Why algae is a forensic biologists best friend

Forensic science is vital in supporting police and the justice system in cases where traditional evidence sources like witnesses are weak.

LJMU forensic biologist Dr Kirstie Scott acted as scientific consultant on the BBC series Body on the Beach: What happened to Annie? which revisits a cold case from 2007.

Kirstie wasn’t involved in the original investigation of the death Annie Börjesson, a 30-year-old from Sweden who lived in Edinburgh until her body was found on Prestwick Beach – but in the new series, she explains how the presence of diatoms in her bone marrow raises interesting questions about some of the facts of the case.

As the series demonstrates, the growing field of forensics can contribute towards revealing new insights into cold cases even decades after the fact.

So what are diatoms and how do they provide evidence?

“Diatoms are eukaryotic microscopic algae. When you're around any kind of body of water, or any kind of damp surface, like soils, or tree bark, more often than not, there'll be these microscopic communities of algae growing and dominating those environments,” explains Kirstie.

“They can be used to diagnose different instances of death, or identify different incidents that might have taken place during crime events.

“A person, whether victim or perpetrator will always leave a trace or take a trace of a location with them, whether it is DNA, fingerprints, pollen or diatoms. If someone has committed a crime around water they may well have diatoms on their clothing – so my work is collecting that algae from people’s clothing, shoes, vehicles. Basically, if they’ve had contact with a location, we will know about it.

“We’ll know whether those diatoms have come from a certain place or environment.”

Why are diatoms such a useful guide?

There are hundreds of thousands of species and they are specific to different environments. For example ponds, rivers, lakes, oceans, and coastal areas will all have different diatom communities. They’re chemically resistant and they change their communities when exposed to different conditions, so can tell us a lot of where they ‘live’ and what goes on there.

They play a big role in the BBC series. How relevant are diatoms to this specific case?

“Annie was found in the water – on the beach to be exact - and whenever there is a death around water, you can expect there to be some algae-based evidence, which could help us understand where the body has been and what it has been exposed to.

“More often than not – drowning is cause of death but there are cases where people have disposed of a body in water to make it look like a drowning or accidents. Diatoms can help us to distinguish these circumstances.

Looking at it years after Annie’s body was first investigated, the programme found two diatoms in Annie’s bone marrow, one freshwater and the other marine. It could be they entered the body in other ways but it could also suggest the body was moved from a river or lake, either by current or a suspect.”

Where else have diatoms been used in forensic investigations?

“They are very good at indicating death by drowning and estimating the time of submersion in water. But there are many other ways we can analyse diatoms in forensics that we are only just starting to understand. For example, diatoms may transfer as trace evidence onto someone’s personal items such as clothing or footwear. We can subsequently use them to link suspects with crime scenes. They can be used to monitor pollution that may result from illegal waste disposal or contamination within our waterways. And very recently, the RSPCA have used diatoms as evidence in animal cruelty cases to understand cause of death by drowning.

“However, it’s tricky when crimes involve water as it is so diverse, so transient, and it changes constantly – it can be very difficult to identify and analyse forensic evidence.”

Body on the Beach: What Happened to Annie? is on the iPlayer.