Diatom Biogenic Silica Research Articles

Increasing iron concentration inhibits the Al-incorporation into the diatom biogenic silica: From laboratory simulation of ocean iron fertilization

Incorporated aluminum (Al) in diatom biogenic silicon (BSi) by biological behaviors shows a dissolution-inhibition effect for BSi, thus affecting the organic carbon export driven by diatoms. Ocean iron fertilization (OIF) aims to reduce the amount of atmospheric CO2 by promoting diatom blooms to enhanced the biological pump. However, BSi dissolved in a short time, leading to low efficacy of the carbon export. Considering the key role of incorporated Al in BSi dissolution, it is necessary to investigate the influence of increasing iron concentrations during OIF on Al incorporation of BSi. A lab simulation experiment, whereby Thalassiosira weissflogii was selected as a model marine diatom, was performed to reveal the effects of increasing iron concentrations on Al incorporation to BSi. Microanalyses including focused ion beams (FIB), high-resolution transmission electron microscopy (HRTEM), and high-resolution energy dispersive X-ray spectrometry (EDS) were used to identify the incorporated Al in BSi and evaluate its content. Results showed that increasing the iron concentration in the culture medium increased the cell yields but decreased the relative content of incorporated Al in BSi. The Al decrease resulted in the reduction of its effects in dissolution-habitation of BSi, which most likely also occurred during OIF. Therefore, the coupled SiAl biogeochemical cycle and the export of organic carbon from the surface ocean driven by diatoms would be influenced by the addition of iron during OIF.

Occurrence of structural aluminium (Al) in marine diatom biological silica: visible evidence from microscopic analysis

Abstract. The global marine biogeochemical cycle of aluminium (Al) is believed to be driven by marine diatoms, due to the uptake of dissolved Al (DAl) by living diatoms from surface seawater. The occurrence of Al in diatom biogenic silica (BSi) can inhibit the dissolution of BSi, thus benefiting the effects of the ballast role of diatoms in the biological pump and forming a coupled Si–Al biogeochemical cycle. However, the occurrence characteristic of Al in marine diatoms is still unclear. In particular, whether or not Al is incorporated into the structure of BSi of living diatoms is unrevealed, resulting in difficulties in understanding the biogeochemical behaviours of Al. In this study, Thalassiosira weissflogii, a widely distributed marine diatom in marginal seas, was selected as the model to evaluate the occurrence of structural Al in BSi based on culturing experiments with the addition of DAl. The structural Al in BSi was detected by combining focused ion beam (FIB) scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDS) mapping analysis. Visible, direct evidence of structural Al in living BSi was obtained, and the distribution and content of this Al were revealed by the EDS-mapping analysis. The effects of structural Al on BSi dissolution–inhibition are discussed based on the content of this Al. The fundamental results indicate the significant contribution of marine diatoms to the biogeochemical migration of marine Al.

Linking silicon isotopic signatures with diatom communities

The use of silicon isotope ratios (expressed as δ30Si) as a paleolimnological proxy in lacustrine systems requires a better understanding of the role of lake processes in setting the δ30Si values of dissolved Si (dSi) in water and in diatom biogenic silica (bSi). We determined the δ30Si of modern dSi (δ30SidSi) and bSi (δ30SibSi) in three lakes in Lassen Volcanic National Park (LAVO), California (USA), and produced diatom assemblage compositional data from the modern system and from sediment core samples. In the modern systems, we observe the largest magnitude diatom Si isotope fractionations yet reported, at −3.4 and −3.9‰ for Fragilaria dominated samples. Using statistical approaches designed to condense multivariate ecological data, we can deconvolve assemblage-specific Si isotope fractionations from the combined diatom assemblage-δ30Si data. For example, samples dominated by generally deeper water euplanktic species have low δ30SibSi values (<−1.10‰). Conversely, samples dominated by shallow water planktic or benthic periphyton have higher δ30SibSi values (>−0.14‰). These data suggest that δ30Si records from LAVO lakes reflect species specific Si isotope fractionations and thus act as paleolimnological proxy for the aquatic-habitat of bSi production. Silicon isotope analysis should be coupled with diatom community composition data and other geochemical proxies for the most robust paleolimnological interpretations. We also construct a Si mass-balance for Manzanita Lake based on elemental fluxes. Despite a short residence time of ∼4 months, it is an efficient Si sink: about 30% of inflowing Si is retained in the lake sediments. An entirely independent Si isotope-based estimate agrees remarkably well. Burial fluxes of bSi derived from radiometrically dated sediment cores yield retention rates of about a factor of three higher, which might suggest groundwater is an important term in the lake Si budget.

