Pollens and Diatoms in Forensic Investigation

Pollens and diatoms are grouped together because both are microscopic, both are recovered by similar microscopy workflows, and both are environmental markers rather than direct physical evidence of a crime. Beyond that, they answer different forensic questions.

Pollen answers the question "was this person or object at a particular place and season?" The pollen rain at a site (the mix of species deposited per unit area per day) varies by latitude, altitude, vegetation type and time of year. Pollen on the cuffs of a suspect's trousers can tie him to a wheat-and-mustard field margin in March even if no eyewitness places him there. The closest book companion on the trace-evidence side is the chapter on soil and botanical evidencewhich covers the broader transfer-and-recovery framework that pollen and seeds both sit inside.

Diatoms answer the question "did this person drown, or were they dumped in water after death?" The classical diatom test is built on a single physiological fact: if the victim was alive when submerged, water (with whatever diatoms it contains) was aspirated into the lungs, crossed the alveolar membrane and was distributed by a beating heart into bone marrow and distant organs. If the victim was already dead, no circulation, no diatoms in marrow. The femur and sternum samples that the test relies on are collected at autopsy under the same SOP that governs viscera in poisoning cases, covered in the book chapter on post-mortem and viscera collection in poisoning.

Both micro-categories share microscopy as the readout. Light microscopy at 400 to 1000 times magnification covers routine identification; scanning electron microscopy in the book chapter on scanning electron microscopy (SEM-EDXRF)is reserved for fine sculpture on pollen exines and for confirming diatom species boundaries when light microscopy is ambiguous. Every sample is logged in the chain of custodyregister from autopsy or scene collection through to the final report.

A pollen grain is the male gametophyte of a seed plant, wrapped in a remarkably tough wall made of sporopollenin. That wall is what survives in soil for thousands of years and what survives a wash cycle on a suspect's jeans. Its morphology is the basis of identification.

Wall layers. The exine (outer wall) is built from two layers. The outer sexine carries the diagnostic sculpture (spines, ridges, reticulum, pores). The inner nexine is smoother and forms the structural backing. The intine (innermost layer) is cellulose and is dissolved during acetolysis, so it does not appear on the microscope slide.

Apertures. Apertures are openings through which the pollen tube germinates. Three examiners-friendly aperture types:

• Porate. Circular pores. Grasses (Poaceae) are typically monoporate, birch is triporate.
• Colpate. Elongate furrows. Many dicots are tricolpate.
• Colporate. A combined furrow with a pore at its centre. Common across many Asteraceae and Fabaceae.

The number, distribution and shape of apertures, together with overall grain shape (spherical, oblate, prolate) and size (typically 10 to 100 micrometres, with most fall pollens at 20 to 50 micrometres), put a grain into a family or genus.

Pollen rain as a geographic and seasonal fingerprint. Each location has a characteristic pollen rain composition. A Doon valley deodar forest in autumn drops a different mix than a Punjab mustard field in February or a Sundarbans mangrove margin in monsoon. A skilled palynologist can read a pollen assemblage off a suspect's clothing or vehicle floor mat and narrow the source area to a region and season.

Sampling surfaces. The standard forensic palynology kit collects from hair, clothing (cuffs, pockets, fibres), footwear treads, vehicle interiors (floor mats, pedals, seats), and at autopsy from nasal cavity and gastric contents (the last two are especially useful in suspected drowning or burial cases).

The standard pollen extraction is Erdtman's acetolysis, published in 1934 and still the reference method. The sample is washed off the surface (or from a soil sub-sample), treated sequentially with hydrochloric acid (to remove carbonates), hydrofluoric acid (to remove silicates if soil is involved), and then with a 9:1 mixture of acetic anhydride to concentrated sulphuric acid at about 100 degrees Celsius for a few minutes. The reagent dissolves cellulose and protoplasm, leaving only the sporopollenin exine. The residue is centrifuged, stained (commonly with safranin or basic fuchsin), mounted in glycerine jelly or silicone oil, and read under a light microscope.

Identification is a reference-collection problem. The analyst compares the unknown grain against a curated slide library and against published atlases. India does not yet have a single national reference pollen atlas at the scale of the European Pollen Database, but the Birbal Sahni Institute of Palaeosciences (BSIP Lucknow) maintains the most comprehensive subcontinental reference collection, and the Wadia Institute of Himalayan Geology in Dehradun anchors Himalayan flora. Light microscopy at 400 to 1000 times handles most identifications; scanning electron microscopy is used for fine surface sculpture when species boundaries are tight.

A canonical case. In 1959 a man in Austria disappeared on a Danube riverboat trip. A suspect (later Joseph Anders) denied ever leaving Vienna. Palynologist Wilhelm Klaus examined mud from the suspect's boots and recovered a distinctive pollen assemblage of spruce, willow, alder and a fossil hickory species that only outcropped at one Danube-side village. Confronted with the pollen evidence, the suspect confessed and led police to the body. This is the first widely cited pollen-based murder conviction.

Indian context. Indian palynology in forensic casework is limited but growing. BSIP Lucknow, Wadia Institute Dehradun and the National Botanical Research Institute (NBRI) hold the reference collections that a state SFSL would consult. Routine examiners-relevant Indian use cases include body-disposal investigations in agricultural belts (matching pollen on a corpse's clothing to the field where it was dumped) and vehicle-interior pollen as supporting evidence in inter-state movement of contraband.

