Pollen – Diamond of the Plant World
You might think staring down a microscope counting pollen grains would be tedious – particularly if you’re a senior lecturer in earth sciences at Massey University in Palmerston North – but for palynologist Dr Katherine Holt the stories a pollen grain can tell exert an endless fascination. Part of that fascination is studying pollen in honeys, but also includes teaching biogeography such as fossil identification, carbon dating and the distribution of plants as it relates to climate change. Nigel Costley takes a look into the wide-ranging work of the Kiwi scientist.
BY NIGEL COSTLEY
Holt’s work gained international recognition during a palynologist conference in Japan she attended in 2015. Her presentation on 3D modelling of pollen grains caused something of a sensation.
“I was bombarded with people – it was the most attention I’ve ever received,” she says.
Concentrating on a few native species, she developed the first set of accurate 3D-printed scale models of pollen up to 3,000 times their actual size. Building on this work, UK scientist Oliver Wilson, in collaboration with Hull and Cardiff Universities, has created the 3D Pollen Project, which provides a fast, free and accessible way for anyone to print replicas of pollen grains.
The project, now based at Reading University, has a global reach and, as of the end of last year, had been downloaded 2,500 times in 45 countries. The UK scientist is quick to recognise Holt’s contribution to the project.
“Kat Holt is an absolute pioneer for 3D pollen. I wouldn’t have been able to make the 3D Pollen Project work without the effort she put in, and I doubt I would have started it without her encouragement,” Wilson says.
The 3D idea came to her when her engineer husband wanted to buy a 3D printer and she thought, “how can I use this for my pollen work?”
A pollen grain viewed through a microscope is seen only in two dimensions. Whereas, by taking photos from different angles a 3D image file can be built. That 3D image is so handy because it otherwise takes a long time for ‘pollen spotters’ to recognise pollen from different angles.
Once well-versed in pollen identification, a scientist has a wonderfully transferable skill.
“Pollen shells are the diamonds of the plant world, one of nature’s toughest materials,” Wilson explains from his UK base.
“They act like a time machine, lasting for millions of years. They tell us how forests are shaped by climate change and people.”
Back in Time
The time machine analogy harks back to how Holt became a palynologist in the first place. Growing up in Napier in a family of keen gardeners, around the age of five she was captivated with dinosaurs. She read everything she could about them, which led on to fossils, and eventually an interest in geological time. What better window into the ancient past than the study of pollen?
As an adult student, the Quaternary Period (the last 2.6 million years) became the focus of her study.
“This is the most recent period, with a lot of dramatic changes that has shaped our modern landscape at a fine scale,” Holt says.
Her PhD thesis was an exploration of the Chatham Island’s Quaternary history via the island’s considerable peat deposits, with peat being an ideal medium for preserving pollen. During her several ‘campaigns’ on the islands she discovered ash deposits from a volcanic eruption that took place 340,000 years ago in the Taupo district – 800 kilometres away. The island’s pollen record traced how the forest has responded to climate change.
The Chatham Island’s landscape, its people, and way of life (such as was detailed in Beekeeping and Business Balance on the Chathams last month) had a lasting impact on Holt and her interest continues with her involvement in beekeeping there to this day. In recent years she has assessed the island’s honey, identifying it as tarahina (Dracophyllum arboreum), mingimingi, and pouteretere (Leptecophylla robusta). All via pollen analysis.
Her expertise will also be invaluable in addressing some of the island’s issues on bee forage as it pertains to land use.
“Gorse is a good pollen source but of course it’s subject to spraying, and some people want to replace the dracophyllum [native tarahina] for ecological reasons. These are questions for the whole island to discuss,” she says.
Honey Analysis Advancements
A big part of her microscope work entails identifying and counting pollen grains for honey analysis. She finds this easy compared to the more “hit and miss” business of scrutinising geological samples where the pollens can be few and far between, random, or squashed.
Developments in artificial intelligence are proving alluring to the pollen analysis world, with Holt and colleagues now working on a robotic microscope to recognise and count pollen types automatically. This entails using 3D images to train up a neural network which mimics the human brain.
“In practise the results are not yet good enough – the sticking point is that as more cases are added the neural networks get confused and make mistakes,” she says.
Anomalies such as debris or squashed and clustered grains can confuse the system. While they are definitely making progress as stronger neural networks become available, Holt has no fears that machines will do her out of a job.
“The real work lies in data interpretation and you still have to know your pollen for that.”
As these systems are perfected the client should get a faster, cheaper, and more statistically-reliable result.
Knowing your pollen also means knowing how they’re represented in specific types of honey. This information is used for correct labelling and marketing, so it is important a minimum percentage of pollen grains are stipulated to qualify a honey as monofloral.
A flower’s design and size can impact the abundance of pollen that ends up in the nectar, and therefore honey, collected. Manuka and kanuka flowers have their stamen close to the nectary and thus 70 percent pollen has to be present to qualify as such in New Zealand. While rata, with a flower better shaped for birds to pollinate, requires only 45% to qualify.
There is more work needed to be done on understanding the pollen make up of New Zealand’s honey if labelling is to be truly accurate, Holt believes.
“There is a big gap in our knowledge of Kiwi honeys, other than manuka,” she says.
Wide Ranging Work
Then there is the study of the nutritional side of pollen and, although she hasn’t had a lot of experience yet in that field of work, it’s an area Holt is keen to explore.
Currently she is analysing some pollen samples used by a bumble bee breeder, to assess its nutritional value for that species. There is interest too in pollen as a human nutrient, but citing the survival of pollen shells in human excrement, she wonders how much nutrient is actually absorbed.
“I suspect pollen may be over-hyped,” Holt says.
Clearly a palynologist’s lot is an ever-flowering one. Yet another of her projects is developing spectroscopy for honey analysis. In this case palynology is used as a control to guide this new technology. Rich in new discoveries, it’s a busy life, and when I spoke to her the academic’s day was rounded off with a three hour Zoom call to Norway.
It’s amazing where a childhood fascination with dinosaurs can take you…
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Dr Holt’s work with pollen is wide-ranging, and so to are the stories pollen can tell us:
· Forensic palynology dates back to a 1950s when a man disappeared, suspected murdered, travelling down the Danube. Mud on the suspect's boots contained 20-million-year-old fossilised pollen grains which could only have come from a small area on the river. The suspect confessed, and led police to the body, which was just where the pollen said it would be.
· New Zealand has its own internationally-recognised forensic palynologists. Most notably Dr Anna Sandiford, and Dr Dallas Mildenhall. The latter worked on the Kirsa Jensen and Mellory Manning murder cases.
· Archaeological studies demonstrate mead as a crust residue on the inside of earthenware used as funeral gifts from at least the 27th-25th centuries BC in Georgia. The main pollen found was from meadowsweet (Fillipendual ulmaria). A comparison of the pollen records of European honey/mead samples strongly suggests that this is indicative of mead.
· Monitoring airborne pollen in response to increased reportage of allergic reactions.
· Evidence of global warming. Studies of ragweed (a well-known allergy-causing plant) in the US show that the flowering season is getting longer because of the warmer autumn weather.
· Because of their abundance and tiny size, fossilized pollen can be extracted from small samples of rock secured in drilling operations. Palynology is therefore used in oil exploration and the search for heavy minerals.
· Ancient farmers as entrepreneurs. Pollen data from sediment cores demonstrates well-connected markets in the Greece Islands 2,600 years ago. Those farmers living close to the Black Sea were reliant on importing grains, in exchange they exported olive oil and wine. This shows a level of specialisation and market integration that previously was thought to have only been reached several hundred years later with the Roman Empire.