Diatom frustules with different silica contents affect copepod grazing due to differences in the nanoscale mechanical properties

AbstractThe silica frustules of diatom are regarded as an important physical defense against predation. While the biogenic silica content of diatoms is a major factor controlling the ingestion rate of copepods, how it affects the three‐dimensional structure and the mechanical properties of the frustules, and consequently copepod grazing, has not been studied. In this study, the diatoms Thalassiosira weissflogii and Amphora coffeaeformis were grown under different light intensities to manipulate their cellular biogenic silica contents. This manipulation generated diatom cells with distinct nano‐structures and mechanical strengths. The diatoms were then fed to the copepod Paracalanus pravus to test the effect of biogenic silica contents on copepod feeding. We demonstrated that the copepods preferred the low‐silica cells of T. weissflogii, but showed a preference for the high‐silica cells of A. coffeaeformis. Atomic force microscopy and X‐ray photoelectron spectroscopy analyses revealed that the chemical composition and mechanical properties of diatom frustules varied with silica contents. High silica containing T. weissflogii had higher frustule elasticity and hardness. In contrast, A. coffeaeformis containing more biogenic silica had lower frustule elasticity but silica level had little effect on the hardness of the frustule. Our results indicate that the nano‐structure and chemical properties of the frustules determine the mechanical strength of the diatom frustules. This nanoscale study improves our understanding of the relationship between diatom biogenic silica content and copepod grazing in marine planktonic food webs.

Diatom Biogenic Silica as a Felicitous Platform for Biochemical Engineering: Expanding Frontiers.

Diatoms are a prolific class of single-celled photosynthetic microalgae, pervasive in aquatic habitats. More than 10 000 species of diatoms are widespread in the world and primarily sustain the food chain in marine ecosystems. An individual diatom has evolved with species-specific distinct microporous to nanoporous rigid siliceous cell walls or frustules. Exceptionally intricate pore patterns of frustules originate from the hierarchical arrangements of silica sheets. Astonishing intrinsic features of diatom frustules, such as chemical inertness, large surface area, mechanical strength, ordered 3D micropatterning to nanopatterning of pores, distinctive optical properties, biocompatibility, etc., have been tailored to utilize them in numerous applications including biomedicine, separation technology, sensing, photonics, energy conversion, and storage. The advantage of biosilica over synthetic nanoporous silica materials for their application in nanotechnology lies in its cost-effective production through in vivo biomineralization that circumvents the usage of hazardous chemicals. Various chemical and biological approaches have been implemented for surface modification, structure mimics, integration, and conversion of diatom silica shells without altering their structural attributes to manufacture nanostructured smart materials with a diverse heterogeneity. Diatom silica shells thus have gained prodigious attention as powerful tools in nanotechnology. We follow an insightful approach in this Review to deal with all different types of applications of diatom silica shells in diverse areas and associated strategies to improve their aptness to fabricate new materials or devices.

Sediment biomarkers elucidate the Holocene ontogeny of a shallow lake.

