Diatom Biology and Taxonomy

A diatom frustule consists of two overlapping halves, the epitheca and the hypotheca, each composed of a valve face and connecting girdle bands. The whole structure is made of amorphous (opaline) silica secreted by the living cell, then deposited in species-specific patterns of pores, ribs, and spines. Those patterns form the primary characters used for identification, visible under a light microscope at 400x magnification and in finer detail under a scanning electron microscope.

The valve face carries two main types of surface features. Areolae are the regular pores through which the cell exchanges substances with the water; their arrangement (radial in centric diatoms, parallel or slightly curved in pennates) is immediately apparent under low magnification. Striae are lines or rows of areolae, and their density (expressed as striae per 10 micrometres) is a key measurement for species identification.

The most fundamental split in diatom classification is between centric and pennate forms. Centric diatoms have radially symmetric valve faces, like the spokes of a wheel or the surface of a drum. They are predominantly planktonic (living suspended in the water column) and are the dominant forms in larger, open lake habitats and in marine and estuarine environments.

Pennate diatoms have bilaterally symmetric valves: elongated, boat-shaped, spindle-shaped, or sigmoid. Many are benthic or epiphytic, living attached to sediment surfaces, submerged plants, or rocks in shallow water. Because they occupy such different niches, finding centric planktonic forms (such as Cyclotella or Stephanodiscus) in a sample suggests mid-water or open-lake exposure, while a community dominated by small raphid pennates suggests a benthic or riffle habitat.

Within the pennates, the presence or absence of a raphe defines two subgroups. The raphe is a longitudinal slit or canal along the valve face, visible under light microscopy as a pale line running the length of the cell. Cytoplasm moves through it and interacts with substrate, allowing the diatom to glide. Raphid species are often dominant in benthic communities because they can actively reposition themselves on surfaces.

Araphid pennates lack this slit entirely. Genera such as Fragilaria and Asterionella are araphid and often live in colonial chains or star-shaped aggregates. They are passively drifted and common in planktonic assemblages of mesotrophic to eutrophic lakes. In a forensic sample, finding a high proportion of araphid chain-forming taxa suggests a planktonic source from a productive water body.

Forensic literature draws on hundreds of diatom genera, but a handful appear most frequently in casework reports because they are ecologically abundant across common drowning environments and morphologically distinctive under a light microscope:

• Navicula: the largest raphid pennate genus; boat-shaped valves with a central raphe and fine transverse striae. Tolerates wide pH and conductivity ranges, so nearly universal in freshwater. Its ubiquity makes genus-level presence weakly discriminating; species-level identification is needed to narrow provenance.
• Pinnularia: a large, strongly silicified raphid pennate with coarse alveolate striae, common in soft, slightly acidic waters including peaty lakes, forest streams, and bog pools. Its preference for oligotrophic, low-pH habitats makes it a useful indicator for those environments.
• Cyclotella: a centric genus with a tangentially striated marginal zone and an often plain or centrally punctate centre. Very common in mesotrophic and eutrophic lakes and reservoirs worldwide. Frequently recovered in forensic samples from still or slow-flowing freshwater drowning sites.
• Stephanodiscus: a centric genus with radiating striae and a ring of marginal spines, strongly associated with eutrophic (nutrient-rich) lake and reservoir conditions. High counts of Stephanodiscus in bone marrow or organ samples have been cited in cases from UK and European freshwater drownings.
• Nitzschia: a raphid pennate with a characteristic off-centre keel (a rib-like thickening housing the raphe canal), tolerant of organic enrichment and low-oxygen conditions. Its presence in high numbers often signals polluted or sediment-disturbed water.

No single genus identifies a water body. Forensic value comes from the assemblage: the proportions and absolute densities of multiple taxa together. A sample containing predominantly planktonic centrics ( Cyclotella, Stephanodiscus) with few raphid pennates looks very different from one with abundant large Pinnularia and sparse centrics, and these two profiles point to different aquatic environments.

Diatom ecology has been studied intensively since the 19th century for paleolimnological purposes (reconstructing past lake chemistry from sediment cores) and for biomonitoring water quality. That research base has produced a large and well-validated reference dataset that forensic analysts can draw on directly. The key environmental variables that shape assemblage composition include:

• pH: acidobiontic taxa (thriving below pH 5.5) such as Eunotia include species absent from neutral water. Alkaliphilous taxa such as Cocconeis prefer pH above 7. pH optima are among the best-calibrated ecological data in paleolimnology.
• Conductivity and dissolved ions: freshwater and brackish assemblages differ markedly; marine and estuarine taxa such as Coscinodiscus and Paralia are never found in low-conductivity inland water. A single marine centric taxon in an alleged freshwater drowning site is a significant anomaly.
• Trophic status: oligotrophic (nutrient-poor) lakes carry high species richness with low biovolume of any single species; eutrophic lakes are often dominated by a few fast-growing planktonic centrics. The ratio of oligotraphentic to eutraphentic taxa provides a rough trophic index.
• rheophilous (current-loving) taxa such as Didymosphenia and Gomphonema indicate lotic (flowing-water) habitats. Lentic (standing-water) assemblages dominated by planktonic centrics and motile pennates reflect still or slow-moving environments.

These ecological preferences translate directly into forensic strategy. An analyst collecting a control sample from the suspected drowning site can compare its assemblage against the taxa recovered from the victim's tissues. Where assemblages show strong similarity, provenance to that water body is supported. Where they diverge sharply, the analyst must consider whether the body was moved, or whether additional water bodies were involved.

Biogenic silica is among the most persistent biological materials in nature. Diatom frustules are recovered from Cretaceous-age sediments with their surface patterning intact. In forensic practice, this means frustules can be identified from extensively decomposed remains, from cremated bone, and from mummified tissue.

There are two conditions that do dissolve frustules. Highly alkaline environments (pH above 10) accelerate silica dissolution, and so do waterlogged acidic sediments where dissolved silica concentrations are high and temperature is elevated. In practice, most forensic remains are recovered from environments that do not reach these extremes, and frustule preservation in bone marrow, where frustules are physically protected inside the compact bone matrix, is generally excellent even in advanced decomposition cases.