Can the presence of structural phosphorus help to discriminate between abiogenic and biogenic magnetites?

The influence of phosphate on the competitive formation of magnetite and lepidocrocite and the properties of magnetite prepared from mixtures of Fe(II) and Fe(III) salts were studied. Products were prepared at 90 °C and pH 12.5 (series 1), 50 °C and pH 7 (series 2) and 20 °C and pH 8 (series 3). The P/Fe atomic ratio in the initial solution ranged from 0 to 3% and the pH was kept at the desired value with NaOH or KOH. Air was used as oxidant in series 2 and 3. All products, which were characterized by X-ray diffraction, transmission electron microscopy, chemical analysis and IR spectroscopy, contained a phase intermediate between magnetite and maghemite (referred to as magnetite in this paper). The products of series 1 consisted only of magnetite at all P/Fe ratios, whereas both magnetite and lepidocrocite formed in series 2 and 3 above a certain P/Fe ratio. On increasing the P/Fe ratio in the initial solution, the magnetite crystals became smaller and more oxidized (i.e. closer to maghemite) and the lepidocrocite/magnetite ratio increased. The P associated with magnetite was partly in the form of occluded P, i.e. non-surface-adsorbed phosphate. IR spectra suggested this P to be structural and occurring as low-symmetry PO₄ units. Because abiogenic magnetites produced in various environments incorporate structural P but some well-characterized biogenic magnetites seem to contain no P or be formed in P-poor environments, we hypothesize that natural magnetites containing occluded P are unlikely to be biogenic. However, more studies are needed to discard the presence of P in biogenic magnetites.     

Affinity chromatography of nucleosides and nucleic acid base derivatives with nucleic acid bases or nitrobenzeneboronic acid substituted silicas

Nucleic acid bases such as adenine and uracil, and nitrobenzeneboronic acid substituted silicas were prepared by the reaction of chloromethylbenzene substituted silica with adenine sodium salt and trimethylsilylated uracil, and nitration of benzeneboronic acid substituted silica, respectively. From the results of HPLC of nucleosides and N-ethyl derivatives of nucleic acid bases using modified silicas, hydrophobic base stacking interaction, selective hydrogen bonding interaction between purine and pyrimidine bases, and reversible cyclic boronate ester formation between diols of nucleosides with boronic acid were effective for the separation of nucleic acid related compounds. Moreover, association constants for hydrogen bonding formation of nucleic acid bases were estimated.     

Cryogenic Minerals in Hawaiian Lava Tubes: A Geochemical and Microbiological Exploration

The Mauna Loa volcano, on the Island of Hawaii, has numerous young lava tubes. Among them, two at high altitudes are known to contain ice year-round: Mauna Loa Icecave (MLIC) and the Arsia Cave. These unusual caves harbor cold, humid, dark, and biologically restricted environments. Secondary minerals and ice were sampled from both caves to explore their geochemical and microbiological characteristics. The minerals sampled from the deep parts of the caves, where near freezing temperatures prevail, are all multi-phase and consist mainly of secondary amorphous silica SiO₂, cryptocrystalline calcite CaCO₃, and gypsum CaSO₄·2H₂O. Based on carbon and oxygen stable isotope ratios, all sampled calcite is cryogenic. The isotopic composition of falls on the global meteoric line, indicating that little evaporation has occurred. The microbial diversity of a silica and calcite deposit in the MLIC and from ice pond water in the Arsia Cave was explored by analysis of ～50,000 small subunit ribosomal RNA gene fragments via amplicon sequencing. Analyses reveal that the Hawaiian ice caves harbor unique microbial diversity distinct from other environments, including cave environments, in Hawaii and worldwide. Actinobacteria and Proteobacteria were the most abundant microbial phyla detected, which is largely consistent with studies of other oligotrophic cave environments. The cold, isolated, oligotrophic basaltic lava cave environment in Hawaii provides a unique opportunity to understand microbial biogeography not only on Earth but also on other planets.     

