophic waters. The 150 m integrated bSi standing stocks in the pico-sized fractions averaged 23 % of the total; the contribution of picoplankton to the total bSi production rate was 44 %. Interestingly, our estimated contributions of Synechococcus alone to the <2 μm bSi standing stock and < 2 μm bSi production rates averaged 14 % and 66 %, respectively, indicating that the significant and persistent contribution of bSi was strongly associated with marine picocyanobacteria. Furthermore, the dynamic changes in nutrient concentrations, especially in DIN and DIP, also potentially affected the variability in picoplankton bSi stocks and production rates, while the effects of temperature and salinity were not obvious. In this study, we have provided new information on measurable bSi in the picoplankton size fraction and its production rate in the SCS. We have demonstrated that picoplankton contributes a measurable, and at times significant, proportion to both the total bSi standing stock and its production rate in the SCS. A high silicon content within picocyanobacteria has important implications for understanding both their ecology and their contribution to biogeochemistry.

Global census of the significance of giant mesopelagic protists to the marine carbon and silicon cycles

Thriving in both epipelagic and mesopelagic layers, Rhizaria are biomineralizing protists, mixotrophs or flux-feeders, often reaching gigantic sizes. In situ imaging showed their contribution to oceanic carbon stock, but left their contribution to element cycling unquantified. Here, we compile a global dataset of 167,551 Underwater Vision Profiler 5 Rhizaria images, and apply machine learning models to predict their organic carbon and biogenic silica biomasses in the uppermost 1000 m. We estimate that Rhizaria represent up to 1.7% of mesozooplankton carbon biomass in the top 500 m. Rhizaria biomass, dominated by Phaeodaria, is more than twice as high in the mesopelagic than in the epipelagic layer. Globally, the carbon demand of mesopelagic, flux-feeding Phaeodaria reaches 0.46 Pg C y−1, representing 3.8 to 9.2% of gravitational carbon export. Furthermore, we show that Rhizaria are a unique source of biogenic silica production in the mesopelagic layer, where no other silicifiers are present. Our global census further highlights the importance of Rhizaria for ocean biogeochemistry.

Purified residues from the production of biogenic silica as resources for the remediation of oilfield produced water: A strategy for phenol removal

The escalating fuel demand generates significant volumes of oilfield-produced water containing complex environmental pollutants such as phenol. This study proposes a purification method for converting activated carbon (CAM), obtained at zero cost as a by-product of biogenic silica production, into an effective adsorbent. The activated carbon was characterized extensively for its morphological, structural, and chemical properties, revealing a substantial surface area exceeding 1000 m2/g. The adsorption process for phenol removal was investigated through kinetics, isotherms, and thermodynamic analyses. Kinetic data analysis favored the PSO models within 150 min, while equilibrium studies showed the best fit for the Sips and Liu models. CAM achieved an adsorption capacity of 23.9 mg g−1 for phenol removal at pH 6, the typical pH of produced water. The adsorption process was found to be exothermic and spontaneous. The produced adsorbent demonstrated high recyclability, with up to 15 reuse cycles achievable through regeneration via thermal desorption at 180 °C. Furthermore, CAM removed 73 % of total organic carbon from real produced water within 150 min. Thus, using activated carbon intrinsically carbonized during silica production is an economically viable and attractive solution for treating phenol-contaminated oilfield-produced water.

Upwelling-driven biogenic silica accumulation in the Yangtze Sea, South China during Late Ordovician to Early Silurian time: A possible link with the global climatic transitions

