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

Abstract

Abstract. Acetate is an important intermediate during the degradation of organic matter in anoxic flooded soils and sediments. Acetate is disproportionated to CH4 and CO2 by methanogenic or is oxidized to CO2 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 (ε) involved, acetate conversion to CH4 and CO2 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 (CH3F), an inhibitor of aceticlastic methanogenesis. Under methanogenic conditions, values of εac for acetate consumption were always in a range of −21 ‰ to −17 ‰ but higher in the lake sediment from the southwestern basin (−11 ‰). Under sulfidogenic conditions εac values tended to be slightly lower (−26 ‰ to −19 ‰), especially when aceticlastic methanogenesis was inhibited. Again, εac in the lake sediment of the southwestern basin was higher (−18 ‰ to −14 ‰). Determination of εCH4 from the accumulation of 13C in CH4 resulted in much lower values (−37 ‰ to −27 ‰) than from the depletion of 13C in acetate (−21 ‰ to −17 ‰ ), 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 mcrA and sulfidogenic dsrB 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.

Fractionation of stable carbon isotopes during microbial propionate consumption in anoxic rice paddy soils

“…The isotopic fractionation of propionate apparently followed Raleigh distillation that is characteristic for kinetic isotope fractionation in a closed system. The isotopic enrichment factor, which was determined from Mariotti plots, was in the range of εprop = -8‰ to -3.5‰, which is less than the enrichment factor for methanogenic acetate consumption, which has been found to be εac = -21‰ to -17‰ (Conrad et al, 2021). The εprop values are on the same order as those predicted from δ 13 C values of propionate, acetate and organic carbon measured in various methanogenic soils and sediments (Conrad et al, 2014).…”

“…In IRRI soil, 13 C depletion was even larger (30-40‰). In both soils, the isotopic enrichment factors for acetate consumption were in a range of -12‰ to -17‰ and for CH4 production from acetate in a range of -37‰ to -27‰ (Conrad et al, 2021).…”

“…The experimental setup was exactly the same as during a previous study on acetate consumption (Conrad et al, 2021). Paddy soil was mixed with autoclaved anoxic H2O at a ratio of 1:1 and incubated under N2 at 25°C for 4 weeks.…”

“…In a second incubation, paddy soil was mixed with autoclaved anoxic H2O at a ratio of 1:1, was amended with 0.07 g CaSO4.2H2O, and then incubated under N2 at 25°C for 4 weeks. These two preincubated soil slurries were sampled and stored at -20°C for later molecular analysis (see data in Conrad et al ( 2021)). The preincubated soil slurries were also used (in 3 replicates) for the following incubation experiments.…”

“…We also used the treatment with methyl fluoride (CH3F) to inhibit the consumption of acetate by methanogenic archaea (Janssen and Frenzel, 1997). Recently, we determined the microbial communities in methanogenic and sulfidogenic rice field soils, which were used for assessment of 13 C isotope fractionation during acetate consumption (Conrad et al, 2021). Here we present analogous data from the same soil suspensions prepared for the propionate degradation experiments.…”