Silica fractionation: a method and differences between two Japanese reservoirs

A new fractionation method of silica was developed in order to investigate its behavior in reservoirs. The method included separation of dissolved and particulate fractions, acid extraction, mild alkali extraction, and alkali fusion. The fraction types were dissolved silica (Si_D), adsorbed silica (Si_Ad), amorphous silica (Si_Am), and crystalline silica (Si_C). Samples were taken from two reservoirs, Iwaya and Kawabe, in the Kiso River in Japan. The inorganic silica (Si_Ad and Si_C) concentrations showed good correlations with the Fe and Al concentrations. The biogenic silica concentration was the difference between total Si_Am and Al-bound Si_Am. In the deep Iwaya Reservoir, inorganic particulate silica derived from silt and clay was transported from the riverine zone to a deep layer of the lacustrine zone. In the shallow Kawabe Reservoir, biogenic silica increased longitudinally with the growth of diatoms.     

Isolation of a wide range of minerals from a thermally treated plant: <Emphasis Type="Italic">Equisetum arvense</Emphasis>, a Mare’s tale

Silica is the second most abundant biomineral being exceeded in nature only by biogenic CaCO₃. Many land plants (such as rice, cereals, cucumber, etc.) deposit silica in significant amounts to reinforce their tissues and as a systematic response to pathogen attack. One of the most ancient species of living vascular plants, Equisetum arvense is also able to take up and accumulate silica in all parts of the plant. Numerous methods have been developed for elimination of the organic material and/or metal ions present in plant material to isolate biogenic silica. However, depending on the chemical and/or physical treatment applied to branch or stem from Equisetum arvense; other mineral forms such glass-type materials (i.e. CaSiO₃), salts (i.e. KCl) or luminescent materials can also be isolated from the plant material. In the current contribution, we show the chemical and/or thermal routes that lead to the formation of a number of different mineral types in addition to biogenic silica.     

Diatom preservation: experiments and observations on dissolution and breakage in modern and fossil material

Selected aspects of diatom preservation in both laboratory and field environments are examined with a view to improving techniques and to help understand why only some lake sediments have good diatom preservation.Laboratory measurements of biogenic silica following diatom dissolution by alkali digestion are questioned because results are shown to be dependant on initial sample size. Diatom breakage experiments identified drying carbonate rich sediment as a major cause of fragmentation of the large robust diatom Campylodiscus clypeus Ehrenb. Diatom dissolution experiments in carbonate media indicated that carbonate rich lakes should preserve diatoms better in order of the particular alkali metal type (Ca > Mg > Na). A preliminary assessment of the role of depth in diatom preservation is made for Lake Baikal where partly dissolved Cyclotella are more common in deep water surface sediments. The effect of time on diatom dissolution is examined in a saline lake sediment core and by comparing dissolution rates of recent and geologically old diatom samples in the laboratory. A simple link between diatom dissolution and sample age was not established. Factors thought to be important in controlling diatom preservation in lake sediments are discussed.     

Biogenic silica recycling in sea ice inferred from Si-isotopes: constraints from Arctic winter first-year sea ice

We report silicon isotopic composition (δ³⁰Si vs. NBS28) in Arctic sea ice, based on sampling of silicic acid from both brine and seawater in a small Greenlandic bay in March 2010. Our measurements show that just before the productive period, δ³⁰Si of sea-ice brine similar to δ³⁰Si of the underlying seawater. Hence, there is no Si isotopic fractionation during sea-ice growth by physical processes such as brine convection. This finding brings credit and support to the conclusions of previous work on the impact of biogenic processes on sea ice δ³⁰Si: any δ³⁰Si change results from a combination of biogenic silica production and dissolution. We use this insight to interpret data from an earlier study of sea-ice δ³⁰Si in Antarctic pack ice that show a large accumulation of biogenic silica. Based on these data, we estimate a significant contribution of biogenic silica dissolution (D) to production (P), with a D:P ratio between 0.4 and 0.9. This finding has significant implications for the understanding and parameterization of the sea ice Si-biogeochemical cycle, i.e. previous studies assumed little or no biogenic silica dissolution in sea ice.     

