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キーワード:
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要旨:
Although the fertilizing effect of elevated atmospheric CO2 on plant
growth is well established, its impact on the structure and activity of
below-ground biota remains elusive. Here, we used metatranscriptomics to
assess possible effects of moderately elevated CO2 (eCO(2)) on grassland
rhizosphere microbiomes at the Giessen free-air CO2 enrichment (GiFACE)
site in August of 2015 and 2017. Since its launch in 1998, the GiFACE
atmospheric CO2 concentration has been maintained at 20% higher than
ambient air levels, thereby suggesting a realistic ecosystem response to
eCO(2). Metatranscriptomics revealed significant eCO(2) effects on the
composition and activity of the grassland microbiomes in 2015, but not
in 2017. The summer of 2015 was characterized by prolonged heat waves,
while the summer temperature in 2017 was close to the long-term average.
In August 2015, rRNA and mRNA abundances of Eukarya relative to Bacteria
were significantly decreased in eCO(2) plots, in both rhizosphere soil
and root-associated microbiomes. On SSU rRNA level, the significant
increase in bacterial abundance was primarily related to an enrichment
of Actinobacteria and Proteobacteria and, most pronounced, a decline in
Fungi. In addition, we observed an enrichment of Acidobacteria,
Bacteroidetes, Chlorofiexi, and Planctomycetes and, among Eukarya, a
decline in Amoebozoa, SAR group, and Metazoa. The decrease in soil
fungal activity was confirmed by RT-qPCR of 18S rRNA; in good agreement
with a significant decrease in fungal mRNA involved in oxidative
phosphorylation (rhizosphere soil), and in folding, sorting and
degradation (root-associated). Among Fungi, the relative abundance of
most groups (e.g., Agaricomycetes [soil) and Leotiomycetes [roots])
decreased, but that of the Glonteromycetes (both compartments)
increased. Functional analysis of root mRNA suggests that the production
of plant secondary metabolites was increased in the summer of 2015.
These may have acted as an effective combat strategy against
phytopathogenic fungi, such as Leotiomycetes. In conclusion, our
metatranscriptomic study suggests that a near-future level of eCO(2)
combined with prolonged heat waves may have a significant impact on the
interactome between rhizosphere microbiomes and plant roots in European
grassland; with a primary effect on fungal activity.
