High-resolution vertical biogeochemical profiles in the hyporheic zone reveal insights into microbial methane cycling
<p>Facing the challenges of climate change, policy making relies on sound greenhouse gas (GHG) budgets. Rivers and streams emit large quantities of the potent GHG methane (CH<span class="inline-formula"><sub>4</sub></span>), but their global impact on atmosphe...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2022-09-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/19/4551/2022/bg-19-4551-2022.pdf |
| Summary: | <p>Facing the challenges of climate change, policy making
relies on sound greenhouse gas (GHG) budgets. Rivers and streams emit large
quantities of the potent GHG methane (CH<span class="inline-formula"><sub>4</sub></span>), but their global impact on
atmospheric CH<span class="inline-formula"><sub>4</sub></span> concentrations is highly uncertain. In situ data from
the hyporheic zone (HZ), where most CH<span class="inline-formula"><sub>4</sub></span> is produced and some of it can
be oxidized to CO<span class="inline-formula"><sub>2</sub></span>, are lacking for an accurate description of CH<span class="inline-formula"><sub>4</sub></span>
production and consumption in streams. To address this, we recorded
high-resolution depth-resolved geochemical profiles at five different
locations in the stream bed of the river Moosach, southern Germany.
Specifically, we measured pore-water concentrations and stable carbon
isotopes (<span class="inline-formula"><i>δ</i><sup>13</sup></span>C) of dissolved CH<span class="inline-formula"><sub>4</sub></span> as well as relevant
electron acceptors for oxidation with a 1 cm vertical depth resolution.
Findings were interpreted with the help of a numerical model, and 16S rRNA
gene analyses added information on the microbial community at one of the
locations. Our data confirm with pore-water CH<span class="inline-formula"><sub>4</sub></span> concentrations of up
to 1000 <span class="inline-formula">µ</span>mol L<span class="inline-formula"><sup>−1</sup></span> that large quantities of CH<span class="inline-formula"><sub>4</sub></span> are produced in
the HZ. Stable isotope measurements of CH<span class="inline-formula"><sub>4</sub></span> suggest that
hydrogenotrophic methanogenesis represents a dominant pathway for CH<span class="inline-formula"><sub>4</sub></span>
production in the HZ of the river Moosach, while a relatively high abundance of
a novel group of methanogenic archaea, the <i>Candidatus</i> “Methanomethyliales” (phylum
<i>Candidatus</i> “Verstraetearchaeota”), indicate that CH<span class="inline-formula"><sub>4</sub></span> production through H<span class="inline-formula"><sub>2</sub></span>-dependent methylotrophic methanogenesis might also be an important CH<span class="inline-formula"><sub>4</sub></span>
source. Combined isotopic and modeling results clearly implied CH<span class="inline-formula"><sub>4</sub></span>
oxidation processes at one of the sampled locations, but due to the steep
chemical gradients and the close proximity of the oxygen and nitrate
reduction zones, no single electron acceptor for this process could be
identified. Nevertheless, the numerical modeling results showed
potential not only for aerobic CH<span class="inline-formula"><sub>4</sub></span> oxidation but also for anaerobic oxidation
of CH<span class="inline-formula"><sub>4</sub></span> coupled to denitrification. In addition, the nitrate–methane
transition zone was characterized by an increased relative abundance of
microbial groups (<i>Crenothrix</i>, NC10) known to mediate nitrate and nitrite-dependent
methane oxidation in the hyporheic zone.</p>
<p>This study demonstrates substantial CH<span class="inline-formula"><sub>4</sub></span> production in hyporheic
sediments, a potential for aerobic and anaerobic CH<span class="inline-formula"><sub>4</sub></span> oxidation, and
underlines the high spatiotemporal variability in this habitat.</p> |
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| ISSN: | 1726-4170 1726-4189 |