Wetland plant tissues as a hidden methane oxidation location?

Abstract

Net methane (CH4) flux from coastal wetlands is an equilibrium product of methanogenesis by anerobic methanogenic microbes and methane oxidation by aerobic and anaerobic methanotrophic microbes. 75-95% of the methane produced is consumed by anaerobic methane oxidation in the deep, anoxic soil layers. Meanwhile, aerobic methane oxidation, which needs both oxygen (O2) and methane as substrates, occurs in the upper soil layer. Aerobic methanotrophy has mostly been studied in temperate grasslands. The well-aerated saltmarshes of the Wadden Sea coast might show a similar effect as such grasslands regarding methane consumption. However, this is a largely understudied process, since salt marshes are known for net methane emission rather than for net methane uptake. We hypothesize that (1) methane consumption vs. emission is driven by the oxygen concentration in the soil and is reflected in the ratio of methanotrophic vs. methanogenic microbes and (2) methane consumption occurs in the rhizosphere of salt marsh soil and in the plant tissue itself. A salt marsh plant-soil pot experiment will be conducted where S. anglica is exposed to different soil oxygen concentrations due to different hydrological conditions (5x waterlogged, 5x intermittently waterlogged, 5x drained), thereby triggering various methane dynamics. To determine methane consumption and emission, chamber measurements, using a MGGA Trace Gas Analyzer, will be performed regularly throughout the experiment. The ratio and location of methanotrophic to methanogenic microbes in the plant-soil system will be determined by quantitative polymerase chain reaction (qPCR) using pmoA189-F/pmoA661-R (methanotrophic bacteria) and mlas-F/mcrA-R (methanogenic archaea) primer pairs. Further analysis will include 16S sequencing on extracted DNA and rRNA from soil and plant tissue to differentiate between the total and the active community, respectively. Results from a pre-study show net methane consumption in the drained pots while net methane emission was measured in the waterlogged pots. We attribute this effect to upregulated methanotrophic processes under oxic conditions, resulting in a higher methane oxidation rate. First results of qPCR reveal methanotrophic bacteria in the rhizosphere and within the plant stem, while methanogenic archaea were only detected in the soil. Our results suggest a previously overlooked role of plant stem associated methanotrophic microbess in salt marshes greenhouse gas dynamics.