Diatoms are unicellular eukaryotic algae of class Bacillariophyceae. The cell wall, called the frustule, is biogenic silica (SiO2) and consists of two interlocking valves: the larger epitheca on top and the smaller hypotheca below, fitting together like a petri dish with its lid. Frustules are species-specific in shape, ornamentation and pore (striae) pattern, and they survive almost everything biological tissue does not, including the concentrated acid digestion used in the drowning test.

Two morphological groups. Pennate diatoms are elongate with bilateral symmetry and dominate freshwater habitats; examples include the Naviculaceae family, Pinnularia and Cymbella. Centric diatoms are circular with radial symmetry and are more common in marine and large-lake plankton; examples include Cyclotella and Aulacoseira. Each water body has a characteristic diatom assemblage shaped by salinity, pH, temperature, nutrient status and season, which is why a diatom species list from a body can be cross-matched to a candidate drowning site.

The diatom test for drowning. The test rests on one piece of physiology.

• Antemortem drowning. Water (with diatoms) is aspirated into the alveoli. Diatoms small enough to cross the alveolar capillary membrane enter pulmonary blood. The beating heart distributes them into systemic circulation, where they lodge in distant capillary beds, including the marrow of long bones, the kidney, the liver and the brain.
• Postmortem submersion. A body dumped in water after death has no circulation. Diatoms in surface water can passively settle on the body and on the lung surface, but they cannot reach the marrow of an enclosed long bone.

The test exploits this asymmetry. The most reliable sample is bone marrow from the femur or sternum, because the bone shell physically excludes environmental contamination. Liver, kidney and brain are useful adjuncts but more prone to false positives from contamination during dissection.

Extraction procedure. A piece of femur or sternum (about 10 to 20 grams of marrow-bearing bone) is opened in the lab. The marrow is scooped out, weighed, and transferred to a digestion flask. A 1:1 mixture of concentrated nitric acid and concentrated sulphuric acid is added, the flask is heated gently in a fume hood until the organic matter is fully dissolved, the residue is centrifuged and washed with distilled water several times to remove the acid, and the silica pellet is resuspended for microscopy. A positive result is the presence of diatom frustules in the marrow whose species composition matches the diatom flora of the suspected drowning site, sampled separately as a reference.

The evidentiary weight of the diatom test is contested in the literature, and analysts should be familiar with the standard objections.

Environmental contamination. Diatoms are ubiquitous in tap water, in laboratory glassware that has been rinsed with river water, in airborne dust near water bodies, and in food. A "positive" marrow test on a body that did not drown can result from contamination introduced during autopsy or laboratory handling. Strict cleanroom protocol and parallel processing of negative-control marrow are required.

Diatom-positive without drowning. Several published Indian and international studies have found diatoms in marrow of non-drowned controls (people who lived near rivers, or who consumed shellfish). A small diatom count in marrow is therefore not by itself proof of drowning; species match with the suspected drowning site is what gives the test its weight.

Diatom-negative drowning. Drowning in cold water (Himalayan rivers in winter), in a chlorinated swimming pool, or in a low-diatom-load environment (a clean village well, a freshly filled water tank) can give a negative test even though the victim genuinely drowned. The absence of diatoms therefore does not rule out drowning.

Sample preference. Femur marrow is preferred over sternum because the femur shell is thicker and excludes contamination more reliably. Some Indian SFSLs run both as a paired sample. Liver, kidney and brain are useful only as adjuncts and only if the marrow is unavailable.

The court treatment in India follows the same trajectory as other presumptive forensic tests under the Bharatiya Sakshya Adhiniyam 2023: the analyst can offer an expert opinion that the marrow diatom assemblage is consistent with antemortem drowning at a specified water body, but a positive test is corroborative rather than conclusive, and a negative test does not exclude drowning.

For palynology, the reference-collection anchors are BSIP Lucknow, the Wadia Institute of Himalayan Geology in Dehradun, and NBRI Lucknow. Most state SFSLs do not have dedicated palynology units, so casework is referred where the trace-evidence question warrants it.

For diatomology, the diatom test is performed in forensic medicine departments and SFSL biology divisions across the country. AIIMS New Delhi forensic medicine, JIPMER Puducherry, AIIMS Bhubaneswar and several state medical college forensic departments run the test routinely, particularly in monsoon-season drowning casework on the Ganga, Yamuna, Brahmaputra, Krishna and Godavari river systems. The National Centre for Polar and Ocean Research (NCPOR, Goa) and other oceanographic centres hold marine diatom reference collections relevant to coastal-drowning casework.

Two notable Indian use patterns. First, monsoon-flooding deaths and river-bank drownings: a positive marrow diatom test consistent with the recovered river's flora is corroborative of drowning rather than postmortem dumping. Second, body-disposal cases in which a homicide victim has been pushed into water to make the death look accidental: a negative marrow diatom test, combined with autopsy findings of antemortem injury, supports the homicide theory. The standard Indian autopsy protocol now routinely collects long-bone segments for diatom testing in suspected drowning cases.