Is it unusual for populations of archaea to be found living inside trees?

Question

The NPR News article and podcast Getting Fire From A Tree Without Burning The Wood begins with

A scientist walks up to a cottonwood tree, sticks a hollow tube in the middle and then takes a lighter and flicks it. A jet of flame shoots out from the tube.

It seems like a magician's trick. Turns out, there's methane trapped in certain cottonwood trees. Methane is the gas in natural gas. It's also a powerful greenhouse gas.

and describes the discovery that quantities of methane have been observed to be produced within cottonwood trees.

There is a video of the methane tapped from a tree being set on fire.

The article continues:

"The wood in this particular species naturally has this condition called wetwood, where it's saturated within the trunk of the tree," says the lighter-flicking scientist, Oak Ridge environmental microbiologist Christopher Schadt.

This wetwood makes for a welcoming home for all sorts of microorganisms.

"You can't actually see a lot of the organisms because we can't grow a lot of these organisms," says Melissa Cregger, a staff scientist at Oak Ridge. "So we're able to identify them using their gene sequences."

Some of those organisms turned out to be species of archaea that are known methane producers. So it's not the trees themselves that are making the methane, it's the microbes living in the trees.

It's not clear to me if the use of "wetwood" refers to the natural condition of cottonwood trees, or if this is meant to refer to some kind of bacterial infection. For example, Mirriam-Webster defines wetwood as:

wood having a water-soaked or translucent appearance because of abnormally high water content sometimes due to bacteria and sometimes to physiological factors

Question: Is it unusual for populations of archaea to be found living inside trees? Is this something new and surprising or is this well documented?

Answer 1

The exact term — wetwood, is used only once in the original article (Yip et al., 2018).

Others have found that water content of heartwood tissue was the best predictor of methane flux across multiple plant species (Machacova et al., 2016; Wang et al., 2017), however this was not a strong predictor of methanogen relative abundance in our analyses. It is possible that within trees characterized by wetwood such as Populus deltoides, other factors may become more important than water content above a certain moisture threshold.

However, if you look at these lines from the introduction of the paper, it seems that wetwood just means wood that has a high water content. It definitely does not refer to a disease or infection.

Indeed, both mechanisms may be important depending on tree species (e.g. those containing dry and dense vs wet and porous wood) and ecosystem under study (e.g. wetland vs upland forest soils).

Based upon past work and our own informal observations of odors associated with tree coring at these sites indicative of fermentative environments, we hypothesized that an anaerobic community including methanogenic archaea would dominate woody tissue, particularly in heartwood tissue which is often water‐soaked.

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Now coming to your question on whether it is unusual for populations of archaea to be found living inside trees.

Roots have been known to harbour microbes; plants establish this symbiotic relationship with the microbes for nitrogen fixation (See mycorrhiza and root nodules). So, it may not be very unusual that archaea interact with plants but it may possibly be unusual that they are living inside this part of the tree. Yip et al. (2018) highlight that although microbial composition of the wood has been studied in the past, these studies have been done ex situ. This means they are the first to study these microbes in situ.

Microbiological investigations of living wood have been conducted in the past (Zeikus & Ward, 1974; Zeikus & Henning, 1975; Ward & Zeikus, 1980; Schink et al., 1981a,b, 1982), but these studies primarily focused on ex situ wood incubations and the isolation of microbial representatives, rather than overall community characterization across tree conditions. For example, Zeikus & Ward (1974) demonstrated that methane production occurred from extracted wood tissue from Populus deltoides and were able to isolate pure cultures of a new species, Methanobacterium arborphilicum (Zeikus & Henning, 1975), later reclassified as Methanobrevibacter arboriphilus (Balch et al., 1979).

So, it seems that people have been working on this area since 1970s.

However, it also appears that there is not much research on archaea as a part of the plant microbiome (Taffner et al., 2018).

IMPORTANCE: Archaea are still an underdetected and little-studied part of the plant microbiome. We provide first and novel insights into Archaea as a functional component of the plant microbiome obtained by metagenomic analyses. Archaea were found to have the potential to interact with plants by (i) plant growth promotion through auxin biosynthesis, (ii) nutrient supply, and (iii) protection against abiotic stress.

So whether this is unusual or not depends on whom you are asking (hence a bit opinion based).