Silica accumulation in the Yangtze Sea during the Ordovician–Silurian (O–S) transition appears to have coincided with global climatic fluctuations, widespread upwelling, and volcanism. There is a need to further evaluate their respective contributions to silica deposition and potential relationships among these factors. The current study selected siliceous deposits in the Wufeng and Longmaxi Formations from four sections spanning the inner to outer Yangtze Sea, South China, to gain a deeper understanding of the climatic and oceanographic evolution associated with silica enrichment. Al/(Al + Fe + Mn) values, the presence of radiolarians, and Si isotope values of samples recovered from the investigated shale successions offer compelling evidence that the silica is largely of biogenic origin with some terrigenous contributions. Further, various productivity and redox proxies suggest that biogenic silica (BSi) accumulated under conditions of enhanced marine productivity and anoxic bottom water conditions. Hg/TOC and Zr/Al2O3 profiles suggest intermittent volcanism during the BSi deposition in the Yangtze Sea. However, the lack of correlation between BSi and Hg/TOC values indicates that volcanic iron fertilization was not responsible for BSi accumulation. Instead, most BSi-rich samples are dominated by low MnEF × CoEF values (<0.5), consistent with BSi deposited in modern upwelling settings. Hydrographic reconstruction based on Mo–U covariation indicates a more open water setting in the outer Yangtze Sea, while the coeval inner Yangtze Sea was relatively restricted. Therefore, upwelling events appear to have been more vigorous in the outer Yangtze Sea. Published and new Chemical Index of Alteration (CIA), BSi, and MnEF × CoEF data for the Wufeng and Longmaxi Formations across the inner to outer Yangtze Sea demonstrate that temporal and spatial variations of BSi were controlled by climate-driven upwelling. In particular, cool-water upwelling contemporaneous with Hirnantian glaciation may have been responsible for the establishment of the cool-water fauna of the shallow-water Guanyinqiao Bed and enhanced silica deposition in deeper water. Moreover, a moderate negative relationship between compiled CIA and BSi contents suggests that enhanced upwelling driving BSi accumulation appears to have been favored during cooling events. Integrated analysis of BSi deposits of the Laurentia and Baltica continental margins further suggests that BSi accumulation on continental margins during the O–S transition was primarily influenced by global cooling. Therefore, we suggest that wind patterns or/and thermohaline circulation, influenced by climate fluctuations, induced widespread cold water upwelling events during the O–S transition. Moreover, elevated BSi production diluted accumulating OM resulting in the observed parabolic relationship of BSi and TOC.

Drivers of diatom production and the legacy of eutrophication in two river plume regions of the northern Gulf of Mexico

In the northern Gulf of Mexico (nGoM), the Louisiana Shelf (LS) and Mississippi Bight (MB) subregions are influenced by eutrophication to varying degrees. Despite recognition that dissolved silicon may regulate diatom productivity in the nGoM, there is only one published data set reporting biogenic silica (bSiO2) production rates for each subregion. We report that bSiO2 production rates on the LS and MB are high and appear to be controlled by different nutrients among seasons. Despite exceptional upper trophic level biomass regionally, which suggests significant primary production by diatoms (as in other systems), gross euphotic-zone integrated bSiO2 production rates are lower than major bSiO2 producing regions (e.g. upwelling systems). However, when normalizing to the depth of the euphotic zone, the bSiO2 production rates on the LS are like normalized rates in upwelling systems. We suggest local river-plume influenced hydrography concentrates diatom productivity within shallow euphotic zones, making production more accessible to higher trophic organisms. Comparison of rates between the LS and MB suggest that the fluvial nitrate within the LS stimulates bSiO2 production above that in the MB, which has a smaller watershed and is less eutrophic (relatively). Beyond understanding the factors controlling regional bSiO2 production, these data offer the most comprehensive Si-cycle baseline to date as the LS and MB will likely exchange freely in the mid to late century due to land subsidence of the Mississippi River delta and/or sea-level rise.

Decoupling of Barium and Silicon at the Congo River‐Dominated Southeast Atlantic Margin: Insights From Combined Barium and Silicon Isotopes

