Diatoms have cell walls made of silica.

Diatoms are a type of phytoplankton that have cell walls composed of silica, indicating they have a biogenic origin. They are crucial to silicon distribution in oceans. The walls have two silica ‘valves’, overlapping bands of silica, and tubing that form up the cellular shape. The system is highly dependent on large amounts of silica for survival and DNA replication.

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Title

Evolution of the diatoms: major steps in their evolution and a review of the supporting molecular and morphological evidence

Authors

Linda K Medlin

Abstract

Over the years, many reviews of different aspects of diatom biology, ecology and evolution have appeared. Since 1993 many molecular trees have been produced to infer diatom phylogeny. In 2004, Medlin & Kaczmarska revised the systematics of the diatoms based on more than 20 years of consistent recovery of two major clades of diatoms that did not correspond to a traditional concept of centrics and pennates and established three classes of diatoms: Clade 1 = Coscinodiscophyceae (radial centrics) and Clade 2 = Mediophyceae (polar centrics + radial Thalassiosirales) and Bacillariophyceae (pennates). However, under certain analytical conditions, an alternative view of diatom evolution, a grades of clades, has been recovered that suggests a gradual evolution from centric to pennate symmetry. These two schemes of diatom evolution are evaluated in terms of whether or not the criteria advocated by Medlin & Kaczmarska that should be met to recover monophyletic classes have been used. The monophyly of the three diatom classes can only be achieved if (1) a secondary structure of the small subunit (SSU) rRNA gene was used to construct the alignment and not an alignment based on primary structure and (2) multiple outgroups were used. These requirements have not been met in each study of diatom evolution; hence, the grade of clades, which is useful in reconstructing the sequence of evolution, is not useful for accepting the new classification of the diatoms. Evidence for how these two factors affect the recovery of the three monophyletic classes is reviewed here. The three classes have been defined by clear morphological differences primarily based on gametangia and auxospore ontogeny and envelope structure, the presence or absence of a structure (tube process or sternum) associated with the annulus and the location of the cribrum in those genera with loculate areolae. New evidence supporting the three clades is reviewed. Other features of the cell are examined to determine whether they can also be used to support the monophyly of the three classes.

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In the Waiotapu, New Zealand in the Champagne Pools, silica was often formed on EPS that was secreted by microorganisms.

When assessing the siliceous sinters formed within the Champagne Pools in New Zealand, it was found that there was significant growth of this silica surrounding exopolymeric substances (EPS). Samples from the pools were collected on site, grown over a year’s time, and analyzed using several different types of microscopy. After testing it was found that often, though not always, EPS production and silicification occurred in areas with colonies of filamentous microorganisms, indicating that the silicification may be a bioproduct of those life forms.

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Title

Silicifying biofilm exopolymers on a hot-spring microstromatolite: templating nanometer-thick laminae

Authors

Kim M Handley, Sue J Turner, Kathleen A Campbell, Bruce W Mountain

Abstract

Exopolymeric substances (EPS) are an integral component of microbial biofilms; however, few studies have addressed their silicification and preservation in hot-spring deposits. Through comparative analyses with the use of a range of microscopy techniques, we identified abundant EPS significant to the textural development of spicular, microstromatolitic, siliceous sinter at Champagne Pool, Waiotapu, New Zealand. Examination of biofilms coating sinter surfaces by confocal laser scanning microscopy (CLSM), environmental scanning electron microscopy (ESEM), cryo-scanning electron microscopy (cryo-SEM), and transmission electron microscopy (TEM) revealed contraction of the gelatinous EPS matrix into films (approximately 10 nm thick) or fibrillar structures, which is common in conventional SEM analyses and analogous to products of naturally occurring desiccation. Silicification of fibrillar EPS contributed to the formation of filamentous sinter. Matrix surfaces or dehydrated films templated sinter laminae (nanometers to microns thick) that, in places, preserved fenestral voids beneath. Laminae of similar thickness are, in general, common to spicular geyserites. This is the first report to demonstrate EPS templation of siliceous stromatolite laminae. Considering the ubiquity of biofilms on surfaces in hot-spring environments, EPS silicification studies are likely to be important to a better understanding of the origins of laminae in other modern and ancient stromatolitic sinters, and EPS potentially may serve as biosignatures in extraterrestrial rocks.