Detecting the bonding type of dithiocarbamate ligands in their complexes as inferred from the asymmetric CS mode

The IR spectra of a number of dithiocarbamate (dtc) complexes (M(R₂dtc)₂, n = 2, M = Ni, Cu, Zn, Cd, Pb, Hg, Se, Te; n = 3, M = Cr, Fe, Co, As, Sb, Bi, R = Et, Prⁿ, Prⁱ, Buⁿ, Brⁱ, as well as the laser Raman spectra of a few colourless compounds (M(Et₂dtc)₂ M = Zn, Cd, Pb, Hg), have been recorded and discussed as to the validity of the Bonati-Ugo (BU) criterion for discerning the dtc bonding type from its ν_as(CS) band (ca. 1000 cm^?1), By comparing these bands for dtc complexes containing different N-substituted ligands, their splittings can be proved to be due to interligand coupling of the CS ligand modes. Further comparison with X-ray diffraction data shows that the dtc ligands, irrespective of the host complex or the ligand bonding type, are at sites of C₁ symmetry, thus ruling out the possibility to detect the ligand bonding type from the solid state vibrational spectra. New evidence is presented that the RN modes are present in the 1000 cm^?1 region, thus making it unsuitable for the determination of the ligand bonding type.     

Tissue concentration changes of amino acids and biogenic amines in the central nervous system of mice with experimental autoimmune encephalomyelitis (EAE)

We have characterized the changes in tissue concentrations of amino acids and biogenic amines in the central nervous system (CNS) of mice with MOG_35-55-induced experimental autoimmune encephalomyelitis (EAE), an animal model commonly used to study multiple sclerosis (MS). High performance liquid chromatography was used to analyse tissue samples from five regions of the CNS at the onset, peak and chronic phase of MOG_35-55 EAE. Our analysis includes the evaluation of several newly examined amino acids including d-serine, and the inter-relations between the intraspinal concentration changes of different amino acids and biogenic amines during EAE. Our results confirm many of the findings from similar studies using different variants of the EAE model as well as those examining changes in amino acid and biogenic amine levels in the cerebrospinal fluid (CSF) of MS patients. However, several notable differences were observed between mice with MOG_35-55-induced EAE with findings from human studies and other EAE models. In addition, our analysis has identified strong correlations between different amino acids and biogenic amines that appear to change in two distinct groups during EAE. Group I analyte concentrations are increased at EAE onset and peak but then decrease in the chronic phase with a large degree of variability. Group II is composed of amino acids and biogenic amines that change in a progressive manner during EAE. The altered levels of these amino acids and biogenic amines in the disease may represent a critical pathway leading to neurodegenerative processes that are now recognized to occur in EAE and MS.     

Origin and modern microbial ecology of secondary mineral deposits in Lehman Caves,Great Basin National Park,NV, USA