AbstractThe correlation between concentrations of dissolved barium (dBa) and silicon (dSi) in the modern ocean supports the use of Ba as a paleoceanographic proxy. However, the mechanisms behind their linkage and the exact processes controlling oceanic Ba cycling remain enigmatic. To discern the extent to which this association arises from biogeochemical processes versus physical mixing, we examine the behavior of Ba and Si at the Congo River‐dominated Southeast Atlantic margin where active biological processes and large boundary inputs override the large‐scale ocean circulation. Here we present the first combined measurements of dissolved stable Ba (δ138Ba) and Si (δ30Si) isotopes as well as Ba and Si fluxes estimated based on 228Ra from the Congo River mouth to the northern Angola Basin. In the surface waters, river‐borne particle desorption or dissolution and shelf inputs lead to non‐conservative additions of both dBa and dSi to the Congo‐shelf‐zone, with the Ba flux increasing more strongly than that of Si across the shelf. In the epipelagic and mesopelagic layers, Ba and Si are decoupled likely due to different depths of in situ barite precipitation and biogenic silica production. In the deep waters of the northern Angola Basin, we observe large enrichment of dBa, likely originating from high benthic inputs from the Congo deep‐sea fan sediments. Our results reveal different mechanisms controlling the biogeochemical cycling of Ba and Si and highlight a strong margin influence on marine Ba cycling. Their close association across the global ocean must therefore mainly be a consequence of the large‐scale ocean circulation.

Iron and silicic acid addition effects on early spring macronutrient drawdown and biogenic silica production of Patagonia estuarine waters

The Patagonia archipelago interior sea (PAIS) of southern Chile is one of the largest fjord systems on earth. These coastal waters include remote and virtually pristine areas where extreme rainfall/runoff and glacial meltwaters intensify the land–ocean interaction impinging on the biological, physical, and chemical characteristics of oceanic subantarctic Surface Water (SAASW) that flood the archipelago basins. The SAASW mix with silicon- and iron-replete continental water and diatom growth would occur concomitantly with a rapid drawdown of SAASW macronutrients. Consequently, phytoplankton metabolism (e.g. macronutrient utilization for primary productivity) in estuaries of southern Patagonian has been previously assumed independent of iron availability (i.e. iron-replete conditions). Experimental results shown here suggest that the nitrate and phosphate drawdown in low salinity (29) water can be enhanced by a 5 nM dissolved iron enrichment (by 13% and 28%, respectively) during the developing phase of a diatom bloom. The simultaneous enrichment in iron (5 nM) and silicic acid (5 µM) in these estuarine waters resulted in a similar macronutrient uptake enhancement, a 119% increment of the production of biogenic silica and a 2-fold rise in the abundance of Pseudo-nitzschia spp (a diatom capable to produce the neurotoxin domoic acid). We suggest that natural freshwater pulses of allochthonous bioavailable forms of iron and silicon to inner waters of the Patagonia archipelago during the onset of the productive season play a potentially significant role modulating macronutrient dynamics (input vs utilization) and influencing coastal phytoplankton assemblages.

Elevated sedimentation of clastic matter in pelagic Panthalassa during the early Olenekian

AbstractThe end‐Permian mass extinction is thought to have greatly altered biogeochemical cycles. The absence of chert and dominance of claystone in low‐latitude pelagic deep‐sea sedimentary sequences of Early Triassic Panthalassa (the deep‐sea chert gap) has been believed to record radiolarian die‐off and consequent decline in biogenic silica production. However, recent studies showed that the upper portion of the deep‐sea chert gap has sedimentation rates higher than bedded chert, meaning that increased clastic inputs, rather than decreased biogenic silica inputs, resulted in the anomalous lithology. In this study, we focus on the Akkamori‐2 section, which preserves a rare sedimentary sequence spanning a large part of the lower portion of the claystone of the deep‐sea chert gap. We obtained conodont fossils that allow correlation with sections in South China that have numerous dated tuffs. By projecting the dates of the tuffs to our measured sections, we show that sedimentation rates of the lower portion of the deep‐sea chert gap is also higher than bedded chert. Hence, most of the deep‐sea chert gap was formed under increased clastic inputs, which likely records disturbance in the terrestrial landscape, probably aridification and/or increased seasonality in arid areas, that lead to elevated dust flux to the pelagic ocean. On the other hand, the idea that the deep‐sea chert gap records lingering effects of the mass extinction event on radiolarians cannot explain the high sedimentation rates of the deep‐sea chert gap. This previously favored scenario needs to be reconsidered, taking into account the burial efficiency of biogenic silica in the Early Triassic ocean, and also effects of increased clay deposition on preservation of radiolarians.

