Diatoms, scientifically known as Bacillariophyceae, are unicellular algae that include more than 100,000 different species of diatoms and 200 genera. The cell wall of the diatoms is constituted by two valves fitting together to enclose the cytoplasm and is made of hard silica, hence the name. Diatoms belong to the kingdom Protista.
Pigments Present in the Diatoms:
Diatoms are classified as algae because they use photosynthesis to generate energy. The cell wall of diatoms consists of chloroplasts. They Chloroplast contains pigments chlorophylls a and c and accessory pigments fucoxanthin and beta-carotene for photosynthesis. These accessory pigments give diatoms their distinct golden color.
Mode of Nutrition in Diatoms:
Diatoms represent the most abundant source of oxygen producers in the biosphere. They produce nearly 50% of the air that we breathe. The diatoms are autotrophic in nature. Diatoms remove carbon dioxide (CO2) from the atmosphere through carbon fixation, a process by which CO2 is converted to organic carbon in the form of sugar, and oxygen (O2) is released. The form of stored food in the diatoms are leucosin (chrysolaminarin) and fats (oils).
Habitat:
Diatom is generally found in both freshwater and marine water habitats. They are found in almost every aquatic environment such as lakes, rivers, oceans, seas, ditches, puddles, and moist soils, in fact almost anywhere moist. Some diatom species prefer the water of specific salinity, thus allowing general distinction between freshwater and marine diatom types.
Shape and Size of Diatoms:
Recent estimates indicate that there are more than 100,000 different species of diatoms and 200 genera. In addition, scientists are discovering new species every year. Being such a species of variety, colors, sizes (typically less than 200um), and shapes, these diverse asexually reproducing, mostly unicellular microorganisms (some form simple colonies) are defined by their unique cell walls.
The Shape of Diatoms:
The diatoms found in nature are of two types. One is round in shape and the other is elongated. On the basis of shape, diatoms can be classified into two types. These types are discussed below:
A. Pennate Diatoms:
These types of diatoms are elongated in shape. These diatoms can be divided bilaterally. Therefore, they have bilateral symmetry. Pennate diatoms are motile in nature. They move by gliding movement.
B. Centric Diatoms:
These types of diatoms are round-circular in shape. They possess radial symmetry. These forms of diatoms are non-motile in nature.
Movement of the Diatoms:
Diatoms are either plank-tonic (free-floating) or benthic (attached to a substratum) in nature. Most diatoms are non-motile given the absence of flagella and are found in suspension in naturally occurring aquatic environments. The movement of some diatoms is done with the help of the slimy secretion. This slimy secretion is an oily material excreted by the diatoms. This provides the lubrication for the movement of the diatoms.
The Unique Shell of Diatom
The unique silica-based cell wall (known as frustules) causes significant variations in different morphological features of different diatom species. It is made up of two distinct halves or valves in all sorts of odd shapes and sizes. In addition, their shells contain distinct patterns and perforations, allowing substances to pass in and out of the cell. This feature allows for diatoms to be easily identified and distinguished from one another.
In addition, these frustules, particularly resistant to decay, enable scientists to detect diatoms in the heavily decomposed bodies of drowning victims.
Reproduction in Diatoms
The diatoms reproduce both by sexual and asexual means. Diatoms mainly reproduce by the asexual mode of reproduction, by binary fission. Sexual reproduction is very rare in them. The asexual reproductive stage is also known as haploid, while the sexual reproductive stage is known as diploid.
Sexually they reproduce through gametic meiosis. The zygote formed during sexual reproduction is called auxospore. Auxospore is a asexual structure. When the diatoms reproduce by asexual means, their size decreases because of silica. To overcome this problem, they reproduce by the sexual mode of reproduction.
Life Cycle of Diatoms
The diatom has a boom and bust life cycle. The species and numbers of diatoms present in a body of water can vary depending on season and the time of year, with diatom populations increasing or decreasing seasonally. For instance, during the spring, there might be a spike in numbers due to the favorable environmental conditions. Then, the numbers decrease in the winter due to the cold. Diatoms are known to multiply rapidly when the water is full of nutrients and sunlight. Because of their photosynthetic properties, they are able to reproduce and grow on a large scale whenever conditions are favorable for the boom. However, after using up all of the nutrients in the water, the diatoms start to bust.
Its Application and Significance:
Due to its life cycle, diatoms can also verify the time of year when samples were taken. They can also identify the species of organisms living in a given region.
Diatom communities are a popular tool for monitoring environmental conditions, past and present, and were commonly used in water quality studies. For example, species have distinct ranges of pH and salinity where they will grow. Diatoms also have ranges and tolerances for other environmental variables, including nutrient concentration, suspended sediment, flow regime, elevation, and different types of human disturbance.
They can also determine the characteristics of the ecosystem. For example, a higher ratio of periphytic diatoms (i.e., those attached to a substrate), the higher the vegetation concentration, and the shallower the water.
Significance of diatoms in Forensic Science
Diatoms, backed by numerous studies, play the role of valuable supportive evidence in drowning cases. Various features of diatoms make these organisms an excellent candidate for analysis in forensic investigations. Their trace size allows them to transfer readily from the crime scene to the victim and suspect. Their peculiar morphology facilitates easier species identification, plus their predictable and constant variable populations produce unique assemblages of diatoms to identify different water ecosystems.
The principle of the Diatoms test is based on the correlation between diatoms present in the medium where the possible drowning occurred and inhalation of water. This inhalation of water causes the penetration of diatoms into the alveolar system and then into the bloodstream, which in turn gets deposited into the brain, kidneys, and other organs.
Conclusion: Diatoms Characteristics in Brief
- They are unicellular organisms, which means they are made of only one cell that provides all functions of the organism.
- Cell size – they are commonly between 20-200 microns in diameter or length, although sometimes they can be 2 mm long.
- They are photosynthetic, which means they use sunlight to create their own food/energy.
- Chloroplasts: Many/cell, 3-5 thylakoid membranes/stack
- Photo-pigments: Chlorophyll a & c, carotenoids
- The cell wall is made up of silica (SiO2) where the outer wall is called the frustule.
- Free moving or not affixed to a surface or substrate.
- Food reserves for energy wherein they store carbohydrates and lipids.
- Shape: Although the shape of the cell is typically circular, some cells may be triangular, square, or elliptical.
- Colour: Few distinct colors due to the chlorophyll, which gives them yellow, green, and brown tones.
- Two-layered shell where outer shell called thecae.
- Reproduction: Asexual & sexual reproduction at some point in their life cycle.
- Ecological roles: Produce deposits (diatomaceous earth), toxic blooms
- Common genera: Chaetoceros, Pseudonitzschia, Skeletonema
