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What are diatoms?
(copy and paste from "The
forensic Value of the Diatom Test for Drowning", M. Pollanen,
and slightly changed)
Diatoms are aquatic unicellular
plants that represent the most abundant single source of oxygen
producers in the biosphere( some say about 35 %). The most distinctive
feature of this unicellular organism is its extracellular coat or
frustule, which is composed of silica. The vast structural diversity
of the frustule leads to a remarkable number of morphologically
distinctive varieties of diatoms. Recent estimates indicate that
there are in excess of 100,000 different species of diatoms (In
Switzerland are around 4000 species). Diatoms are most often encountered
in naturally occurring bodies of water such as lakes, rivers, oceans,
seas, ditches, and puddles. Some diatom species have preference
for water of specific salinity thus allowing general distinction
between freshwater and marine diatom types. In addition, some diatom
species are more frequently associated with soil and puddles than
lakes. Fossilised diatoms are another major source of frustules
in the biosphere, although, these frustules are derived from long
dead diatoms. Such diatoms are mined for commercial use and are
as forensically important as live freshwater contemporary diatoms.
Some important features of diatoms
are their population dynamics and ecology. Diatom populations are
constantly in flux and these fluxes are the result of complex and
poorly understood nutrient and aquatic cycles. The net result is
a monthly periodicity in the abundance of live aquatic diatoms with
blooms of diatom populations in the spring and autumn (i.e. seasonal
maxima). In addition, there are temporal and spatial variations
in diatom concentration in any body of water in response to local
factors including mineral content of water, temperature, water stratification,
and acidity. A poorly understood aspect of diatom ecology is the
variation in the species and genus distribution over seasons. These
ecological characteristics of diatom populations have great, and
under utilized, forensic significance. Among the most important
forensically relevant feature of diatom populations is the monthly
variation in water concentration of diatom frustules that should,
a priori, effect the outcome of the diatom test for drowning during
various times of the year.
Samples of diatoms:
source of these pictures: Aqua
- Plus Switzerland
More information:
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