An improved method for the production of biogenic silica from cornhusk using sol–gel polymeric route

Porous silica was synthesized from cornhusk using the sol–gel polymeric route and compared with ash obtained from the direct combustion process under laboratory conditions. The unmodified ash from the direct combustion process was dissolved in NaOH for 1 h to form sodium silicate, which was subsequently hydrolyzed with citric acid to yield a silica xerogel. The obtained xerogel was characterized using inductively coupled plasma–optical emission spectrometry (ICP-OES), Fourier transforms infrared (FTIR) spectroscopy, X-ray diffraction (XRD), simultaneous thermal analysis (STA), gas sorption techniques to determine their elemental constituents, functional groups, crystalline phases, thermal stability, and porosity, respectively. The results showed that the synthesized silica xerogel exhibited porous network structures with a high-specific surface area and mesopore volume of 384 m2/g and 0.35 cm3/g, respectively. The pore size distribution revealed a complete transformation of the pore network structures of the unmodified ash from a monomodal to a bimodal pore system, with micro- and mesopore peaks centered around 1.5 and 3.8 nm, respectively. The ICP-OES results showed that the silica content significantly increased from 52.93 to 91.96 wt.% db after the sol–gel treatment. XRD diffraction confirmed the amorphicity of the silica particles obtained from the sol–gel extraction method. In addition, the STA data showed that the silica xerogel has high thermal stability compared to the unmodified ash, as the latter exhibited poor thermal stability and low textural properties. The high surface area and narrow pore cavity size distribution of the porous silica xerogel make it an ideal substrate for catalysts and an excellent template for growing other nanoparticles within the pores.

Benthic diatoms modify riverine silicon export to a marine zone in a hypertidal estuarine environment

Riverine dissolved silicon (DSi) and biogenic silica (BSi) are modulated along the estuarine gradient by several biotic and abiotic processes governed by physical forcings. An important area controlling silicon transport in alluvial estuaries with large intertidal mudflats is the benthic diatom-dominated biofilm system. Here, the hypertidal Severn Estuary, UK, has been used as a case study to improve our understanding of silicon transport in these benthic-dominated systems. We present the first time-series dataset of Si concentrations in the Severn. River and tidal hydrodynamics drove spatio-temporal changes in DSi. The longitudinal profile of DSi followed the classical view of dilution with downstream transport. Despite low riverine supply of BSi and low siliceous-phytoplankton production, relatively high BSi concentrations were measured in the Severn Estuary (maximum of 14.9 mg/L), which accounted for over 70% of the total bioavailable silicon present and were characterised by isotopically heavy waters (δ30Si of + 0.9 to + 1.1‰). Benthic biofilms (microphytobenthos) on the intertidal mudflats contained significant biomass (measured as chlorophyll a concentration with a maximum of 116.8 ± 16.2 µg/g dw. sed) with high productivity, driven by their photoprotective adaptions to these harsh intertidal environments, contributing to isotopically heavy mudflat water (δ30Si of + 1.19 to + 2.03‰), and resulting in high benthic BSi content in the spring (0.74 ± 0.03%) and summer (0.76 ± 0.05%). The fast-flowing tidal currents resulted in high bottom shear stress which likely exceeded the erosion thresholds of the biofilms, transporting the sediment-BSi matrix into the water column. Suspended particulate matter (SPM) and BSi remained tightly coupled in the estuarine water column (bioflocculation), and experienced the series of erosion–deposition events, burial/dissolution and export out of the estuary. Our novel observations improve understanding of the complex processes governing Si transport in hypertidal, benthic-dominated estuaries, and highlights the importance of tightly coupled benthic-pelagic systems in influencing the terrestrial silicon export to a marine zone.

