Rhizosphere of grassland plants: A hot spot of methanol consumption driven by unusual methylotrophs

Managed grasslands are global sources of atmospheric methanol, which is one of the most abundant biogenic volatile organic compounds (bVOCs) in the atmosphere and promotes oxidative capacity for tropospheric and stratospheric ozone depletion. The phyllosphere is a favoured habitat of plant-colonizing methanol-utilizing methylotrophs, but their quantitative relevance for methanol consumption and ecosystem fluxes in the rhizosphere is unclear. Methanol utilizers of the plant-associated microbiota are key for the mitigation of methanol emission through consumption. However, information on grassland plant methylotrophs, their biodiversity and, metabolic traits, and thus key actors in the global methanol budget is largely lacking.

Two common plant species (Festuca arundinacea, Taraxacum officinale) of a grassland were investigated in pot experiments using soil as a growth substrate. We used radiotracers (¹⁴C-methanol) to evaluate potential methanol oxidation rates and ¹³C-methanol RNA stable isotope probing (SIP) and metagenomes to identify methanol utilizers.

Intact plants unveiled different methanol utilizer communities between plant compartments (phyllosphere, roots, and rhizosphere) but not between plant host species. Methanol utilizers of Gamma- and Betaproteobacteria colonized the phyllosphere. Whereas,Deltaproteobacteria, Gemmatimonadates, and Verrucomicrobiae were predominant in the rhizosphere. Metagenome assembled genomes (MAGs) revealed bacterial methanol dehydrogenases of known but also unexpected genera, such as Methylomirabilis, Methylooceanibacter, Gemmatimonas, and Verminephrobacter. Divergent methanol oxidation rates in both plant species but similarly high rates in the rhizosphere and phyllosphere were determined by ¹⁴C-methanol tracing of alive plant material.

Our study revealed eventually the rhizosphere as a hotspot for methanol consumption in grasslands. Differences between the methanol utilizer communities of the two plant species were not evident suggesting a negligible host effect. Our results suggest a model for methanol turnover in which both the sources (plants) and sinks (microbiota) of a bVOC are separated but in the same ecological unit.