Lehman Caves is an extensively decorated high desert cave that represents one of the main tourist attractions in Great Basin National Park, Nevada. Although traditionally considered a water table cave, recent studies identified abundant speleogenetic features consistent with a hypogenic and, potentially, sulfuric acid origin. Here, we characterized white mineral deposits in the Gypsum Annex (GA) passage to determine whether these secondary deposits represent biogenic minerals formed during sulfuric acid corrosion and explored microbial communities associated with these and other mineral deposits throughout the cave. Powder X-ray diffraction (pXRD), scanning electron microscopy with electron dispersive spectroscopy (SEM-EDS), and electron microprobe analyses (EPMA) showed that, while most white mineral deposits from the GA contain gypsum, they also contain abundant calcite, silica, and other phases. Gypsum and carbonate-associated sulfate isotopic values of these deposits are variable, with δ³⁴S_V-CDT between +9.7‰ and +26.1‰, and do not reflect depleted values typically associated with replacement gypsum formed during sulfuric acid speleogenesis. Petrographic observations show that the sulfates likely co-precipitated with carbonate and SiO₂ phases. Taken together, these data suggest that the deposits resulted from later-stage meteoric events and not during an initial episode of sulfuric acid speleogenesis. Most sedimentary and mineral deposits in Lehman Caves have very low microbial biomass, with the exception of select areas along the main tour route that have been impacted by tourist traffic. High-throughput 16S rRNA gene amplicon sequencing showed that microbial communities in GA sediments are distinct from those in other parts of the cave. The microbial communities that inhabit these oligotrophic secondary mineral deposits include OTUs related to known ammonia-oxidizing Nitrosococcales and Thaumarchaeota, as well as common soil taxa such as Acidobacteriota and Proteobacteria. This study reveals microbial and mineralogical diversity in a previously understudied cave and expands our understanding of the geomicrobiology of desert hypogene cave systems.     

Review of methodologies for extracting plant-available and amorphous Si from soils and aquatic sediments

There is a variety of methodologies used in the aquatic sciences and soil sciences for extracting different forms of Si from sediments and soils. However, a comparison of the published extraction techniques is lacking. Here we review the methodologies used to extract different Si fractions from soils and sediments. Methods were classified in those to assess plant-available Si and those to extract Si from amorphous silica and allophane. Plant-available Si is supposed to comprise silicic acid in soil solution and adsorbed to soil particles. Extraction techniques for plant-available Si include extractions with water, CaCl₂, acetate, acetic acid, phosphate, H₂SO₃, H₂SO₄, and citrate. The extractants show different capabilites to desorb silicic acid, with H₂SO₃, H₂SO₄ and citrate having the greater extraction potential. The most common extractants to dissolve amorphous silica from soils and aquatic sediments are NaOH and Na₂CO₃, but both also dissolve crystalline silicates to varying degrees. In soils moreover Tiron is used to dissolve amorphous silica, while oxalate is used to dissolve allophanes and imogolite-type materials. Most techniques analyzing for biogenic silica in aquatic environments use a correction method to identify mineral derived Si. By contrast, in the soil sciences no correction methods are used although pedologists are well aware of the overestimation of amorphous silica by the NaOH extraction, which is most commonly used to extract silica from soils. It is recommended that soil scientists begin to use the techniques developed in the aquatic sciences, since it seems impossible to extract amorphous Si from soils completely without dissolving some of the crystalline silicates.     

Cobalt(II) Oxidation by Biogenic Mn Oxide Produced by Pseudomonas sp. Strain NGY-1

Oxidation of Co by Mn oxide has been investigated using abiotically synthesized Mn oxide. However, oxidation of Co by biogenic Mn oxide is not well known. In this study, we isolated a Mn-oxidizing bacterium (Pseudomonas sp.), designated as strain NGY-1, from stream water. Sorption experiments on Co were carried out using biogenic Mn oxide produced by strain NGY-1. Similar sorption experiments were also conducted using a synthetic analogue of δ-MnO₂. Sorption of Co on δ-MnO₂ was faster and stronger than that on biogenic Mn oxide, which was possibly due to their structural difference and/or the presence of bacterial cells in biogenic Mn oxide. X-ray absorption near-edge structure spectra clearly demonstrated that Co was oxidized from the divalent to the trivalent state on biogenic Mn and δ-MnO₂. The oxidation property of both the biogenic Mn oxide and δ-MnO₂ was stronger under circumneutral conditions than under acidic conditions. Linear combination fitting using divalent and trivalent Co reference materials suggested that ～90% of Co was oxidized at pH ～ 6, whereas ～80% was oxidized at pH ～ 3. Oxidation properties of the biogenic Mn oxide and δ-MnO₂ were similar, but Co(II) oxidation by biogenic Mn oxide was slower than that by δ-MnO₂. The difference of Co oxidation may be caused by the coexisting bacterial cells or structural differences in the Mn oxides.

Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the supplemental file.     

Biogeochemical silica mass balances in Lake Michigan and Lake Superior

Silica budgets for Lake Michigan and Lake Superior differ in several respects. Mass balance calculations for both lakes agree with previous studies in that permanent burial of biogenic silica in sediments may be only about 5% of the biogenic silica produced by diatoms. Because dissolution rates are large, good estimates of permanent burial of diatoms can not be obtained indirectly from the internal cycle of silica (silica uptake by diatoms and subsequent dissolution) but must be obtained from the sediment stratigraphy. The annual net production of biogenic silica in Lake Michigan requires 71% of the winter maximum silica reservoir which must be maintained primarily by internal cycling in this large lake whereas the comparable silica demand in Lake Superior is only 8.3%. The greater silica demand in Lake Michigan is the result of phosphorus enrichment which has increased diatom production. It is hypothesized that steady-state silica dynamics in Lake Michigan were disrupted by increased diatom production between 1955 and 1970 and that a new steady state based on silica-limited diatom production developed after 1970. Mass balance calculations for Lake Michigan show in contrast with previous work that the hypothesized water column silica depletion of 3.0 g · m^–3 could have occurred even though 90% or more of the biogenic silica production is recycled.     

Silica nanoparticles aid in structural leaf coloration in the Malaysian tropical rainforest understorey herb Mapania caudata

Background and Aims

Blue-green iridescence in the tropical rainforest understorey sedge Mapania caudata creates structural coloration in its leaves through a novel photonic mechanism. Known structures in plants producing iridescent blues consist of altered cellulose layering within cell walls and in special bodies, and thylakoid membranes in specialized plastids. This study was undertaken in order to determine the origin of leaf iridescence in this plant with particular attention to nano-scale components contributing to this coloration.

Methods

Adaxial walls of leaf epidermal cells were characterized using high-pressure-frozen freeze-substituted specimens, which retain their native dimensions during observations using transmission and scanning microscopy, accompanied by energy-dispersive X-ray spectroscopy to identify the role of biogenic silica in wall-based iridescence. Biogenic silica was experimentally removed using aqueous Na₂CO₃ and optical properties were compared using spectral reflectance.

Key Results and Conclusions

Blue iridescence is produced in the adaxial epidermal cell wall, which contains helicoid lamellae. The blue iridescence from cell surfaces is left-circularly polarized. The position of the silica granules is entrained by the helicoid microfibrillar layers, and granules accumulate at a uniform position within the helicoids, contributing to the structure that produces the blue iridescence, as part of the unit cell responsible for 2 ° Bragg scatter. Removal of silica from the walls eliminated the blue colour. Addition of silica nanoparticles on existing cellulosic lamellae is a novel mechanism for adding structural colour in organisms.     

Surface photochemistry: benzophenone as a probe for the study of silica and reversed-phase silica surfaces.

This work reports the use of benzophenone, a very well characterized probe, to study new hosts: two reversed-phase silicas. Laser-induced room temperature luminescence of argon purged solid powdered samples of benzophenone adsorbed onto the two different reversed-phase silicas, RP-18 and RP-8, revealed the existence of a low energy emission band in contrast with the benzophenone adsorbed on 60 A pore silica, where only triplet benzophenone emits. This low energy emission band was identified as the fluorescence of the ketyl radical of benzophenone, which is formed as the result of a hydrogen atom abstraction reaction of the triplet excited benzophenone from the alkyl groups of the surface of the reversed silicas. Such emission does not exist for benzophenone adsorbed onto 60 A pore silica. Room temperature phosphorescence was obtained in argon purged samples for all the surfaces under use. The decay times of the benzophenone emission vary greatly with the alkylation of the silica surface when compared with "normal" silica surface. A lifetime distributions analysis has shown that the shortest lifetimes for the benzophenone emission exist in the former case. Triplet-triplet absorption of benzophenone was detected in all cases and is the predominant absorption in the case of 60 A pore silica, while benzophenone ketyl radical formation occurs in the case of the reversed silicas. Diffuse reflectance laser flash photolysis and gas chromatography-mass spectrometry techniques provided complementary information, the former about transient species and the latter regarding the final products formed after laser irradiation, both at 266 nm or 355 nm. Product analysis and identification show that the degradation photoproducts are dependent on the excitation wavelength, the photochemistry being much more rich and complex in the 266 nm excitation case, where an alpha-cleavage reaction occurs. A detailed mechanistic analysis is proposed.     