The Sediment Green‐Blue Color Ratio as a Proxy for Biogenic Silica Productivity Along the Chilean Margin

AbstractSediment cores recently collected from the Chilean Margin during D/V JOIDES Resolution Expedition 379T (JR100) document variability in shipboard‐generated records of the green/blue (G/B) ratio. These changes show a strong coherence with benthic foraminiferal δ18O, Antarctic ice core records, and sediment lithology (e.g., higher diatom abundances in greener sediment intervals), suggesting a climate‐related control on the G/B. Here, we test the utility of G/B as a proxy for diatom productivity at Sites J1002 and J1007 by calibrating G/B to measured biogenic opal. Strong exponential correlations between measured opal% and the G/B were found at both sites. We use the empirical regressions to generate high‐resolution records of opal contents (opal%) on the Chilean Margin. Higher productivity tends to result in more reducing sedimentary conditions. Redox‐sensitive sedimentary U/Th generally co‐varies with the reconstructed opal% at both sites, supporting the association between sediment color, sedimentary U/Th, and productivity. Lastly, we calculated opal mass accumulation rate (MAR) at Site J1007 over the last ∼150,000 years. The G/B‐derived opal MAR record from Site J1007 largely tracks existing records derived from traditional wet‐alkaline digestion from the south and eastern equatorial Pacific (EEP) Ocean, with a common opal flux peak at ∼50 ka suggesting that increased diatom productivity in the EEP was likely driven by enhanced nutrient supply from the Southern Ocean rather than dust inputs as previously suggested. Collectively, our results identify the G/B ratio as a useful tool with the potential to generate reliable, high‐resolution paleoceanographic records that circumvent the traditionally laborious methodology.

Cultivation of Navicula sp. on rice straw hydrolysate for the production of biogenic silica

Rice straw hydrolysate (RSH) prepared at room temperature was found to be rich in silica (140 ± 4.1 mg L-1) and other nutrients (nitrate-N: 160 ± 4.3 mg L−1, total dissolve phosphate: 164 ± 6.7 mg L−1, ammoniacal-N: 439.8 ± 17 mg L−1). The aim of this work was to study four RSH dilutions (10, 30, 50, 70% v/v) to cultivate Navicula sp. with modified ASN-III as a control. The best result was achieved in 30% RSH in terms ofdoubling time (d = 1.49 days) and growth rate (µmax = 0.46 day−1). Compared to control, specific growth rate and biomass productivity were increased by 2.93 folds and 1.85 folds, respectively. Cultivation in 5 L reactor with optimized 30% RSH yielded frustule (54.2 ± 1.9%), carbohydrate (12.4 ± 1.2%), lipid (18.9 ± 1.4%), and protein (8.2 ± 0.6%). The residual solid fraction showed 18.99% increased theoretical methane yield than raw rice straw. Overall, the present process offers a sustainable solution to manage rice straw residue and recover nanoporous silica.

New fuel indexes to predict ash behavior for biogenic silica production

One of the main challenges in biomass combustion for energy and material applications are existing uncertainties for the prediction of slag formation in the bottom ash. Slagging indexes, which are calculated based on the fuel ash composition, can play an important role as simple tools to address this challenge. However, in order to use a fuel index, it is pivotal to consider its relevant scope and its applicability according to its chemical background. In this work, a self-developed Python code was employed to find relevant fuel indexes for the prediction of slag formation with high correlation coefficient based on experimental data from the combustion of rice husk and rice straw. Pre-treatment and fuel blending were utilized to mitigate slag formation in these silica-rich biomass fuels. Results showed that the selected newly defined fuel index, i.e. (K + Na + Mg) / P [mol/mol], which includes ash forming elements relevant for slag formation during the combustion of silica-rich biomass, improves average coefficient of determination for the prediction of the slag formation between 800 and 1100 °C to 0.9187. By employing gas sorption as well as a scanning electron microscopy analysis, it was also shown that the fuel indexes are able to classify the silica-rich ashes for material applications in terms of slag formation on microscopic level and the changes in silica purity. At temperatures higher than 900 °C, porosity of the ashes decreases substantially because of ash sintering on microscopic level. This also applies for acid-leached fuels although their slag formation tendency on macroscopic level is lower than 30 %. These findings are highly relevant to secure sustainable operation of biomass boilers and high-quality biogenic silica production from silica-rich biomass assortments for material applications.

