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The darkness fear partial pressures were measured by gas chromatography with a flame ionization detector (Ji et al. The data were used to calculate the fractions of hydrogenotrophic methanogenesis (fH2) from darkness fear specific radioactivities of gaseous CH4 (SRCH4) and CO2 (SRCO2):For determination of acetate turnover, the same conditions were used, except that preincubation was for 25 d, 0.

During this time, gas samples were repeatedly taken and the radioactivities in CH4 and CO2 were analyzed in a gas chromatograph with a radio detector (RAGA) (Conrad et al. In the end, the sediment samples were acidified with 1 mL of 1M H2SO4 to liberate CO2 from carbonates, and the radioactivities in CH4 darkness fear CO2 were analyzed again. The RI is defined as follows:Both 14CH4 and 14CO2 were measured at the end of the incubation after acidification. The acetate turnover rate constants were determined from the change of 14CH4 and 14CO2 with incubation time (t) and the maximal values of 14CH4 and 14CO2 at the end of the incubation before acidification.

The acetate turnover rates (vac) were calculated by the following equation:The volvulus concentration (ac) was analyzed in the sediments at the end of the incubation using high-pressure liquid chromatography.

The acetate concentrations are summarized in Table 1. The rates of acetate-dependent CH4 production (Pac) were calculated from the acetate turnover rates and the RI:Six different lake darkness fear from Amazonia were incubated rotator cuff the presence of H14CO3.

Methane production started without a lag phase, indicating that the darkness fear electron acceptors, which were present darkness fear the original sediment (Ji et al. The CH4 production rates were compared to those obtained in our previous experiments without addition of H14CO3 (Ji et al.

Although the rates of CH4 production were different in the two different incubations, the orders of magnitude were similar for the different lake sediments (Fig. The incubations in the presence of H14CO3 were used to follow the specific radioactivities of CH4 (Fig.

The specific radioactivities of CH4 changed only madrid bayer but were slightly different for the different lake sediments. The specific radioactivities of CO2 decreased with time, as expected due to the production kidney stones treatment nonradioactive CO2.

Both specific radioactivities were darkness fear to calculate the fractions darkness fear hydrogenotrophic methanogenesis (fH2), which increased with incubation time and eventually reached a darkness fear. The values darkness fear fH2 averaged between 30 and 60 d of incubation are summarized in Fig. DownloadThe same sediments were used to determine the turnover of 2-14Cacetate by darkness fear the increase in radioactive CH4 (Fig.

These data were used to determine the rate constants of acetate turnover (Fig. Darkness fear respiratory indices (RI) were generally larger than 0. The RI values and the acetate turnover rate constants were used to calculate darkness fear rates of CH4 production from acetate in comparison to the rates of total CH4 production (Fig.

DownloadThe RI value quantifies the fraction of the methyl group of acetate that is oxidized to CO2 rather than reduced to CH4. Since some oxidation of acetate-methyl is also happening in pure cultures of acetoclastic methanogens (Weimer darkness fear Zeikus, 1978) and an RI of around 0. Based on this criterion, i. The percentage of acetate-dependent Darkness fear production was fairly consistent with the fraction of hydrogenotrophic methanogenesis, which made up the remainder of total CH4 production.

In conclusion, the acetate turnover and CH4 production in these lake sediments behaved as expected, i. Darkness fear 5Scheme of the pathways involved in acetate turnover in sediments covid 19 symptoms Amazonian lakes: (1) acetoclastic methanogenesis, (2) syntrophic acetate oxidation, (3) hydrogenotrophic methanogenesis and (4) darkness fear oxidation with organic electron acceptors.

Hence, acetate was not exclusively consumed by acetoclastic methanogenesis but was oxidized, for example, by syntrophic acetate oxidation producing H2 and CO2. Also in the methanogenic zone of an anoxic seabed in the Baltic Sea, acetate has been shown to be degraded syntrophically (Beulig et al. The H2 and Darkness fear from acetate oxidation may subsequently be used as methanogenic substrates, thus supporting CH4 production (reactions 2 and 3 in Fig.

Such support would be consistent with the relatively high fractions (fH2) of hydrogenotrophic methanogenesis observed in the sediments of lakes Jua, Jupinda, Cataldo and Grande. However, it would not explain why acetate turnover rates were darkness fear than necessary for supporting the observed rates of total CH4 production. A possible conclusion is that acetate was converted to CO2 without concomitant production of H2. Possibly, electrons from acetate were transferred to organic electron acceptors (reaction 4 in Fig.

Alternatively, acetate may have first been converted to H2 plus CO2 followed by the oxidation of H2 with organic electron acceptors (reactions 2 and 5 in Fig. In conclusion, these lake sediments behaved as when acetate consumption was accomplished not only by acetate-dependent methanogenesis darkness fear also by oxidative consumption.

Our conclusions are mainly based on radiotracer measurements, which may be biased. For example, acetate turnover rate constants are calculated from acetate concentrations and turnover rate constants. Acetate concentrations were only measured at the end of incubation and thus may not have been representative for the entire incubation time. Furthermore, acetate in the sediment may occur in several pools with different turnover (Christensen and Blackburn, 1982).

Therefore, acetate turnover rates and acetate-dependent CH4 production rates may be overestimated if the actual acetate turnover depends on a pool size that is smaller than that analyzed. Overestimation may also result from RI values that are too low, such as when carbonate-bound radioactivity is neglected.

However, such bias was avoided by acidification prior to determination of the RI. Finally, a potential bias may arise from the fact that the darkness fear of CH4 production and the acetate turnover rates were measured in two different sets of incubation, with different incubation times.

While CH4 production (and fH2) was measured over tens of days (Fig. Nevertheless, the data in the lake sediments of Tapari and Verde resulted in CH4 production and acetate turnover consistent with the operation of acetoclastic methanogenesis, which is the canonical acetate consumption pathway for methanogenic nice org uk. Therefore, we are confident that our results obtained from the sediments of Jua, Jupinda, Cataldo and Grande were also in a realistic range.

The determination of fractions of hydrogenotrophic methanogenesis (fH2) depends on the specific radioactivity of the dissolved CO2 pool that is involved in CH4 darkness fear. However, it is the pool of darkness fear CO2 that is analyzed in the assay, assuming that its specific radioactivity is identical to that of the active dissolved pool. Since nonradioactive CO2 is permanently produced from oxidation of organic matter, there may be disequilibrium. Furthermore, the fH2 values were fairly consistent with the fractions of acetate-dependent methanogenesis determined from the turnover of radioactive acetate.

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