Biosilicification: the role of the organic matrix in structure control

Silicon (although never in the elemental form) is present in all living organisms and is required for the production of structural materials in single-celled organisms through to higher plants and animals. Hydrated amorphous silica is a mineral of infinite functionality and yet it is formed into structures with microscopic and macroscopic form. Research into the mechanisms controlling the process have highlighted proteins and proteoglycans as possible control molecules. Such molecules are suggested to play a critical role in the catalysis of silica polycondensation reactions and in structure direction. This article reviews information on silica form and function, silica condensation chemistry, the role of macromolecules in structure control and in vitro studies of silica formation using biomolecules extracted from biological silicas. An understanding of the mechanisms by which biological organisms regulate mineral formation will assist in our understanding of the essentiality of silicon to life processes and in the generation of new materials with specific form and function for industrial application in the 21st century.     

Abiotic–biotic controls on the origin and development of spicular sinter: in situ growth experiments, Champagne Pool, Waiotapu, New Zealand

Abiotic–biotic mechanisms of microstromatolitic spicular sinter (geyseritic) initiation and development were elucidated by in situ growth experiments at Champagne Pool (75 °C, pH 5.5). Siliceous sinter formed subaerially on glass slides placed along the margin of the hot spring. Environment–silica–microbe interactions were revealed by periodic collections of incremental sinter growth that formed under a range of environmental conditions including quiescence vs. wave turbulence, and wind–evaporation vs. steam–condensation. Sinter surfaces were intermittently colonized by voluminous networks of filamentous micro‐organisms, with submicron diameters, that provided an extensive surface area for silica deposition. The subaerial distribution of sinter and its textures reflected micron‐ to centimetre‐scale differences in environmental conditions, particularly relating to the balance between wave‐supplied dissolved silica and its precipitation, forced by cooling and evaporation. A continuum of sinter textures formed, representing rates of silica precipitation that either out‐paced biofilm growth or regulated the structural development of biofilms, and hence also the nature of microbially templated sinter. Massive laminae of porous, filamentous‐network sinter and/or fenestrae (up to 10's of microns in thickness and diameter) formed at relatively low rates of silica deposition (approximately 0.2 mg slide^?1 day^?1). At high rates (>1.9 mg slide^?1 day^?1), densely packed, granular or nonporous sinter formed, with filament networks disappearing into the siliceous matrix and becoming imperceptible under scanning electron microscopy (SEM). Furthermore, spicules were nucleated by filamentous microcolonies, where their discrete conical morphologies were preserved by accretion of thin sinter laminae. Microstromatolitic spicular growth ensued at fluctuating low to high rates of silica precipitation. Greater apical sinter build‐up, and hence upward polarity, resulted from focused microbial recolonization and progressively greater subaerial exposure at microspicule tips. The biogenic origin of spicular sinter at Champagne Pool clearly demonstrates that micron‐scale biofilms, displaying self‐organization patterns common to both biofilms and microbial mats, can be an essential factor in shaping characteristic centimetre‐scale sinter macrostructures. These findings suggest that a biogenic origin for geyserites elsewhere should also be considered. Moreover, results corroborate the supposition that microbially generated surface roughness may be significant for stromatolite morphogenesis in cryptic Precambrian carbonates.     

