CHAPTER EIGHT

GENERAL FEATURES AND USES

8.1Diatom Interactions and Association

Generally, the basic involvement for all diatoms species is that they function as the base of their food webs. This is because they are extremely prevalent and are efficient primary producers, contributing an estimated 20 to 25 percent of the world’s net primary production every year. There are very few species that form symbiotic associations with other organisms.

Several different types of associations exist between diatoms and other organisms. True parasites have been reported in some species. The fungus Olpidium phycophagum, an encysted parasite, has been found in Eucampia zoodiacus and in many other species of diatoms. Other fungi also have been observed. Both Chaetoceros and Rhizosolenia have been found containing the alga Richelia intercellularis, one of the Nostocaceae (fig. 48-A, c). The dinoflagellate Paulseniella chaetocerates has been found on Chaetoceros borealis and C. decipiens. The cells are fixed by small stalks inside the setae and appear as round prominences outside.

Commensalism between the tube-building species of the genus Cymbella and Nitzschia dissipata, freshwater diatoms, was described by Cholnoky (1929). In this association Nitzschia utilizes the metabolic products of Cymbella.

Several cases of symbiosis have been reported, but the existence of a true symbiotic.

condition is difficult to establish in diatoms. Some species are known to form a unique symbiosis with polar sponges. The species are almost always pennate, benthic, species of diatoms. The symbiosis consists of two parts: a mutualism and a parasitism. The two parts vary on time of year and the location in which the sponge is growing. If the diatom has enough light it grows without any problems and will produce extracellular sugars that the sponge can feed on, while the sponge provides protection to the diatom. However, if there isn’t enough light present then the diatom begins to consume the sponge to prevent itself from going into a vegetative state. For the diatom, becoming a parasite allows it to continue to grow without any problems and derives energy to consume the sponge that it is living on. So, if enough light is present the diatom carries out the process of photosynthesis because it is what is best for it. In this condition, the sponge doesn’t matter to the diatom, which normally produces the extracellular sugars risk of being forced into an unproductive vegetative state.

Figure 11: Symbiotic association with polar sponges

8.2Adaptations of Diatoms for Flotation

Diatoms are adapted for flotation in a variety of ways. Gran (1912), following Schütt, who studied the various types of suspension organs carefully, placed the different cell forms with their many adaptations for floating under four headings:

The bladder type: Here the cell is relatively large, the cell wall and protoplasm merely thin membranes around a large central cavity. The cavity is filled with fluid of about the same specific gravity as sea water. Species of the genus Coscinodiscus are examples of this group.

The ribbon type: The cell wall is thin, and the cell is flattened to give a greater surface. The cells may be joined into ribbon-shaped colonies. The genus Fragilaria are examples of this type.

The hair type: The cells are very long in one direction or united into narrow, elongated colonies. These long cells may be slightly curved or have sloping terminal faces which help maintain them in a horizontal position and thus keep them floating. Thalassiothrix and Rhizosolenia are examples.

The branching type: The genus Chaetoceros with its four long setae from each cell is a good example of this group. These cells are usually found in chains. Other outgrowths such as knobs and spines also help increase the surface.

The secretion of long filaments or masses of mucus from special pores probably aids in flotation. The extrusion of protoplasmic threads or streams probably serves the same purpose by stream friction against the surrounding water and by increase of surface. In Planktoniella sol, widely distributed in the oceanic plankton of warmer seas, the epivalve is provided with a broad, hollow, slightly silicified wing divided by septa into a series of chambers. Sometimes this wing extends straight out from the cell, at other times it is bent to resemble an umbrella.

The production of gases and oils by the living diatom cells undoubtedly has a marked effect upon their ability to float. The specific gravity of diatom cells must be very near that of sea water. Any slight variation in amount of oil stored, amount of gas produced, length and number of protoplasm threads extruded, or other factors possibly too unobtrusive to have been considered, may be reflected in the ability of the cells to maintain themselves at a favourable level in the sea. Conditions external to the individual cells themselves, differences in density and viscosity of the surrounding water, abundance of other organisms or inorganic material in their environment, movement of the water, and other factors likewise influence flotation (Gran, 1912; Allen, 1932).

8.3Uses and Applications of Diatoms

The diatoms are used in various purposes either directly or indirectly as follows:

8.3.1Diatomite:

The most common of uses of diatoms by humans now, is in the form of diatomaceous earth or diatomite, which is made of fossilized diatoms.

