Diatoms definition
Diatoms are microscopic, single-celled algae that range from 10 to 100 nm in length, or a colony. The cell wall of a single cell consists of two intervening valves made of silica instead of the cellulose in the cell walls of higher plants. Under the microscope, they appear radially or bilaterally symmetrical. Scientific estimates indicate that there are 100,000 species of diatoms, of which only about 8,000 species have been identified and described.
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General characteristics of diatoms
- Habitat
Diatoms are found in
freshwater and marine water
- External structure of diatoms
Diatoms contain
silica in their outer wall. The outer wall consists of two overlapping valves.
The color of the bivalves is generally transparent. Its cell wall generally
consists of cellulose. In this cellulosic cell wall, silica molecules are
embedded in many places. For this reason, the cell wall appears to be composed
of silica.
- Diatom nucleus
The nucleus of
diatoms is diploid (2 n).
- Pigments found in diatoms
The cell wall in
diatoms is composed of chloroplasts. In the chloroplasts of diatoms, pigments
are found. These pigments are chlorophyll a, chlorophyll c, and xanthophyll.
This xanthophyll is also known as fucoxanthin. Because of these pigments,
diatoms appear golden in color.
- Movement in diatoms
Diatoms are
non-motile, due to the lack of flagella. They generally float on the surface of
water with the help of stored lipids of low molecular weight. The movement of
some diatoms is achieved with the help of a sticky secretion. This sticky
secretion is an oily substance secreted by diatoms. This substance facilitates
the movement of diatoms.
- Method of nutrition in diatoms
Diatoms are
autotrophic in nature. The food stored in diatoms is leucosin (chrysolaminarin)
and fats (oils).
- Nutrition in diatoms
Diatoms are
autotrophic organisms because they contain plastids. The plastids are the molecules
responsible for the process of photosynthesis. They live in marine, brackish,
and fresh waters, and each of these environments is characterized by different
groups of diatoms. This is why diatoms are used to determine the water source
from which they came
- Diatoms shape
Diatoms found in
nature are of two types. One is round in shape and the other is elongated.
Types of diatoms
On the basis of shape,
diatoms are classified into:
- Pennate diatoms
These types of
diatoms are feather-shaped, with bilateral symmetry, and have the ability to
move, but it is a sliding movement.
- Central diatoms
These types of
diatoms are circular in shape, radially symmetrical and do not have the ability
to move.
Reproduction in diatoms
Diatoms reproduce by
sexual and asexual means. Diatoms reproduce mainly asexually, by binary
fission. Sexual reproduction is very rare among them. They reproduce sexually
by gamete meiosis. The zygote formed during sexual reproduction is called an
auxospore. The auxospore is an asexual structure. When diatoms reproduce
asexually, their size decreases due to the presence of silica. To overcome this
problem, they reproduce sexually.
Industrial uses of diatoms
Diatoms are used in many industries, such as: detergents, polishes, toothpastes, and other removers. Because of its industrial importance, countries are excavating ancient deposits of marine diatoms known as diatomaceous earth. The use of diatoms in industry is due to their wall, which consists of silica.
The use of diatoms in forensic evidence
Diatoms have been
used in many drowning cases to confirm whether death occurred as a result of
drowning or before the victim was thrown into the water. It was also used to
confirm whether the victims were thrown into the water, or were immersed under
the surface of the water until they drowned. or whether they drowned by mistake.
In addition, it was used to estimate how long the body had been in the water. And
it was also used to link the victim or suspect to the crime scene.
When a person drowns
in water, diatoms enter the lungs with the water, and spread through the blood
vessels to other internal organs. Its presence in the lungs, stomach, or bone
marrow indicates that the person was alive before drowning. But, its absence in
the drowned person’s body, confirm that he may have died from another cause
before being thrown into the water. Or he drowned in another place and then was
thrown at the crime scene. The failure in matching the diatoms in died body
with the diatoms at the site, from which the body was recovered from confirm
that the victim may have died from another cause before being thrown into the water.
In the event that the
soft tissues of the corpse have decomposed, diatoms are extracted from the
femur to be used to verify that death was due to drowning or not.
The investigator should
be careful, as diatoms may be found in the lungs and other tissues of people
who die from causes other than drowning, and their source may be from the
detergents the person uses. Therefore, the proportions of the same types of
diatoms at the drowning site are among the important factors that the forensic investigator
must take into consideration. These proportions are supposed to be similar to
those in the drowning body.
Cases occurred in the United States in which diatoms were used in criminal investigations
- The case of a teenage mother who killed her daughter and claimed that she had been kidnapped
In this case teenage
mother in Northern California claimed that her infant daughter was kidnapped by
a group of boys. They kidnap her daughter while she and a young child were
playing near a fountain in a city park. But while the police were searching for
the child, some eyewitnesses said that they saw the mother at the fountain and
at the watercourse near the city. Meanwhile, the child's body was found
floating in the nearby watercourse. Due to the contradiction between the
mother’s statements and the statements of eyewitnesses, the mother was arrested
and brought for investigation. When she was confronted with the witnesses’
statements during the investigation, she denied her presence at the watercourse
and concealed her statements. To solve the mystery of this case, the
investigators sent samples of the fountain water, the watercourse, and the
girl’s stomach to a specialist in identifying and classifying diatoms. After
examination, the specialist found diatoms in the three samples. But what was
striking was that the diatoms in the sample taken from the girl’s stomach
contained diatoms from the fountain and stream water. When investigators told the
mother that the child had swallowed water from both the fountain and the
stream, she collapsed and confessed that she had tried to drown the child in
the fountain. Then she believed that the girl was already dead. Then she threw
her into the stream and fabricated the kidnapping story.
- New England case
Another case in which
diatoms were used in forensic evidence was a case known as the “New England”
case. In the summer of 1991 in New England, two boys were attacked by a number
of teenagers while they were fishing in a pond on the outskirts of the city. The
attackers tied the two boys with duct tape. They beat them with a baseball bat,
dragged them into the pond and left them to drown. But one of the boys was able
to free himself, rescue his companion, and call for help. Police later
identified three suspects. By taking samples from their sports shoes and
sending them to a specialist in identifying and classifying diatoms. Upon
examination, the specialist found in the samples taken from the suspects’ shoes
the same types of diatoms as those in the pool in which the boys were tied and
thrown, and in the same proportions. For the police, this was sufficient
evidence to prove their involvement in the crime.
References
https://en.wikipedia.org/wiki/Diatom
https://www.sciencedirect.com/science/article/abs/pii/B978012370626300135
https://www.frontiersin.org/articles/10.3389/fmars.2021.636613/full
https://www.sciencedirect.com/science/article/abs/pii/S0015736877711302
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