Explanation made by the forensic medicine institute: presentation_diatomees.pptx
They are present in all the aquatic circles such as oceans,lakes and ponds, with a preference for cold and little salted waters. Living remotely or in colonies, their concentration in water is very variable. Their sensibility in seasonal variations such as temperature, luminous intensity or surrounding pollution makes that we can find, in the same stream, two different species of diatoms separated from some meters only. It's the same for two periods different from the year. We can thus qualify them as biological markers specific to an environment. Exchanges with the outside environment are made through numerous very fine openings which are arranged according to a pattern different according to the species. These ornamentations allow of to divide the family of diatoms with kinds, themselves were subdivided in species.The number of known species approaches 100 000, but in European aquatic circles, we find only 500 - 600 of these species.
We distinguish two categories among the kinds of diatomées:
We see that Cyclotella presents intermittent shelves, contrary to Cyclostephanos. These have little visible thorns, contrary to Stephanodiscus. In spite of these differences, the similarities are strong between species of centriques, and it is difficult to associate in a certain way a diatom with a species in particular.
In case of drowning, some water is inhaled and invades respiratory tracts up to lungs. When the water penetrates, by osmosis, in blood system, a part of diatoms penetrates there also and spreads in various organs (liver, loins, brain, bone marrow). Another part finds itself in lung fabrics. Given that diatoms present a siliceous external skeleton which makes them heat-resistant, in acids, as well as in decay, we can observe diatoms on cups of fabric of a body, even damaged, found in the water.
If it is the corpse which is thrown to the water, diatomées cannot spread in all the organs because blood circulation is stopped. We can thus find diatoms only in lungs. According to the quantity and to the location of diatoms found in the examination, we can support or not the diagnosis of drowning.
In particular, we can support this diagnosis if:
By comparing diatomées to those found around the place of discovery of the body, we can possibly locate the place of the drowning. The analysis of diatoms is not however an exact science. The diagnosis must be confirmed by other histological tests or anatomical examinations.