We carried out geochemical analyses on a sediment core from Lake Harris, Florida (USA) to identify sources of organic matter to the sediment throughout the Holocene, and relate changes in those sources to shifts in past climate and environmental conditions. We hypothesized that the sources of organic matter changed in response to regional hydrologic shifts following de-glaciation, and to human population expansion in the state during the 20th century. Hydroclimate shifts in Florida were related to: 1) a steady rise in relative sea level and the fresh water table that began in the early Holocene, 2) wetland formation and expansion ca. 5,000 cal yrs BP, and 3) the onset of the modern El Niño (ENSO) cycle ~3,000 cal yrs BP. Stratigraphic changes in sediment variables from Lake Harris reflect each of these hydroclimate periods. Early in the Holocene, Lake Harris was a marsh-like system in a relatively dry, open-prairie environment. Organic sediments deposited at that time were derived largely from terrestrial sources, as inferred from high TOC/TN ratios, a dominance of longer-chain of n-alkanes (n-C29-31), relatively negative organic carbon isotope values (δ13CTOC), and low biogenic silica concentrations. In the middle Holocene, a positive shift in δ13CTOC coincided with the onset of wetter conditions in Florida. Submerged macrophyte biomarkers (n-C21-23) dominated, and during that period bulk organic carbon isotope values were most similar to δ13C values of mid-chain-length n-alkanes. In the late Holocene, δ13CTOC values declined, CaCO3 levels decreased to trace amounts, organic carbon concentrations increased and diatom biogenic silica concentrations increased from 10 to 120 mg g-1. Around 2,900 cal yrs BP, the effects of ENSO intensified and many Florida lakes deepened to their current limnetic state. Concentrations of algal and cyanobacterial biomarkers in the Lake Harris core increased by orders of magnitude after about AD 1940, in response to human-induced eutrophication, an inference supported by values of δ15N that fluctuate around zero.

Post-mortem oxygen isotope exchange within cultured diatom silica.

RationalePotential post‐mortem alteration to the oxygen isotope composition of biogenic silica is critical to the validity of palaeoclimate reconstructions based on oxygen isotope ratios (δ18O values) from sedimentary silica. We calculate the degree of oxygen isotope alteration within freshly cultured diatom biogenic silica in response to heating and storing in the laboratory.MethodsThe experiments used freshly cultured diatom silica. Silica samples were either stored in water or dried at temperatures between 20 °C and 80 °C. The mass of affected oxygen and the associated silica‐water isotope fractionation during alteration were calculated by conducting parallel experiments using endmember waters with δ18O values of −6.3 to −5.9 ‰ and −36.3 to −35.0 ‰. Dehydroxylation and subsequent oxygen liberation were achieved by stepwise fluorination with BrF5. The 18O/16O ratios were measured using a ThermoFinnigan MAT 253 isotope ratio mass spectrometer.ResultsSignificant alterations in silica δ18O values were observed, most notably an increase in the δ18O values following drying at 40–80 °C. Storage in water for 7 days between 20 and 80 °C also led to significant alteration in δ18O values. Mass balance calculations suggest that the amount of affected oxygen is positively correlated with temperature. The estimated oxygen isotope fractionation during alteration is an inverse function of temperature, consistent with the extrapolation of models for high‐temperature silica‐water oxygen isotope fractionation.ConclusionsRoutinely used preparatory methods may impart significant alterations to the δ18O values of biogenic silica, particularly when dealing with modern cultured or field‐collected material. The significance of such processes within natural aquatic environments is uncertain; however, there is potential that similar processes also affect sedimentary diatoms, with implications for the interpretation of biogenic silica‐hosted δ18O palaeoclimate records.

