Question

What are Diatoms. Explain the identification methods of Diatoms and its specificity. 

Answer 1

Diatoms are a major group of eukaryotic algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although some form chains or simple colonies. A characteristic feature of diatom cells is that they are encased within a unique cell wall made of silicate.

Role of diatoms 

In cases, where the cause of death cannot be ascertained by conventional post-mortem examination in those cases presence of diatoms in lungs and other body tissues plays an important role in determining whether the death is due to drowning or not.

Investigation of Drowning

One of the primary goals of forensic work is to determine the post-mortem interval (PMI), or time since death.

• Diatoms are ubiquitous in both lotic (flowing water) and lentic (standing water) systems

• Algal populations, though variable, persist throughout the year

• Most diatoms can be identified to species with only a light microscope

Examining Diatoms: The diatom test for drowning relies not only upon the identification of diatoms in the bone marrow, but also upon the identification of the same species of diatoms as found in a sample of water obtained from the place of recovery of the body.

Diatoms as Biological Evidence 

The post-mortem examination of drowning, is one of the most difficult problems in forensic. The post-mortem analysis of drowning is a classical problem in the pathology of sudden violent death.

Extraction of diatoms from water samples using acid digestion methods:

Collected water samples from various water bodies like lakes, ponds, wells and drains.

Water samples were treated with conc. HCI acid and supernatant was discarded.

Then addition of conc. H2SO4 turned it blackish (conc. H2SO4 charred present organic material).

The supernatant was cooled and added with solid NaNO3.

Suspension was reheated until its color turned brown and finally cleared.

Distilled water was used for the washing of the resultant suspension of silica diatom cells and residue was re-suspended in acetone.

OR

Extraction of diatoms from tissue samples using acid digestion method:

Dissolve sternum bone marrow in a Kjeldahl flask containing 50 cc of nitric acid.

After half an hour the yellow fluid turned transparent.

This solution was allowed to cool and then centrifuged.

The residue was put on the slide and examined under a microscope.

OR

Soluene-350 Method:

Soluene-350 method for the extraction of diatoms from fresh water and seawater samples.

At the time of collection water samples were fixed with formalin.

30 ml. of both fresh and seawater samples were washed thrice with distilled water and at every wash, samples were centrifuged at 3000 rpm for 5 min and the supernatant was discarded every time.

Pellet was suspended in 8 volumes of Soluene-350.

The solutions were then incubated at 50 °C for 2 hours and subsequently at room temperature overnight.

Following centrifugations at 3000 rpm for 60 minute, samples were analyzed using the light microscope.

Treatment was excessively destructive for seawater diatoms, probably because their frustules were less silicized and have less resistance as compare to the fresh water diatoms.