Fractionation of stable carbon isotopes during acetate consumption by methanogenic and sulfidogenic microbial communities in rice paddy soils and lake sediments

<p>Acetate is an important intermediate during the degradation of organic matter in anoxic flooded soils and sediments. Acetate is disproportionated to <span class="inline-formula">CH₄</span> and <span class="inline-formula">CO₂</span> by methanogenic or is oxidized to <span class="inline-formula">CO₂</span> by sulfate-reducing microorganisms. These reactions result in carbon isotope fractionation, depending on the microbial species and their particular carbon metabolism. To learn more about the magnitude of the isotopic enrichment factors (<span class="inline-formula"><i>ε</i></span>) involved, acetate conversion to <span class="inline-formula">CH₄</span> and <span class="inline-formula">CO₂</span> was measured in anoxic paddy soils from Vercelli (Italy) and the International Rice Research Institute (IRRI, the Philippines) and in anoxic lake sediments from the northeastern and the southwestern basins of Lake Fuchskuhle (Germany). Acetate consumption was measured using samples of paddy soil or lake sediment suspended in water or in phosphate buffer (pH 7.0), both in the absence and presence of sulfate (gypsum), and of methyl fluoride (<span class="inline-formula">CH₃F</span>), an inhibitor of aceticlastic methanogenesis. Under methanogenic conditions, values of <span class="inline-formula"><i>ε</i>_ac</span> for acetate consumption were always in a range of <span class="inline-formula">−21</span> ‰ to <span class="inline-formula">−17</span> ‰ but higher in the lake sediment from the southwestern basin (<span class="inline-formula">−11</span> ‰). Under sulfidogenic conditions <span class="inline-formula"><i>ε</i>_ac</span> values tended to be slightly lower (<span class="inline-formula">−26</span> ‰ to <span class="inline-formula">−19</span> ‰), especially when aceticlastic methanogenesis was inhibited. Again, <span class="inline-formula"><i>ε</i>_ac</span> in the lake sediment of the southwestern basin was higher (<span class="inline-formula">−18</span> ‰ to <span class="inline-formula">−14</span> ‰). Determination of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M18" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">ε</mi><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="243653f26a5b3ea941b9d635223537b1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-6533-2021-ie00001.svg" width="23pt" height="12pt" src="bg-18-6533-2021-ie00001.png"/></svg:svg></span></span> from the accumulation of <span class="inline-formula">¹³C</span> in <span class="inline-formula">CH₄</span> resulted in much lower values (<span class="inline-formula">−37</span> ‰ to <span class="inline-formula">−27</span> ‰) than from the depletion of <span class="inline-formula">¹³C</span> in acetate (<span class="inline-formula">−21</span> ‰ to <span class="inline-formula">−17</span> ‰ ), especially when acetate degradation was measured in buffer suspensions. The microbial communities were characterized by sequencing the bacterial 16S rRNA (ribosomal ribonucleic acid) genes as well as the methanogenic <i>mcrA</i> and sulfidogenic <i>dsrB</i> genes. The microbial communities were quite different between lake sediments and paddy soils but were similar in the sediments of the two lake basins and in the soils from Vercelli and the IRRI, and they were similar after preincubation without and with addition of sulfate (gypsum). The different microbial compositions could hardly serve for the prediction of the magnitude of enrichment factors.</p>