The role of oxygen conditions in the microbial dissolution of biogenic silica under brackish conditions

Regeneration of biogenic silica (BSi) is essential for the constant supply of dissolved silica (DSi) to diatoms in aquatic ecosystems, where these primary producers play an important role in carbon (C) drawdown. Increased DSi concentrations have been detected in the oxygen (O2) depleted bottom waters of e.g. the brackish Baltic Sea, which is suffering from one of the largest anoxic or hypoxic marine areas in the world. As hypoxia is likely to become more frequent also in coastal areas, the effects of suboxic conditions on the silicon (Si) cycle should be explored. To examine the effects of hypoxia on microbially mediated DSi release from diatom detritus and sediment to brackish seawater, a batch laboratory experiment was conducted. Hypoxic conditions facilitated more complete dissolution of diatoms than oxic conditions. However, the age, origin and physiological condition of the BSi significantly affected the dissolution rate, and the presence of sedimentary material decreased BSi dissolution. Inhibition of bacterial activity reduced the dissolution of diatoms under both oxic and hypoxic conditions, and T-RFLP of 16S rRNA genes confirmed that O2 conditions affected the composition of the bacterial community. Taken together, these results demonstrate that oxygen conditions and microbes play a fundamental role in regeneration of diatom BSi. Moreover, the results may explain the elevated DSi concentrations associated with O2 depletion in the bottom waters of the Baltic Sea.

Contribution of phytoliths to total biogenic silica volumes in the tropical rivers of Malaysia and associated implications for the marine biogeochemical cycle

The contribution of phytoliths to total biogenic silica (BSi) volumes in rivers worldwide, and the associated implications for the biogeochemical cycle, require in-depth study. Based on samples from rivers in Peninsular Malaysia, this project investigated the source and characteristics of BSi found in Asian tropical rivers, as well as the process of reverse weathering taking place in these fluvial systems. Results indicated that BSi samples collected in sediments consisted of phytolith, diatom and sponge spicules. Phytoliths, predominantly of the elongate form, comprised 92.8%–98.3% of BSi in the Pahang River. Diatom BSi in this river consisted mainly of pennatae diatoms, but represented a relatively small proportion of the total BSi volume. However, diatom BSi (predominantly of the Centricae form) was more prevalent in the Pontian and Endau Rivers with shares of 68.8% and 79.3% of the total BSi volumes, respectively, than Pahang River. Carbon contents of the BSi particulates ranged from 1.85% to 10.8% with an average of 4.79%. These values are higher than those recorded in other studies to date, and indicate that BSi plays a major role in controlling permanent carbon burial. This study suggests that phytoliths from terrestrial plants are the primary constituents of BSi in the rivers of Peninsular Malaysia, and therefore represent a significant proportion of the coastal silica budget.

A Holocene Sediment Record of Phosphorus Accumulation in Shallow Lake Harris, Florida (USA) Offers New Perspectives on Recent Cultural Eutrophication.

We studied a complete Holocene sediment record from shallow (zmax = 9.7 m) Lake Harris, Florida (USA) to infer the historical development of the lake and its current eutrophic status. We used 210Pb and 14C to date the 5.9-m sediment sequence (core LH-6-13) and determined accumulation rates for bulk sediment, organic matter, calcium carbonate, phosphorus fractions and biogenic silica fractions. The chronology of changes in sediment characteristics for LH-6-13 is consistent with the general paleoenvironmental framework established by core studies from other Florida lakes. Lake Harris began to fill with water in the early Holocene, ca. 10,680 cal a BP. A shift from carbonate-dominated to organic-rich sediments ca. 5,540 cal a BP corresponds to a transition to wetter climate in the middle Holocene. A rapid increase in diatom biogenic silica concentrations and accumulation rates ca. 2,600 cal a BP signals that the lake had deepened to its modern limnetic state. In LH-6-13, an up-core decrease in rates of accumulation for several sediment variables indicates time-course oligotrophication of the lake through the Holocene. In near-surface sediments, abrupt increases in the accumulation rates of these same variables indicate progressive cultural eutrophication after ca. AD 1900. Comparison of the modern state of Lake Harris to its condition 50–100 years ago provides a measure of the impact of recent cultural eutrophication. Because the pre-disturbance trajectory of this lake was one of oligotrophication, the true impact of cultural eutrophication is even greater than what is inferred from the changes over the past century.