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The biotic environment of El Tatio, Chile hosts opaline silica deposits that have many similarities to the ‘Home Plate’ deposits on Mars.

El Tatio is a site that hosts acid-sulfate-chloride hot springs which lead to the formation of opaline silica sinters in nodular clumps within discharge channels for the springs. Biofilms often can form on these deposits as they support the growth of life. These deposits have a surface fabric that appears smooth, but on the undersides have microporosity similar to the Home Plate deposits. Spectroscopy has shown that a thin layer of halite exists on Home Plate deposits, which is similarly seen on inters at El Tatio as the springs evaporate and condensates form. Both sites are also suspected to be volcanic hydrothermal environments. Despite this evidence, proof of biotic origin has not been established for these features on Mars, likely due to the current harsh environment.  

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Title

Silica deposits on Mars with features resembling hot spring biosignatures at El Tatio in Chile

Authors

Steven W Ruff, Jack D Farmer

Abstract

The Mars rover Spirit encountered outcrops and regolith composed of opaline silica (amorphous SiO2·nH2O) in an ancient volcanic hydrothermal setting in Gusev crater. An origin via either fumarole-related acid-sulfate leaching or precipitation from hot spring fluids was suggested previously. However, the potential significance of the characteristic nodular and mm-scale digitate opaline silica structures was not recognized. Here we report remarkably similar features within active hot spring/geyser discharge channels at El Tatio in northern Chile, where halite-encrusted silica yields infrared spectra that are the best match yet to spectra from Spirit. Furthermore, we show that the nodular and digitate silica structures at El Tatio that most closely resemble those on Mars include complex sedimentary structures produced by a combination of biotic and abiotic processes. Although fully abiotic processes are not ruled out for the Martian silica structures, they satisfy an a priori definition of potential biosignatures.

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Many species of bacteria, algae, and micro-organisms are silicified in laboratory conditions in a manner that is consistent with authentic microfossils found in Precambrian cherts.

Francis et. all chose 30 different species of bacteria, algae, and micro-organisms that are suspected to create fossils similar to those in the eukaryotic database from Precambrian cherts and attempted to silicify them in order to compare the laboratory-derived fossil to naturally occurring sample. Using an equal amount of culture medium and ethyl silicate, each sample was given time to silicify for between 2 weeks to 6 months. At the end of this process, it was found that, though not all the bacteria silicified, some of the silicified bacteria were reminiscent of microfossils from Precambrian cherts.

Evidence is Sourced from:

Title

On the experimental silicification of microorganisms II. On the time of appearance of eukaryotic organisms in the fossil record