00After the death of diatom cells, the outer coverings i.e., the silicified walls become accumulated at the bottom of water, constituting diatomaceous earth. The accumulation may be thicker during favourable conditions. Diatomaceous earth is a substance composed of fossil diatoms. It is used in filters, insulation, abrasives, paints, and varnishes. Diatomaceous Earth is a strong natural insecticide that can be a practical alternative to synthetic pesticides in some applications. It is used in different industrial application as follows:

8.3.2Dietary Supplement

When taken by mouth, diatomaceous earth is used as a source of silica, for treating high cholesterol levels, for treating constipation, and for improving the health of skin, nails, bones and hair. When applied to the skin or teeth, diatomaceous earth is used to brush teeth or remove unwanted dead cells.

8.3.3Filter

A common use of diatoms is for filtration. The fine structures of diatom shells trap foreign particles in fluids, such as dirt, lint, hair and some other microscopic organisms. Diatoms are often used to filter water, particularly water in hot tubs and swimming pools. Diatoms are used as filters in different industries like sugar (to filter microorganism), oil and chemical industry. Diatomite is also used as filter for battery boxes.

8.3.4Insulator

It is used for its heat-resistant ability as insulator in boilers, steam pipes and blast furnaces, and as a base in dynamite. After drying, the diatomite insulation brick is fired at a temperature between 900 and 1000oC. Diatomite brick is available from 0.4 to 1.0 bulk density; it is mainly used as insulating materials for industrial furnaces and other thermal equipments, heating pipelines.

8.3.5Absorbent

It is used as an absorbent of liquid nitroglycerine in explosives.

8.3.6Abrasives

Diatomite is used as abrasive (scrubber) substance for the manufacture of scoring powder/vim, metal paints, polish, varnish, toothpaste etc. It is also used with bake-lite for electrical fuse and switch boxes.

8.3.7Paints

They are used in paints to cause them to sparkle. Diatomaceous earth fillers are used as flattening agent for paint. When parts of the diatoms protrude from the coating surface, they diffract light efficiently and promote a uniform sheen. By mixing different grades and qualities, one can create the finish you’re looking for, whether flat or shining.

8.3.8In forming the Basis of some Household Goods such as Pest/Mite Prevention.

Diatomaceous Earth (DE) is sprinkled lightly on the surfaces where unwanted pests will come in contact with the dry particles. Use a bulb puffer to blow DE into crevices where bugs are likely to hide.

8.3.9Used as a Mixture of Cement

Diatomite is currently being used as raw material in various industrial sectors worldwide and is also in use in the construction industry, aimed at improving characteristics of concrete and cement, and in producing heat-insulating bricks, using various binders

Recently though diatoms have expanded beyond diatomaceous earth for humans in their use.

8.3.10Diatoms as Food

Diatoms occur in large populations in the different oceans, seas as well as freshwater ecosystems and serve as food for microorganisms, other animals and animals’ larvae. Diatoms produce long-chain fatty acids, thus are an important source of these energy rich molecules that are food for the entire food web, from zooplankton to aquatic insects to fish to whales. Such animals may be consumed as food by higher members of the food chain and ultimately man.

8.3.11Testing of Microscopic Lenses

Due to the fine markings on shell, the diatom cells are used to test microscopic lenses. Diatoms make very interesting specimen under the microscope. They show complex patterns with very fine punctures on their surfaces. With some of the species, fine pores on the frustules are used for testing the resolving power of the lens of a microscope.

8.3.12Water Quality Evaluation

In the past three decades or so, diatoms have been used worldwide to determine the ecological quality of freshwaters (Whitton et al, 1991, Whitton & Rott, 1996; Prygiel et al, 1999). Diatoms fulfil the requirements of good indicator organisms in aquatic ecosystems. They occur in a wide variety of environments and show a broad range of tolerance along several gradients of abiotic factors, while individual species have specific water chemistry requirements (Round, 1991). Finally, sampling and processing are relatively simple and cost-effective.