Identifying sources of organic matter in sediments of shallow lakes using multiple geochemical variables

We analyzed 210Pb-dated sediment cores from four relatively shallow lakes (zmax < 10 m) in the Upper Ocklawaha River Basin, Florida, USA to compare primary producer community structure before and after anthropogenic impacts. We measured physical and chemical sediment variables including density, organic matter (OM), water-soluble phosphorus, polyphosphate (Poly-P), total P (TP), total carbon to total nitrogen mass ratios of OM (TC:TN), biogenic silica (diatoms, sponge spicules), total amorphous silica, and stable carbon and nitrogen isotope ratios of bulk OM. Principal component analysis showed that diatom biogenic silica, TC:TN, Poly-P and TP displayed discernible stratigraphic changes associated with the shift in the primary producer community. We applied k-means cluster analysis to these variables to identify macrophyte-derived, transitional, and phytoplankton-derived sediments. Our approach provides an objective method for identifying sediment sources that may be applied to shallow lakes in other regions. The four study lakes shifted from a macrophyte-dominated state to a transitional state before major anthropogenic disturbances, and became phytoplankton-dominated after ~1950.

Net biogenic silica production and nitrate regeneration determine the strength of the silica pump in the Eastern Equatorial Pacific

The high-nitrate, low-silicic acid character of the eastern equatorial Pacific (EEP) has been attributed to the preferential export of diatom biogenic silica (bSiO 2) over particulate organic nitrogen due to less efficient recycling of Si in surface waters. To gain insight into the strength of this silica pump, we examined [Si(OH) 4] and [bSiO 2] distributions and net bSiO 2 production rates in two regions of the EEP, one spanning the equator from 4°N to 3.25°S at 140°W and the other along a tropical instability wave (TIW) at 0.5°N between 132.5 and 123.4°W, ending within a cold vortex at 1.75°N by 125°W. Large uncertainty in the net bSiO 2 production rate measurement precluded a detailed examination of trends at high spatial resolution, but averaged data revealed clear differences in Si cycling between these two sampling areas. Surface [Si(OH) 4] generally remained at levels <4 μM across both, but [bSiO 2] nearly doubled to values as high as 226 nmol Si L −1 along the TIW. The mean integrated net rate of bSiO 2 production along the meridional transect was no more than 0.29 mmol Si m −2 d −1 to the 0.1% light level depth and −0.31 mmol Si m −2 d −1 to a depth of 300 m, implying net loss of bSiO 2 to dissolution in the upper 300 m in this area. In contrast, integrated net bSiO 2 production rates were five times higher on average in the zonal sampling area, exhibiting a mean of 1.45 mmol Si m −2 d −1 within the euphotic zone that declined by only ∼15% to a depth of 300 m, suggesting a significant potential for silica export to deeper waters along the TIW. In total, the fraction of bSiO 2 produced in the euphotic zone that was supported by new inputs of Si(OH) 4 was at least 3.7 times greater on average than the fraction of inorganic nitrogen taken up as NO 3 − , consistent with expectations for a silica pump. However, the mean integrated rate of NO 3 − uptake exceeded that of new Si(OH) 4 uptake by at least five times, implying preferential nitrate depletion in contradiction to nutrient distributions that indicated preferential Si(OH) 4 drawdown. This discrepancy can be resolved if, on average, ∼70% of the nitrate taken up was regenerated in the euphotic zone rather than being supplied by ‘new’ upwelled sources. These results suggest that the silica pump in the EEP arises only in part from the less efficient recycling of biogenic silica compared to organic matter, but also requires efficient recycling of nitrate in surface waters to reduce the rate of new nitrogen production well below the rate of gross nitrate uptake. Additional work will be necessary to confirm this apparent dependency and its relationship to variations in the physical and biological processes driving new production in this system. A relationship between bacterial protease activity and rates of silica recycling was also identified, but, unlike in coastal systems, the contribution of particle-associated bacteria was relatively unimportant in effecting bSiO 2 dissolution in the EEP.