Authors

S Francis, L Margulis, E S Barghoorn

Abstract

On the basis of ultrastructural, biochemical and genetic studies, bacteria and blue green algae (Kingdom Monera, all prokaryotes) differ unambiguously from the eukaryotic organisms (Fungi, plants sensu stricto) and protists or protoctists, (Copeland, 1956). The gap between eukaryotes and prokaryotes is recognized as the most profound evolutionary discontinuity in the living world. This gap is reflected in the fossil record. Fossil remains of Archaean and Proterozoic Aeons primarily consist of prokaryotes and the Phanerozoic is overwhelmingly characterized by fossils of the megascopic eukaryotic groups, both metazoa and metaphyta. Based on the morphological interpretation of microscopic objects structurally preserved in Precambrian cherts, the time of appearance of remains of eukaryotic organisms in the fossil record has been claimed to be as early as 2.7 · 109 years ago, (Kaźmierczak, 1976). Others suggest chronologies varying between 1.7 to 1.3 · 109 (Schopf et al., 1973) or a time approaching 1.3 · 109 years (Cloud, 1974). There is general agreement that many of the Ediacaran faunas, which have been dated at about 680 m.y. are fossils of megascopic soft-bodied invertebrate animals. Since all invertebrates are eukaryotic, the ca. 680 m.y. date for deposition of these animal assemblages may represent the earliest appearance of eukaryotic organisms. But the question remains as to whether there is definitive evidence for eukaryotic cells before this “benchmark” of the late Precambrian. An excellent discussion of this particular problem as especially relating to acritarchs extending from rocks of Upper Riphean through Vendian and into the basal Cambrian is presented in recent studies by Vidal (1974, 1976) in Late Precambrian microfossils from the Visingsö rocks of southern Sweden. Previous work on the laboratory silicification of wood and algal mat communities (Leo and Barghoorn, 1976) suggested that further analysis of “artificial fossils” might be of aid in the interpretation of fossil morphology toward the ultimate solution of this problem. Thus the procedure developed by one of us (ESB) for laboratory wood silicification was adapted to various smaller objects. By successive immersions of wet cellular aggregates, colonies of various organisms and abiotic organic microspheres in tetraethyl orthosilicate, silicified cells and structures are produced which bear an interesting resemblance to ancient chert-embedded microfossils. Our observation of these microorganisms and proteinoid microspheres silicified in the laboratory as well as of degrading microorganisms, both eukaryotic and prokaryotic, have led us to conclude that many, if not all, of the criteria for assessing fossil eukaryotic microorganisms are subject to serious criticism in interpretation. We studied a large variety of prokaryotic algae, some eukaryotic algae, fungi, protozoa, and abiotic organic microspheres stable at essentially neutral pH. In some cases, degradation and/or silicification systematically altered both size and appearances of microorganisms. By the use of monoalgal cultures of blue-green algae, features resembling nuclei, chloroplasts, tetrads, pyrenoids, and large cell size may be simulated. In many cases individual members of these cultures show so much variation that they may be mistaken as belonging to more than one species. The size ranges for silicified prokaryotic and eukaryotic algae overlap. Several prokaryotes routinely yielded spherical or filamentous structures that resembled large cells. Because of genuine large sizes (e.g., Prochloron), shrinkage, systematic alteration or congregation of unicells to form other structures we find sizes to be of very limited use in determining whether an organism of simple morphology was prokaryotic or eukaryotic. Although some “prebiotic proteinoid microspheres” (of Fox and Harada, 1960) are impossible to silicify with our laboratory methods, those stable at neutral pH (Hsu and Fox, 1976) formed spherical objects that morphologically resemble silicified algae or fungal spores. Many had internal structure. We conclude that even careful morphometric studies of fossil microorganisms are subject to many sources of misinterpretation. Even though it is a logical deduction that eukaryotic microorganisms evolved before Ediacaran time there is no compelling evidence for fossil eukaryotes prior to the late Precambrian metazoans.

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Silicification in the Proterozoic commonly occurred in locations that had good silica availability, sediment permeability, and significant amounts of organic matter.

Any environment with large amounts of silica available for use, such as areas with volcanism or large deposits or dissolved SiO2, has a potential to undergo silicification, however the process favors areas with large amounts of organic matter. This was first noticed during analysis of the Draken Conglomerate Formation, where organics-rich areas were silicified and poor areas had instead been calcified, despite the full area being exposed to the materials that could have triggered either process. Many models have been made to try and characterize how presence of organics affects silica growth patterns, but no one solution appears to be favored at the time of writing. But this still indicates that there is a direct connection between silicification and presence of organisms.

