Diatom frustules are examined routinely during autopsies of deaths due to drowning. Presence of same species of diatoms (in similar concentrations) as that of the putative drowning medium from the internal organs of drowning victims constitute a corroborative or even conclusive evidence to support the diagnosis of death. This paper highlights the relationship of diatoms with that of drowning victims and provides an overview as to the digestion techniques and microscopic examination of siliceous residues of diatoms. Role of diatoms in identification of a site of drowning has been emphasized with suitable case studies from literature.
Death by drowning is the result of hampering of respiration by complete or partial submersion and subsequent entry of water into the air passages. Drowning can occur in any medium; a six inches puddle or deep sea.1 Drowning in shallow waters is seen in alcoholics, epileptics or infants. Since the facts pertaining to the events in a drowning victim are primarily obtained from animal experiments and to lesser extent from observations of eye witnesses, our knowledge is by no means complete.
A diagnosis of death due to drowning is normally based on the presence of foreign particles like twigs / leaves or soil in the lobar bronchioles (deep air passages of lungs) on dissection of overinflated and waterlogged lungs during autopsy examination and further conclusively by exclusion of all other unnatural causes of death. It is difficult to furnish anopinion as to the cause of deathin cases of highly putrefied bodies (recovered from drowning medium and exhumations). The rate of decomposition of drowned bodies will be at a quicker pace due to imbibition of water.2It is not possible to identify classical autopsy findings (Paltauf sign, emphysema aquosum or pleural effusion) in a decomposing body. In such situations the forensic pathologist has to rely on somelaboratory investigations. Diatom, a unicellular alga with a siliceous cell wall is found in almost all water bodies. In wet drowning these diatoms along with water enter the lungs and reach various internal organs. Acid digested extracts of various internal organs demonstrates the presence of diatoms similar to those found in the drowning medium (Diatom Test).
The distribution of diatoms widely varies between different water bodies. It depends on the various parameters of the water body such as temperature, salinity, pH, other algal matter, impurities etc.3Such parameters can vary between different water bodies. Even within one water body the same parameters can vary from time to time. Thesevariations are the fundamental reasons behind the prevalence of different types of diatoms between water bodies and within a single source of water. Planktonologists (algal specialists) are the experts who study in depth about relationship of such parameters with the seasonal or temporo-spatial variations of diatoms.
Identification of the site of drowning is important in drowning investigations. This can be more efficiently located based on the above mentioned parameters. This is because such parameters (for a particular season) will be specific to a drowning medium. Quite often the site of discovery of a dead body will be far from the actual site of drowning, because water currents, tides etc. transfer the body for some distance. A diatom test must not only demonstrate the mere presence or absence of diatoms in the internal organs of a drowned victim, but must also demonstrate the same type of diatoms in similar concentrations in the drowning medium.
Biology of diatoms
Marine plant life consists largely of minute, free-floating forms collectively known as phytoplankton. The word ‘plankton’ is derived from Greek word planktos, which means a wanderer. This shows that planktonic organisms are drifters rather than powerful swimmers.
Their horizontal distribution is mostly governed by currents in water than by the outcome of their own efforts. On land, plants of different species are present everywhere, but in sea and other water bodies majority of the plants are these phytoplankton. They are present in abundance everywhere, particularly in the sunlit water surfaces where the environmental conditions are favourable for them to photosynthesize and multiply. Phytoplankton produces half of the world’s oxygen through photosynthesis. Since mostly autotrophic, they form the foundation of all the food webs in sea. Diatoms or Bacillariophyceae are conspicuous members of the phytoplankton. The other members include Dinophyceae, Cyanobacteria, Chlorophyceae, Haptophyceaeetc.4 Diatoms range in size from one-thousandth of a millimetre (1 micron) to nearly two millimetre. Some are single-celled while others exist in the shape of filaments, ribbons, or stellate colonies. There are more than 200 genera of living diatoms comprising approximately of 1,00,000 species.5 They are found in oceans, freshwater bodies, soils etc. Their cell walls are hardened to a glassy consistency by siliceous material, intricately sculptured in miraculous patterns of pits and hollows. Some of the diatoms, especially marine forms are extremely beautiful. Therefore diatoms are better known as ‘Ornaments of the Sea’ or ‘Grasses of the Sea’.
The cell wall of a diatom consists of two nearly equal, overlapping halves: hence diatom, meaning ‘two atoms’. The box so formed may be circular (centric kinds) or oblong (pennatekinds) on the basis of their symmetry. Their cell wall is also called as frustule. The frustule consists of two parts called valves (thecae). The upper half (valve) is called epitheca and the lower half is called hypotheca. The joint between the two thecae is supported by bands of silica (girdle bands) that hold them together. This overlapping allows for some internal room expansion which is essential during reproduction. The epitheca closes on the hypotheca (smaller) like the lid of a pill box orPetri Dish.
The centrales are radially symmetrical, either circular or triangular whereas the pennates are bilaterally symmetrical. The linear (pennate) diatoms may be wedge-shaped, boat-shaped or keel-like in appearance. Most of the diatoms are non-motile while some move via flagellation. Many of them, have intracellular lipid vacuoles which help them to reduce the weight of cells and thereby their rate of sinking. Diatoms have a constant shape and size due to the presence of a siliceous wall. These walls are marked by minute pores or depressions that allow it to access to its environment. The frustule encloses a mass of living protoplasm which is demarcated into peripheral and central masses connected by strands.
Drowning and its medico-legal importance
The medico – legal system in our country is active to discover the real truth behind instances of a person being murdered and then disposed in water bodies to simulate drowning. In cases of death due to drowning, the Judiciary attributes as much importance to diatom test as DNA profiling. When a dead body is recovered from water, there is usually a suspicion as to whether death was due to drowning or if it was a case of post-mortem submersion. Detection of diatoms from internal organs of victims of wet drowning is considered as a positive proof of antemortem immersion by the forensic pathologists nationwide. The types of diatoms that are found in various body tissues / organs are compared with those present in the putative (suspected) drowning medium. This further helps in the reconstruction of events that had occurred prior to the death of an individual.
Drowning occurs when a person submerged in water attempts to breathe and instead aspirates water resulting in decreased oxygenation, hypoxemia and hypoxic brain damage finally leading to death. It is mostly accidental. In a relatively small number of cases it is suicidal. In some other cases death in the water is due to natural causes such as myocardial infarction (cardiac arrest due to absence of blood supply) or cerebral haemorrhage (bleeding from blood vessels of brain) during bathing or swimming.