Influence of bacteria and salinity on diatom biogenic silica dissolution in estuarine systems

Dissolution of diatom biogenic silica (bSiO2) in estuaries and its control by water salinity and bacteria were investigated using the river euryhaline species Cyclotella meneghiniana as a model. Laboratory-controlled bioassays conducted at different salinities with an estuarine bacteria inoculum showed a faster dissolution of diatom bSiO2 at the lowest salinity where bacteria were the most abundant. However in another experiment, salinity increase clearly enhanced the dissolution of cleaned frustules (organic matter free). The presence of active bacteria might therefore predominate on the effect of salinity for freshly lysed diatoms whereas salinity might rather control dissolution of organic-matter-free frustule remains. Incubation of cultivated diatoms at different protease concentrations revealed that high proteolytic activities had little effect on bSiO2 dissolution at a 1-month scale in spite of an efficient removal of organic matter from the frustules. Altogether it is hypothesized that bacterial colonization increases bSiO2 dissolution by creating a microenvironment at the diatom surface with high ectoproteolytic activity but also via the release of metabolic byproducts since the presence of organic matter seems generally to facilitate diatom bSiO2 dissolution.

Holocene climatic change in Swedish Lapland inferred from an oxygen-isotope record of lacustrine biogenic silica

Holocene climatic variability was studied in a 9500-year lake-sediment sequence from the Abisko region in Swedish Lapland, using the oxygen-isotope ratio in diatom biogenic silica (d18Osi). Oxygen-and hydrogen-isotope ratios of waters from the Abisko area suggest that in this region the evaporative flux is small and the isotopic composition of most lakes reflects that of the local precipitation. The hydrological setting of the region and sensitivity analysis of isotopic response to changing climatic parameters such as humidity, inflow and evaporation show that the downcore diatom d18Osi record is primarily controlled by changes in the summer isotopic composition of the lake water. The overall 3.5‰ depletion in d18Osi since the early Holocene is interpreted as an increase in the influence of the Arctic polar continental air mass that carries depleted precipitation. We estimate that this change is associated with a 2.5–4°C cooling that has occurred since the early Holocene. In general, the diatom d18Osi record resembles the average annual air temperature reconstructed for the Greenland ice core GISP2, especially during the past 4000 years, with a pronounced cooling starting at 2000 years BP.

Potential Role of Sponge Spicules in Influencing the Silicon Biogeochemistry of Florida Lakes

Amorphous silica, e.g. biogenic silica (BSi), contained in diatoms and in sponge spicules was estimated by time course extraction from surficial sediment samples of 82 Florida lakes. Separation of diatom BSi from sponge BSi was based on the observation that diatoms completely dissolve within 2 h of digestion at 85 °C in 1% Na2CO3 whereas sponge spicules, which are generally larger than diatoms, take longer to dissolve. Sponge samples from four lakes in northern Wisconsin ranged widely in the time required to dissolve completely (1.5–12 h), but no significant differences were observed in the rates of dissolution among the lakes. In Florida lake sediments, diatom BSi averaged 49.2 (± 48.4) mg∙g−1 and sponge BSi averaged 31.5 (± 35.8) mg∙g−1, with sponge BSi comprising on average 40% of the total amorphous silica extracted. The procedure for separating diatom BSi from sponge BSi underestimates sponge BSi because smaller and/or lightly silicified components of sponges are completely dissolved early in the digestion. However, because sponge spicules comprise a significant fraction of total amorphous silica extracted, we hypothesize that sponge spicules, which on average are larger than diatoms and require a longer time for complete dissolution, may constitute an important sink for BSi in Florida lakes.