Evidence is Sourced from:

Title

Exceptional preservation of photosynthetic organisms in silicified carbonates and silicified peats

Authors

A H Knoll

Abstract

Carbonaceous cherts in Proterozoic carbonate sequences provide an exceptionally clear record of early microbial life, but one that is significantly biased with respect to the range of environments inhabited by contemporary organisms. Many of the best preserved Proterozoic microfossil assemblages come from microbial mats and organicrich muds that accumulated in protected coastal areas where a combination of high productivity, limited water circulation, and, often, hypersalinity limited post mortem degradation. The close distributional relationship between early diagenetic silica and organic matter can be explained in terms of a model developed by Leo and Barghoorn for the silicification of wood. Three factors appear to control the distribution of early diagenetic chert in Proterozoic sequences: sediment permeability, availability of silica in ground water solution, and locally high concentrations of organic matter in near-surface sediments. Of these, organic content appears to impose the major environmental bias. In terms of their excellent preservation, geochemistry of formation, and limited environmental coverage, Phanerozoic silicified peats bear comparison with their Proterozoic counterparts. Swamp dwellers may be the plants most likely to be preserved exceptionally well, but they may also be the plants least likely to give rise to new populations that will become ecologically widespread and evolutionarily important in subsequent periods. Allochthonous elements in permineralized peats may be unusually important to palaeobotany because they combine the exceptional preservation conferred by peat permineralization with ecological representation of floodplain and upland evolutionary cradles rather than swampland museums.

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Carbonaceous material in cherts may exist in different phases (from different time periods), possibly resulting from pulses of hydrothermal silica of different temperatures.

This paper discusses the use of Raman spectroscopy to identify carbonaceous material in the Apex Chert. The result of the analyses indicates that there are two different phases of CM in the rock samples analyzed. The different generations add ambiguity to the potential biogenicity of the CM. After viewing the different phases, the authors note that one phase of CM is more thermally altered than the other, hinting at possible scenarios where one phase was deposited and preserved after a thermal event. Regardless of whether the CM was transported to the chert from somewhere else, or if the hydrothermal vent pulsed silica out at different times, these scenarios do not support biogenicity.

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Title

Multiple Generations of Carbon in the Apex Chert and Implications for Preservation of Microfossils.

Authors

Alison Olcott Marshall, Julienne R. Emry, Craig P. Marshall.

Abstract

While the Apex chert is one of the most well-studied Archean deposits on Earth, its formation history is still not fully understood. Here, we present Raman spectroscopic data collected on the carbonaceous material (CM) present within the matrix of the Apex chert. These data, collected within a paragenetic framework, reveal two different phases of CM deposited within separate phases of quartz matrix. These multiple generations of CM illustrate the difficulty of searching for signs of life in these rocks and, by extension, in other Archean sequences.

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Silica samples in the Apex Chert are found at depths where photosynthetic organisms can’t exist. 

The argument of hydrothermal silica is used again to emphasize the abiotic nature of Apex Chert samples. When judging different forms of silica, certain stratiforms are noted to be derived from seawater silica, while silica filaments in black chert veins are shown to be from hydrothermal fluids mixing with seawater at depth. The depth is the key part of this argument, as photosynthetic organisms would not be found on sub-seafloor hydrothermal vents. It is too deep a place to get an accurate amount of sunlight to undergo photosynthesis. Instead, Fischer-Tropsch synthesis is proposed for how the CM was created. This process is abiotic in nature, supporting the argument that silica, and therefore the CM was abiotic in nature.  

Evidence is Sourced from:

Title

Self-Assembled Silica-Carbonate Structures and Detection of Ancient Microfossils.

Authors

J.M Garcia-Ruiz, S. T. Hyde, A. M Carnerup et al.

Abstract

We have synthesized inorganic micron-sized filaments, whose microstucture consists of silica-coated nanometer-sized carbonate crystals, arranged with strong orientational order. They exhibit noncrystallographic, curved, helical morphologies, reminiscent of biological forms. The filaments are similar to supposed cyanobacterial microfossils from the Precambrian Warrawoona chert formation in Western Australia, reputed to be the oldest terrestrial microfossils. Simple organic hydrocarbons, whose sources may also be abiotic and indeed inorganic, readily condense onto these filaments and subsequently polymerize under gentle heating to yield kerogenous products. Our results demonstrate that abiotic and morphologically complex microstructures that are identical to currently accepted biogenic materials can be synthesized inorganically.

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