Synthesis, immobilization, and solid-state NMR of new phosphine linkers with long alkyl chains

Monodentate and chelating phosphines with long alkyl chains, incorporating ethoxy- or chlorosilane functions for immobilizations, have been synthesized and fully characterized. The new compounds (EtO)₃Si(CH₂)_xPPh₂, Cl₂Si(CH₂CH₂PPh₂)₂, and (EtO)₂Si[(CH₂)_xPPh₂]₂ (x = 7, 11) could be prepared in high yields from cheap starting materials, and they have been characterized by multinuclear NMR spectroscopy and X-ray crystallography. The phosphines have been immobilized on silica in a well-defined manner, and the modified silicas have been studied by ³¹P and ²⁹Si solid-state NMR of the dry materials and of the suspensions.     

Lorica Production in Choanoflagellates–A Model System for Investigating the Mechanics, Controls and Dynamics of Secretion

BIOMINERALIZATION is the process by which living organisms assemble structures from naturally occurring inorganic compounds. Mineral deposition is common and widespread amongst Protozoa and in most instances the mineralized structures provide skeletal support and protection for softer organic parts [10]. The 2 most common minerals to be deposited by Protozoa are silica and calcium carbonate. Groups of Protozoa that deposit silica, which we are concerned with here, include the diatoms, chrysophytes, choanoflagellates, Radiolar-ia, Heliozoa and testate amoebae [10]. In the majority of silica-depositing protista, silica is taken up from the medium in the form of monomelic orthosilicic acid Si(OH)₄ (soluble reactive silicate) and deposited as amorphous, polymerised biogenic silica or opal within membrane-bounded vesicles known as silica deposition vesicles (SDV). Often biogenic silica is characteristically patterned and ornamented and for most protozoan groups the morphology of silicified parts is of prime taxonomic importance. By far the most extensively studied group of silica-depositing organisms are the diatoms [1, 12, 13]. To date most of our knowledge of silica metabolism in protists has been based on investigations into this group. Diatoms require silica for the production of their frustules. Uptake and deposition of silica occurs within a closely denned portion of the cell cycle, between nuclear division and cell separation. It occupies about ± of the cell cycle and without an adequate supply of silica diatoms are unable to produce new frustule valves with the result that cell division cannot be completed. Diatoms, therefore, have an obligate requirement for silica and without this nutrient they cease to grow [11]. In contrast to diatoms a number of other silica-depositing protistan groups, such as loricate choanoflagellates and certain chrysophytes, have a facultative requirement for silica. In the past decade the ultras true ture, physiology and ecology of loricate choanoflagellates have been extensively studied by a number of different workers [7] and the significance of these studies to our understanding of the mechanisms, controls and dynamics of silica secretion is summarised and discussed here.     

Prebiotic Synthesis of Methionine and Other Sulfur-Containing Organic Compounds on the Primitive Earth: A Contemporary Reassessment Based on an Unpublished 1958 Stanley Miller Experiment

Original extracts from an unpublished 1958 experiment conducted by the late Stanley L. Miller were recently found and analyzed using modern state-of-the-art analytical methods. The extracts were produced by the action of an electric discharge on a mixture of methane (CH₄), hydrogen sulfide (H₂S), ammonia (NH₃), and carbon dioxide (CO₂). Racemic methionine was formed in significant yields, together with other sulfur-bearing organic compounds. The formation of methionine and other compounds from a model prebiotic atmosphere that contained H₂S suggests that this type of synthesis is robust under reducing conditions, which may have existed either in the global primitive atmosphere or in localized volcanic environments on the early Earth. The presence of a wide array of sulfur-containing organic compounds produced by the decomposition of methionine and cysteine indicates that in addition to abiotic synthetic processes, degradation of organic compounds on the primordial Earth could have been important in diversifying the inventory of molecules of biochemical significance not readily formed from other abiotic reactions, or derived from extraterrestrial delivery.