Diatoms have been used as bio-indicators for various environmental factors such as eutrophication (Datta et al., 2019), impoundment (Krajenbrink et al., 2019), acidity (Korhola et al, 1999), nutrient loading (Winter & Duthie, 2000), and climatic factors (Rûhland et al., 2015; Soininem & Teittinen (2019)

8.3.13Environmental History Indicators

Though fish and macro invertebrates can serve as important measures, diatoms were found to be best indicators of nutrient pollution in aquatic ecosystems. Diatom species in waters considered unimpaired were largely made up of surface-attached species, while those in water considered impaired were motile, having the ability to swim. The skeletons or cell walls of diatoms are persistent and remains preserved for a long period in many sedimentary environments, providing records of past changes in the aquatic systems. They therefore are a great potential for paleo-environmental reconstruction and the examination of past diversity of the environment, making diatoms excellent ecological indicator species.

8.3.14Primary Producers

A primary producer converts an abiotic source of energy (e.g., light) into energy stored in organic compounds, which can be easily used by other organisms (e.g., heterotrophs). In the process they give rise to molecular oxygen which they release into the atmosphere. Primary producers are at the lowest trophic level and are the reasons why Earth is sustainable for life to this day. Diatoms are a major source of atmospheric oxygen responsible for 20–30% of all carbon fixations on the planet (Nesara and Bedi (2019).

8.3.15Environmental Indictors of Climate Change

Diatoms are an effective proxy for climate change due to their sensitivity to a variety of ecological conditions. Past changes in climate can be inferred from changes in species abundance within a sediment core, as the ecological requirements are well known for several indicator species. Diatoms have precise ecological requirements; consequently, can be used as environmental indicators, revealing what is happening in the environment.

8.3.16Forensic Application

Forensic limnology is a sub-field of forensic botany which examines the presence of diatoms in crime scene samples and victims. Diatoms have been used in forensic science in a variety of ways, the most frequent being the diagnosis of death by drowning. When a person drowns, water will enter the lungs and then enter the blood stream through ruptures in the peripheral alveoli before being carried to the other organs such as the liver and heart. Diatoms can be analyzed in organs of a human being and that can indicate where drowning occurred in the aquatic environment (Fucci et al., 2015; Williams et al., 2017; Coyle et al., 2001; Domemelen, 2005; and Horton, 2007).

8.3.17Solar Panels

Diatoms have been used to increase the likelihood that light would be trapped in solar panels. Possibly this increases a new form of solar panel effectiveness by three times.

The new solar panels are worked by having a photosensitive dye on them along with some titanium. The dye when hit by light releases electrons that the titanium takes up to create an electrical current. Next the silicon dioxide is effectively replaced with titanium dioxide, causing the frustules to be made from titanium instead of silicon. Then the organic material is removed from the diatoms and the titanium frustules remain in the panel with the dye. The idea is that the frustules would increase the likelihood that light would come in contact with the dye because the frustules could also trap the light. This increased the panel’s effectiveness roughly three times. With the use of diatoms, it allowed cheaper more effective solar cells compared to the previously used silicon cells that were also environmentally unsound (Jeffreyes, et al., 2011).

Figure 12: Solar panels impregnated with diatom silica wall (Courtsey Jeffreyes et al, 2011)

8.3.18Biofuel

Diatoms containing large oil deposits can be used in biofuel production. Humans are also interested in diatoms because of the possibility of using them as a source of fuel. Development and progressive investigations are ongoing using diatoms to produce a sustainable natural carbon energy source. This oil can be used to run cars and power plants without taking up a large amount of space or using food crops as an energy source. With a little genetic engineering they could be modified to secrete the oils much as they do with many polysaccharides. This way the oil that they produce can be harvested daily. This way they don’t have to be destroyed to harvest the oil allowing the process to occur extremely quickly and at a constant rate (Wang and Seibert, 2017).

8.3.19Potential for the use in nanotechnology

There is great potential for the use of diatoms in nanotechnology (Bradbury, 2004). This potential lies in the pores and channels which give rise to a greatly increased surface area, and the silica structure which lends itself to chemical modification. Diatom cells repeatedly and reliably manufacture valves of various shapes and sizes, potentially allowing diatoms to manufacture micro- and nano-scale structures which may be of use in a range of devices, including optical systems; semiconductor nanolithography; and even vehicles for drug delivery. With an appropriate artificial selection procedure, diatoms that produce valves of particular shapes and sizes might be evolved for cultivation in chemostat cultures to mass produce nanoscale components. (Drum and Gordon, 2003). It is in this way that diatoms are used as a component of solar cells by substituting photosynthetic titanium dioxide for the silicon dioxide that diatoms normally use to create their cell walls (Johnson, 2009). Diatom biofuel producing solar panels have been proposed (Ramachandra et al, 2009).