Diatoms are unicellular, photosynthetic, eukaryotic organisms, but many more are yet to be discovered. Diatoms are diverse microscopic organisms with silica cell walls. They are photosynthesizing algae. They vary in different characteristics such as colour, shape, and size. There are 15,000 known species of diatoms.
These microscopic organisms mainly inhabit freshwater, marine water and soil as well. They can’t survive in domestic pools due to cleaning agents such as chlorine. They may be simple or branched, filamentous, and have a gelatinous envelope. All diatoms are enclosed by a frustule formed from two valves together by a connective at the girdle. They are mainly non-motile.
Forensic Limnology focuses on the study of diatoms in crime scene samples and bodies. In forensic science, diatom testing is a technique used for the determination of death due to drowning. A body recovered from the water does not always infer that death was due to drowning. When an individual drowns, as the diatoms are microscopic contents of the water, they will pass into the bloodstream through ruptures in the alveoli of lungs before being carried to the other organs such as the brain, liver, heart, kidney and bone marrow.
When a body is recovered from the water, and if suspicion arises if the case is of antemortem (before death) or postmortem (after death) drowning, a diatom test can be done. Diatom in organs can be diagnosed as death by drowning by conducting a ‘diatom test‘.
This article talks about diatom’s importance in forensic science and instrumentation. Forensic pathologists identify diatom species based on visual examination of their structure and patterns, which are preserved well even following chemical digestion. As some species unique to habitats, they can be used as evidence to identify a crime scene.
Use of Diatoms at Crime Scene
Diatoms are extremely sensitive to changes in environmental conditions, such as temperature and nutrients. Hence, the diatom species population varies from water to water source.
Based on the study of drowning victims, when diatoms are present in the water, the diatoms flow into the alveolar system, and the bloodstream has been caused by the inhalation of water by the drowning victims and so ends up in different organs like bone marrow, brain, kidneys and lungs.
Bones can be sent to the FSL for the detection of this phytoplankton. In the resolution of drowning cases, a correlation between the diatoms extracted from these tissue samples and the samples obtained from the drowning site must be established to determine the drowning site.
The prevalence of diatoms within the bone marrow is proof that the individual was alive once they entered the water. This implies that the death was caused by drowning. Even though diatom testing plays a crucial role in drowning cases, there is a necessity to develop better techniques for identification and classification, as current testing takes a long time and has a low detectable rate.
Traditional Chemical Digestion Method for Diatom Detection
Diatom cell walls are made up of silicon dioxide, which is not destroyed even in strong acid digestion. This allows diatom profiles to be analyzed when chemical digestion methods remove tissues. Under chemical digestion, tissues are digested, they are centrifuged and visually examined under microscopy.
Due to their simplicity and low value, chemical digestion methods are widely utilized in forensic casework. Digestion with acids like nitric acid, sulfuric acid is the most commonly used method for diatom detection. This method effectively eliminates organic components, and the remaining waste liquid may cause severe environmental pollution.
The integrity of diatom structures can be destroyed by excessive digestion, often leading to false-negative results. In order to look at the morphology of diatoms, transmission and scanning electron microscopes is used for an additional elaborate image than lightweight microscopes.
These microscopes were necessary for classifying and identifying diatoms, distinguishing between species being very minute at times. Electron or dark field microscope is presently the most used for analysis. This gives much more detailed imaging than easy light microscopy.
DNA Sequencing in Diatom Testing
DNA sequencing can also be used in testing diatoms. DNA sequencing analyzes the base sequence of a DNA fragment from the diatom and can identify species by DNA markers. PCR technique can be used to amplify the gene which is to be analyzed.
In addition to diatoms, other algae in the water can be detected in tissue samples by analyzing the DNA, which is unfamiliar with the human genome. In distinction to chemical digestion ways, sequence-based alga testing has higher sensitivity and is simpler to implement.
The primary task in cases of suspected rowing is to identify if drowning was antemortem or postmortem. Changes in the body cannot be relied upon due to submerging in water for a long time and predation by fish etc., making it difficult to identify drowning cases.
Diatoms are found in most aquatic environments. They are small enough to be taken up into the bloodstream by the lungs due to submersion. Diatoms settle in organs such as the lungs, kidney and bone marrow.
The presence of diatom in tissues shows that person was alive when he/she was submerged in water. Tissues can be analyzed in FSL for the presence of diatoms. They vary depending on location, temperature etc. Hence an approximate location of crime can be found out.
To extract diatoms from tissues, acid extraction is done. Forensic pathologists can identify the different species using light microscopy or electron microscope. DNA analysis can also be done on diatoms for accurate results. Diatom testing is an essential part of investigation in drowning cases.
By Chaitra M Deshpande
Chaitra M Deshpande is a Graduate in BSc (H) Forensic Science from Jain University. She has also completed her internship in a Police Station within Bangalore jurisdiction where she helped in the
investigation review of several criminal cases. She has also assisted in conducting forensic workshops for wildlife police officers to educate them about forensic science, wildlife crime scene investigation, how to distinguish between human crime and animal assault etc. She has participated in several conferences and competitions and won awards and appreciations in research categories. She aspires to further her field of knowledge by pursuing masters and to explore research ideas in the field of forensic anthropology through a PhD program.