CHAPTER TWO

GENERAL INTRODUCTION

2.1General Characteristics

The word ‘Diatom’ is a group name derived from their two symmetrical sides with a split between them. Diatoms are unicellular protists that consist of an estimated 200,000 species. The range of forms of diatoms is only unicellular and colonial forms (very few filamentous species). They live mostly individually or singly (unicellular) but in a few species they form chains colonial or filamentous. Almost all, with only a few exceptions undergo photosynthesis. Diatoms are widespread and dominate many diverse habitats. Diatoms are among the most important and prolific micro-organisms in the seas and serve directly or indirectly as food for many animals.

They occur in both marine and freshwater environments dominating the phytoplankton. They are also prominent in surface and bottom flora of lakes and ponds. They are prevalent and dominant in global ecosystems, contributing an estimated 20 to 25 percent of the net global primary production (Malviya et al, 2016).

Diatoms are a relatively well studied species because of a key feature that they possess; their silica cell walls (frustules), which are considered beautiful. They were also once used to determine the quality of microscope lenses. There is great potential for the use of diatoms in nanotechnology. This potential lies in the pores and channels which give rise to a greatly increased surface area, and the silica structure amenable to chemical modification.

Diatoms have two general orientations for their frustules: Pennales, which are bilaterally symmetrical, and Centrales, which are radially symmetrical. The frustules are composed of two parts often compared to a Petri dish because the top part, or valve, of the frustules is larger and has a slight overhang over the bottom valve. They also have ornate pores, spikes, and other constituents in them that assist in protection from predation and nutrient transport in and out of the cell; as well as, their classification, because the structures are species-specific.

Diatoms are characterized by the following features.

Cells of diatoms are eukaryotic

Silicified cell walls which show characteristic secondary structures like spines, bristles etc.

Photosynthetic pigments are chlorophylls a, c, beta carotene, xanthophyll and golden- brown pigment - fucoxanthin, which masks the chlorophylls and carotenoid pigments.

Food storage products are fats, oil, volutin and crysolaminarin.

The motile forms have a single pantonematic flagellum.

Based on electron microscopy, four basic types of secondary structures have been observed in diatoms. These are: Punctae (small perforations on valve surface), Canaliculi (tube-like narrow channels which run through the valve surface), Areolae (large boxlike depressions) and Costae (rib-like structures on the valve surface) [Hendey, 1971].

2.2Facts

Diatoms are unique organisms. Diatom walls show a wide diversity in form, some quite beautiful and ornate. In addition, there is a huge variety in the sizes and shapes of diatoms. They have an incredible assortment of shapes (round, oval, elongated, rod-shaped, triangular, ribbons, fans, zigzags, stars, disc-shaped etc.) and sizes, with many interesting and beautiful patterns. Their shapes and structure are usually highly regular and symmetrical, and these features are used to identify and classify them. They are very diverse. Diatoms also can exist in just about every ecosystem in some shape or form. Diatoms have a range of potentially useful attributes, such as a rigid silicified cell wall, the presence of vacuoles for nutrient storage, fast responses to changes in ambient light, resting stage formation, proton pump-like rhodopsins, ice-binding proteins, and a urea cycle (Bowler et al, 2010, Malviya et al., 2016).

Diatoms in addition, have something that no other organism has, a glass cell wall called a frustule. The presence of frustules gives diatoms many advantages and causes them to be of special interest to humans because of their unique beauty and because they are the only microorganisms that will fossilize, being a hard tissue. The frustule is also rather complex and deals with a large amount of what the diatom can do.

The frustule is made from silicon—transparent opaline silica. The diatoms obtain the silicon in the form of silicic acid present in high concentrations in waters worldwide. Silicon is one of the most prevalent elements on earth and is the principal part of glass and gets consciously washed into the oceans via rivers and runoff water. If the pectin and organic cell content is destroyed, then there is a translucent silica shell.

The diatom frustule is composed of two overlapping parts, halves or valves, or theca. They are uneven in size and fit together like a Petri dish or a pill box. The larger part is called the epitheca and the smaller part is the hypotheca. They are produced in two ways in the diatom life cycle. The most common form is when the diatom is reproducing asexually and each of the daughter cells will take one of the valves as the epitheca and secretes the hypotheca; this causes one of the diatoms involved in the division to gradually get smaller and smaller. The other form in which the cell produces the frustules is during sexual reproduction when the frustules are formed by the auxospore. Within the auxospore both valves are produced at the maximum size that the original frustule for that diatom species takes; this compensates for the gradual reduction in size caused by asexual reproduction.

The function of the frustule is rather diverse and can vary to a degree depending on species and habitat. The primary functions of the frustule shared across the species are rather fundamental. The functions include, but are not restricted to, providing support, protection from predation, attachment to surfaces and substrates, changing the buoyancy of the organism (placement in water column); in terrestrial habitats it allows organisms to help maintain water, and it helps refract and reflect light on the inside of the organism causing a greater amount of light contact to the photosystems and increasing the effectiveness of the cell to fix carbon. The frustules perform a lot more functions although not all of these apply to all species of diatoms. For example, such organisms that attach to surfaces of substrates don’t have frustules that affect the organism’s buoyancy.

The different attributes of the frustules are ascribed to the various features that are present on the diatom frustules. The features that are present on the frustules of diatoms are various and include different combinations, sizes and ordinations of pores, spines, and some other features. The spines provide some protection from predation and are also used for attachment to various substrates. The pores allow for transportation of various solutions in and out, as well as slimes out of the cell. The pores allow the diatom to obtain some nutrients from the different sources as well as to allow them to secrete some of their extracellular polysaccharides. Some species of diatoms can also use the pores to assist in the locomotion via gliding.

Beyond the structure and use of the frustules, diatoms have other promising features that can be put to use. They thrive abundantly in the open ocean; however, their growth here is limited by lack of iron. Because diatoms acquire carbon dioxide from the atmosphere, plans have been brought together to fertilize open ocean areas with iron that will facilitate growth of diatoms in the area. Consequently, this will cause them to take up carbon dioxide from the atmosphere, reduce carbon emissions and combat global warming.