May light microscopy observations of algae play a significative role in forensic investigations of soils?

Contrarily to land plants, which display complex anatomical features useful in diagnostics, algae are problematic to identify at the species level. Taxonomic identifications classically are focused on morphological observations at the light microscopy, but current research showed extensive phenotypic plasticity and cryptic diversity resulting in different phylogenetic assemblages. Modern taxonomic approaches also include ultrastructural (SEM and/or TEM), phylogenetic and phylogenomics information, all methodologies that may be expensive and need the involvement of skilled experts. In criminal investigations, such methodologies may not be always applicable by the judicial system because of the costs, and morphological identification of algae at the light microscope is usually the standard method. Consequently, scientific data coming from algae are often neglected in forensic investigations, with the notable exceptions of the diatoms in drowning victims.

This research deals with a traditional morphological investigation on the detection and identification of soil microalgae in a case of disappearance. The method was useful in forensic investigations to associate control samples from the scene of the events to detected traces of unknown origin found on the victims.

Morphological characteristics (shape, size, color, taking into consideration the different state of conservation of the algae) and cellular characteristics (wall, unicellular, colonial, multicellular organization) were observed at the light microscopy.  Where species identification was not achievable with certainty, the smallest identifiable taxonomic level was recorded. A comparison of identified morphotypes as well as of the peculiar associations of taxonomic entities was made between sample of unknown origin to those of known origin and was used to evaluate similarity degree.

Observations of microalgae, in association with other geological (shape, size, color, composition of mineral grains) and botanical (shape, size, color of leaves and seeds) analyses, allowed investigators to: i) associate the walking of a person under investigation in specific sites of the scene of the events; ii) exclude that the bodies of two victims were submerged under water; iii) exclude the contact of any surface of the persons’ belongings, other than the soles of their shoes, with water basins of any kind.

The present investigation proved how a traditional light microscopic approach could be decisive to associate field samples to detected traces, basing on the identification of associations of morpho-types of microalgae.