Can Phytoliths Save the World?

Can Phytoliths Save the World?

Or: Plants do Carbon Capture and Storage Quite Naturally

Can Phytoliths….. Can what?? Many people have never heard of phytoliths, but I have worked on them for nearly 40 years, so let me tell you a little about them, and then I will describe some of my recent research. Yes, these unheard of phytoliths might help in the fight against climate change. But read on.

Pampas grass

Have you ever cut your hand pulling up grass? Have you noticed how sharp the edges of Pampas grass are? Have you ever been stung by a stinging nettle? This is all due to phytoliths! Soluble silica is taken up by the plants and is deposited in some of the cells as hard, solid phytoliths. If you know a little Greek you will know phyton is plant and lithos is stone or rock, so phytoliths are plant stones. They take the shape of the cells they are deposited in. So if you had a microscope you would be able to see that the edges of Pampas grass had razor sharp prickles made of silica. If you looked at the lower surface of nettle leaves you would see silica hairs which act like minute hypodermic syringes to inject poison into your skin.

When a plant dies the leaves, stems and flowers fall into the soil and are incorporated into the soil organic matter (humus). But the phytoliths are much more resistant to breakdown in the soil and can persist for hundreds or thousands of years. Because they last a long time, and have shapes and sizes that are characteristic for the plants they come from, phytoliths are used by archaeologists and palaeoecologists to work out what people grew and ate, and past environments and climates.

There is increasing interest in carbon sequestration in soils. It is recognised that the soil is a huge carbon store and that if we could find ways of increasing that storage then it could really help to suck carbon dioxide out of the atmosphere. But one of the big problems with this idea is that carbon sequestration in the soil is reversible. So once plant materials enter the soil and form humus it is susceptible to breakdown, releasing the carbon dioxide back to the atmosphere.

Back in 2005 some Australian phytolith experts, Parr and Sullivan, had a brilliant idea. They realised that phytoliths encapsulate carbon within their structures. According to their calculations, phytoliths store a lot of carbon in the soil and potentially sequester it within the silica for a very long time. Their paper and their idea created a whole new area of phytolith research. The idea has not been without controversy, particularly over how much carbon can be stored in phytoliths. Nonetheless, the area remains a major focus for phytolith research.

In the 1980s I spent a long time looking at how phytoliths developed within the plant, and I have kept up this interest, publishing a major review on the topic in 2016. Plants have two main types of phytolith: those developing in the cell lumen; and those that form in the cell wall on a carbohydrate (largely cellulose) matrix. If you did biology at school you might remember the cell wall as a kind of box around the lumen. The lumen contains the cytoplasm and all of the organelles, including the chloroplasts, nucleus etc.

I was invited by the Frontiers journal organisation in 2017 to be a guest associate editor for a special collection of papers, "Frontiers in Phytolith Research". I assembled an editorial team of experts from around the world and started to invite potential authors. But what would I write my own paper on? I decided that I wanted to look at one aspect of the carbon sequestration in phytoliths story that I felt had been neglected. Which types of phytolith are most important in storing carbon in the soil? Is it the cell lumen or the cell wall types?

Wheat inflorescence phytolith

To answer this question I needed to consult a wide range of literature. I began by outlining the history of carbon sequestration in phytoliths and discussing the major methodological controversy over how much carbon they contain. Next, I wanted to determine exactly which phytoliths were cell wall phytoliths. In most cases, it is pretty obvious, but there is one important type where we lack clarity. Then I looked at what is known about carbon concentrations in the two types of phytolith, and not surprisingly the cell wall types have much more carbon than the lumen types.

I then needed to find out what happened to the two types of phytolith in the soil. The received wisdom is that cell wall phytoliths break down faster in soil than lumen types. But I carried out a major literature survey looking for evidence to support this contention, and I couldn't find any! Moreover, when I investigated the archaeological and palaeoecological literature I found that cell wall phytoliths were present in a wide range of contexts and could be found in samples that were thousands of years old. Having done all this, I then constructed two hypotheses: one to consider what happens to phytoliths when they are prepared in the laboratory (this also addressed the question of how much carbon is stored in phytoliths); and the second concerned what happens in the soil.

Can phytoliths save the world? Probably not! But I think we need to look far more carefully at the rather neglected cell wall phytoliths. As I say in my paper, phytoliths are unlikely to be a "silver bullet" for climate change, but they may have a role to play. We are spending large amounts of time, money and energy on trying to get carbon capture and storage to work on power stations. Why not see if plants can do it naturally? Can we find ways to increase carbon sequestration in phytoliths and in soils? In my paper, I have outlined a whole lot of work that we need to do over the next few years. Let's get on and do it!

Martin J. Hodson
(July 2019)

Images:
1) Pampas grass image- Shirley Hirst on Pixabay:
https://pixabay.com/photos/grass-pampas-grass-pampas-56993/
2) Wheat inflorescence phytolith- MJH