Extraction and Characterization of Biogenic Silica Obtained from Selected Agro-Waste in Africa

Increased amounts of available biomass residues from agricultural food production are present widely around the globe. These biomass residues can find essential applications as bioenergy feedstock and precursors to produce value-added materials. This study assessed the production of biogenic silica (SiO2) from different biomass residues in Africa, including cornhusk, corncob, yam peelings, cassava peelings and coconut husks. Two processes were performed to synthesize the biogenic silica. First, the biomass fuels were chemically pre-treated with 1 and 5% w/v citric acid solutions. In the second stage, combustion at 600 °C for 2 h in a muffle oven was applied. The characterization of the untreated biomasses was conducted using Inductively coupled plasma—optical emission spectrometry (ICP-OES), thermal analysis (TG-DTA) and Fourier-transform infrared spectroscopy (FTIR). The resulting ashes from the combustion step were subjected to ICP, nitrogen physisorption, Energy dispersive X-ray spectroscopy (EDX) as well as X-ray diffraction (XRD). ICP results revealed that the SiO2 content in the ashes varies between 42.2 to 81.5 wt.% db and 53.4 to 90.8 wt.% db after acidic pre-treatment with 1 and 5 w/v% acid, respectively. The relative reductions of K2O by the citric acid in yam peel was the lowest (79 wt.% db) in comparison to 92, 97, 98 and 97 wt.% db calculated for corncob, cassava peel, coconut husk and cornhusk, respectively. XRD analysis revealed dominant crystalline phases of arcanite (K2SO4), sylvite (KCl) and calcite (CaCO3) in ashes of the biomass fuels pre-treated with 1 w/v% citric acid due to potassium and calcium ions present. In comparison, the 5 w/v% citric acid pre-treatment produced amorphous, biogenic silica with specific surface areas of up to 91 m2/g and pore volumes up to 0.21 cm3/g. The examined biomass residues are common wastes from food production in Africa without competition in usage with focus application. Our studies have highlighted a significant end-value to these wastes by the extraction of high quality, amorphous silica, which can be considered in applications such as catalyst support, construction material, concrete and backing material.

Ash transformation mechanism during combustion of rice husk and rice straw

Biomass is an alternative energy resource to fossil fuels because of its potential to reduce greenhouse gas emissions. However, ash-related problems are serious obstacles for this development, especially for the use in combustion plants. Thus, design and operation of biomass boilers require detailed understanding of ash transformation reactions during thermochemical conversion. To evaluate ash transformation in silica-rich biomass fuels, rice husk and rice straw were selected because of their abundance, limited utilization conflicts with the food sector, as well as their potential in both energy and material applications. This paper reveals ash transformation mechanisms relevant for the ash melting behaviour of silica-rich biomass fuels considering chemical and phase composition of the ashes. In this regard, several advanced spectroscopic methods and diffractometry were employed to characterize the materials. The ash transformation reactions and the viscosity were simulated using thermodynamic equilibrium calculations and a slag viscosity modeling toolbox. The results illustrate the impact of impurities on the atomic structure of the silica resulting in an altered ash melting behaviour and viscosity of the silica-rich ashes. Chemical water washing, acid leaching, and blending of rice straw with rice husk strongly influenced the chemical composition of the ashes and improved ash melting behaviour. The analysis also revealed the correlation between the crystalline fraction and the porosity in silica-rich biomass ashes, as well as a crystallinity threshold. These findings are highly relevant for future investigations in boiler designs and production of biogenic silica for material applications.

Diatom growth, biogenic silica production, and grazing losses to microzooplankton during spring in the northern Bering and Chukchi Seas

