Stable carbon isotope discrimination and microbiology of methane formation in tropical anoxic lake sediments

Abstract

Methane is an important end product of degradation of organic matter in anoxic lake sediments. Methane is mainly produced by either reduction of CO<sub>2</sub> or cleavage of acetate involving different methanogenic archaea. The contribution of the different methanogenic paths and of the diverse bacteria and archaea involved in CH<sub>4</sub> production exhibits a large variability that is not well understood. Lakes in tropical areas, e.g. in Brazil, are wetlands with high potential impact on the global CH<sub>4</sub> budget. However, they have hardly been studied with respect to methanogenesis. Therefore, we used samples from 16 different lake sediments in the Pantanal and Amazon region of Brazil to measure production of CH<sub>4</sub>, CO<sub>2</sub>, analyze the content of <sup>13</sup>C in the products and in intermediately formed acetate, determine the abundance of bacterial and archaeal microorgansisms and their community composition and diversity by targeting the genes of bacterial and archaeal ribosomal RNA and of methyl coenzyme M reductase, the key enzyme of methanogenic archaea. These experiments were done in the presence and absence of methyl fluoride, an inhibitor of acetoclastic methanogenesis. While production rates of CH<sub>4</sub> and CO<sub>2</sub> were correlated to the content of organic matter and the abundance of archaea in the sediment, values of <sup>13</sup>C in acetate, CO<sub>2</sub>, and CH<sub>4</sub> were related to the <sup>13</sup>C content of organic matter and to the path of CH<sub>4</sub> production with its intrinsic carbon isotope fractionation. Isotope fractionation was small (average 10‰) for conversion of C<sub>org</sub> to acetate-methyl, which was hardly further fractionated during CH<sub>4</sub> production. However, fractionation was strong for CO<sub>2</sub> conversion to CH<sub>4</sub> (average 75‰), which generally accounted for >50% of total CH<sub>4</sub> production. Canonical correspondence analysis did not reveal an effect of microbial community composition, despite the fact that it exhibited a pronounced variability among the different sediments

Response of Methanogenic Microbial Communities to Desiccation Stress in Flooded and Rain-Fed Paddy Soil from Thailand

“…This assumption is consistent with the occurrence of Woesearchaeota and MEG archaea in the Thai soil, albeit they are not expected in soil environments. Another feature is the fraction of hydrogenotrophic methanogenesis, which after desiccation was comparable to the relatively high values ( f H2 > 0.5) typically found in lake sediments (Conrad et al, 2011; Ji et al, 2016), while those in paddy soils are usually lower ( f H2 < 0.4) (Yao and Conrad, 2000). Finally, we don’t know to which extent the resemblance of Thai paddies to lake sediments was effected by the flood catastrophe the year before sampling.…”

Climate change in Brazil: perspective on the biogeochemistry of inland waters

“…3 (suppl. ), p. 709-722 Roland, F. et al 714 in the water column, reducing the amount stored in the sediment (Ploug et al, 1997).In tropical ecosystems, the organic-matter degradation processes (aerobic and anaerobic respiration and fermentation) in the sediment, which result in CO 2 or CH 4 production, are related to the amount and availability of organic matter (Conrad et al, 2010(Conrad et al, , 2011. Because organic matter in the sediment originates from the water column, changes in temperature and rainfall may affect the quality and quantity of organic-matter sedimentation, which could directly affect CO 2 and CH 4 produced in the sediment, due to changes in temperature and rainfall patterns.…”

Natural events of anoxia and low respiration index in oligotrophic lakes of the Atlantic Tropical Forest

“…Indeed, our results consistently suggest a natural susceptibility of deep waters in oligotrophic lakes of the Atlantic Tropical Forest to anoxia and low RI (reaching values < 0) mainly during the summer. These conditions in aquatic ecosystems typically result in low biological diversity (Brewer and Peltzer, 2009;Diaz and Rosenberg, 2008;Vaquer-Sunyer and Duarte, 2008) and high production of CO 2 and other more powerful greenhouse gases (Conrad et al, 2011;Bastviken et al, 2011). Here, the natural water mixing during the beginning of the dry winter showed a reversal oxygenation and increase of RI in deep waters, coupled to the opposite trend at the surface without reaching severe hypoxia throughout the water column.…”

“…These weakly significant and non-significant correlations suggest that dynamics other than the balance between aquatic photosynthesis and respiration might drive high fluctuations in metabolic gases, strongly reducing the negative relationship between metabolic gases, pO 2 and pCO 2 , in both lakes. The C inputs from the watershed (Marotta et al, 2010b), and anaerobic (Conrad et al, 2011) or physical-chemical (Amado et al, 2007) organic degradation processes may enhance CO 2 without consuming O 2 in natural waters. In addition, anoxygenic photosynthesis may be responsible for the decoupling between CO 2 fixation and O 2 production (Fontes et al, 2011).…”

“…The O 2 depletion following high respiration of the excessive organic loading is a typical cause of organism death and severe decline in the species (VaquerSunyer and Duarte, 2008), which also stimulates the anaerobic organic decomposition in natural waters (Conrad et al, 2011). These anaerobic processes have important implications to global warming, producing more powerful greenhouse gases than CO 2 (Bastviken et al, 2011), as well to create "dead zones" by releasing toxic substances for major aquatic organisms (Diaz and Rosenberg, 2008).…”