Mention of cave-dwelling plants living in extremis [see Uncovering underground Botany] brings to mind the real extremophiles of planet Earth, known – unsurprisingly! – as extremophiles. Although organisms in that category include representatives of all three domains of living things – the eukaryotes, bacteria, and Archaea, it’s the latter category that are the most extreme of extremophiles.
Amongst their ranks are members that produce methane (the methanogens), acidophiles who live at pH extremes, and halophiles that thrive in very salty environments. Now, dear readers, being an erudite bunch, what environments do you think of at the mention of these organisms? Extremely salty pools, yes; very low pH sulphur springs, yes, again; and, high-pressure, high temperature deep-sea vents, yes thrice. But, who amongst us would have suggested the interior of a tree? Not I for one. Yet, that is the unlikely habitat where methanogenic archaea have been found by Daniel Yip et al..
For an indeterminate length of time, these methane-producing microbes have been quietly doing whatever it is they do right under our very noses. But it’s because the methane (or so-called natural gas) that they release famously doesn’t have any odour that these unseen unicells could exist where they do without us really being aware of them*. Using 16S rRNA gene sequence analyses**, Yip et al. found the wood of Populus deltoides to be dominated by anaerobic (“existing in the absence of free oxygen”) microbiomes (“a community of microorganisms”). Amongst which, methanogens were prominent in the heartwood (“the central cylinder found in tree trunks”) where the “dominant operational taxonomic units” (OTUs***) were classified as Methanobacterium sp..
Quite how prevalent these microbes might be in trees of P. deltoides other than the 13 individuals sampled, or amongst other tree species is not yet known. However, this finding has relevance to understanding the flux of carbon between the atmosphere and living organisms. For example, I was generally led to believe that, as such long-lived entities, trees are a major sink for atmospheric CO2. However, if they also harbour methanogenic microbes, they must also now be viewed as a source of carbon in respect of the methane (CH4) they release. And, crucially, methane is not only another greenhouse gas (GHG) that contributes to global warming – like CO2 – but is also a far more potent (i.e. ‘effective’) heat-trapping molecule than CO2.
Although individual amounts of methane from one tree may be tiny, multiply that by the number of individuals in a population, then by the number of other methanogen-harbouring tree species, etc., and it can be a rather large amount. And is yet another hitherto unrecognised source of planet-warming organics to add to the increasingly complex carbon cycle.
Are arbor ‘arboured methanogens turning up the heat on planet Earth? Indeed, could flammable microbe-manufactured methane be a contributory factor to the spread of the wildfires that scorched huge areas of the planet during the summer of 2018?[Ed. – A video clip showing the burning of methane released from the heartwood can be seen at New Phytologist].
* The authors of the New Phytologist paper do acknowledge that accounts of methanogens in tree-wood were first reported decades ago, the distinction made is that their 21st century work is the first to ‘revisit’ such stories using modern-day, molecular ecology approaches.
** This technique doesn’t actually reveal the actual bacteria as one might do with a microscope as an aid to their identification. Instead it is based upon detection of sequence differences in a highly variable regions of the 16S r(ribosomal)RNA gene whose product is part of the particular protein-synthesizing ribosome that is unique to and present within all ‘bacteria’. Comparison of the different sequences identified in a sample with published sequences for identified organisms thus permits identification of the bacteria present, albeit without actually seeing them with one’s own eyes – as workers like Zeikus and Ward would have had to do in the ‘good old days’…
*** An OTU is a substitute form of identification of a species without actually observing the organism, and here assumes that similar 16S rRNA sequences are from the same organism, and that different sequences are therefore from different species.