It is unclear how warming polar marine systems will alter the magnitude of diatom productivity and its fate within the food web. We examined diatom productivity and size-fractionated phytoplankton grazing losses to protozoan grazers in the northern Bering and Chukchi seas during June 2017. Sea ice was nearly absent and water temperatures were unseasonably warm; such conditions may be considered normal in future decades. Among 28 experiments conducted, five were in bloom conditions. Diatom biomass and production rates were similar to previous studies, suggesting the early ice retreat did not lead to appreciably reduced diatom growth. Statistical analyses showed that 77% of the variance in diatom growth rate could be explained by a combination of nutrients, light, and their interaction, but the interactive effect was most important (explaining 66% of the variance). Protozoan grazing intensity on phytoplankton was largely affected by size, specifically, grazing on larger phytoplankton (e.g. diatoms) was highly variable among stations, with many stations having unquantifiable rates. Protozoan grazers consumed an average of 23 ± 35% of growth at bloom stations and 55 ± 102% among non-bloom stations. For smaller phytoplankton, grazing was persistent and less variable spatially, consuming 64 ± 38% of growth at bloom stations and 79 ± 63% at non-bloom stations. Although previous studies (that did not size-fractionate samples) inferred that protozoan grazers control diatom biomass during blooms, our results suggest that diatom productivity largely escaped protozoan grazing losses, especially in bloom conditions, likely due to temporal lag between phytoplankton and protist biomass accumulation. Thus, during bloom conditions, it was estimated that 20–50 times more diatom organic matter was available for higher trophic levels and/or export (as opposed to water column remineralization) than under non-bloom conditions, despite only a 12-fold increase in gross diatom production in the bloom.

Amorphous biogenic silica production and utilization in experimental dental composites: Effect of silica gel formation pH on silica and composite properties

AbstractThe purpose of this present study was to synthesize biogenic silica from rice hull ash (RHA) and investigate the effect of silica gel formation pH on the properties of silica powders and light‐cured experimental dental composites (EDCs) prepared using biogenic silica powders. Silica extracted from RHA and silica powders with different properties were obtained by the gelation of silicate solutions at pH 4, 7, and 9. Silica samples were characterized using inductively coupled plasma/optical emission spectrometry, X‐ray diffraction spectroscopy, Fourier‐transform infrared spectroscopy, scanning electron microscopy, laser particle sizer, and N2 adsorption analysis. Silanated silica powders were mixed with organic matrix (bisphenol A‐glycidyl methacrylate 50 wt%—2‐hydroxyethyl methacrylate 50 wt%) and EDC obtained were subjected to Vickers hardness, three‐point bending, and degree of conversion (DC) tests. EDC paste prepared with the inclusion of silica powder produced from pH 9 gel (9SiO2) could not be cured. EDC prepared with silica produced from pH 4 gel (4SiO2) had higher flexural strength than composite prepared with silica produced from pH 7 gel (7SiO2) at the same wt%. EDCs prepared using 9SiO2 showed significantly lower DC compared to EDCs prepared with 4SiO2 and 7SiO2. These observations reveal that EDC properties are susceptible to silica gel formation pH just as the obtained biogenic silica properties are. Furthermore, this study indicates the importance of optical properties of filler designed for light‐cured dental composites. Biogenic silica powders obtained from RHA may be low‐cost alternatives to current amorphous silica fillers depending on the production conditions.

Role of small Rhizaria and diatoms in the pelagic silica production of the Southern Ocean

We examined biogenic silica production and elementary composition (biogenic Si, particulate organic carbon and particulate organic nitrogen) of Rhizaria and diatoms in the upper 200 m along a transect in the Southwest Pacific sector of the Southern Ocean during austral summer (January–February 2019). From incubations using the 32Si radioisotope, silicic acid uptake rates were measured at 15 stations distributed in the Polar Front Zone, the Southern Antarctic Circumpolar Current and the Ross Sea Gyre. Rhizaria cells are heavily silicified (up to 7.6 nmol Si cell−1), displaying higher biogenic Si content than similar size specimens found in other areas of the global ocean, suggesting a higher degree of silicification of these organisms in the silicic acid rich Southern Ocean. Despite their high biogenic Si and carbon content, the Si/C molar ratio (average of 0.05 ± 0.03) is quite low compared to that of diatoms and relatively constant regardless of the environmental conditions. The direct measurements of Rhizaria's biogenic Si production (0.8–36.8 μmol Si m−2 d−1) are of the same order of magnitude than previous indirect estimations, confirming the importance of the Southern Ocean for the global Rhizaria silica production. However, diatoms largely dominated the biogenic Si standing stock and production of the euphotic layer, with low rhizarians' abundances and biogenic Si production (no more than 1%). In this manuscript, we discuss the Antarctic paradox of Rhizaria, that is, the potential high accumulation rates of biogenic Si due to Rhizaria in siliceous sediments despite their low production rates in surface waters.

Biogenic matter content in marine sediments in the vicinity of the Antarctic Peninsula: Recent sedimentary conditions under a diverse environment of production, transport, selective preservation and accumulation

Burial fluxes of organic carbon and biogenic silica were determined in 17 continental shelf sediment cores collected from the northern Weddell Sea, the Bransfield Strait, and the southern Drake Passage. Coring sites included open-shelf stations as well as slope and glacial trough environments, with water depths varying from 220 to 760 m. Apparent 210Pb accumulation rates from these cores ranged from 0.04 g m−2y−1 to 0.21 g m−2y−1 (1 to 3 mm y−1), with organic carbon burial rates ranging from 3 to 15 g OC m−2y−1 and biogenic silica accumulation rates ranging from 15 to 126 g SiO2 m−2y−1. OC contents below the surface mixed layer ranged from 0.26 to 1.51 wt. % (avg. 0.64 %). Biogenic silica contents at depth ranged from 2.3 to 11.2 wt. % (avg. 7.5%), with an average bSi/OC ratio (wt. %/wt. %) at depth of 12. Annual OC primary production rates and biogenic silica production rates in the euphotic zone were estimated from satellite chlorophyll-a data in the literature and from a seasonal model for biogenic particle export from surface waters. Based on these biogeochemical data, preservation efficiencies (i.e., mass burial rate/water column production rate) were calculated for organic carbon and biogenic silica. These preservation efficiency values ranged from 2 to 18% (avg. 9%) for OC and 8 to 106% (avg. 54%) for bSi. These relatively high preservation efficiencies resulted from extensive lateral sediment focusing (210Pb Psi (Ψ) values [burial flux/water column production rate] ranging from 2 to 33; avg. of 16), cold bottom water temperatures (2 to −2°C), and relatively high biogenic Si and OC production rates in the euphotic zone. The enhanced preservation efficiency for bSi relative to OC (i.e., 54% vs. 9%) in these Antarctic settings is consistent with the change in the phytoplankton bSi/OC (wt. %/wt. %) value of 2 for this area to the burial bSi/OC value of 12. Excess 210Pb activities in surface sediments varied from 4 to 47 dpm g−1. The surface mixed layer in the seabed varied in thickness from 0 to 4 cm. The penetration of excess 210Pb into these Antarctic Peninsula sediments varied from 6 to 28 cm (avg. 18 cm). The inventory of excess 210Pb in the seabed varied from 13 to 230 dpm cm−2 (avg. 110 dpm cm−2). Although 210Pb was the only radionuclide measured in this study, “apparent” 210Pb sediment accumulation rate (SAR) values from these 17 cores (assuming that deep bioturbation is negligible) are believed to be accurate SAR values because of good agreement between 210Pb and 14C chronologies from nearby cores reported in the literature.

Reviews and syntheses: The biogeochemical cycle of silicon in the modern ocean

Abstract. The element silicon (Si) is required for the growth of silicified organisms in marine environments, such as diatoms. These organisms consume vast amounts of Si together with N, P, and C, connecting the biogeochemical cycles of these elements. Thus, understanding the Si cycle in the ocean is critical for understanding wider issues such as carbon sequestration by the ocean's biological pump. In this review, we show that recent advances in process studies indicate that total Si inputs and outputs, to and from the world ocean, are 57 % and 37 % higher, respectively, than previous estimates. We also update the total ocean silicic acid inventory value, which is about 24 % higher than previously estimated. These changes are significant, modifying factors such as the geochemical residence time of Si, which is now about 8000 years, 2 times faster than previously assumed. In addition, we present an updated value of the global annual pelagic biogenic silica production (255 Tmol Si yr−1) based on new data from 49 field studies and 18 model outputs, and we provide a first estimate of the global annual benthic biogenic silica production due to sponges (6 Tmol Si yr−1). Given these important modifications, we hypothesize that the modern ocean Si cycle is at approximately steady state with inputs =14.8(±2.6) Tmol Si yr−1 and outputs =15.6(±2.4) Tmol Si yr−1. Potential impacts of global change on the marine Si cycle are discussed.