EVIDENCE FOR ISOTOPIC EXCHANGE DURING METABOLISM OF STABLE-ISOTOPE-LABELED FORMATE IN A METHANOGENIC SEDIMENT

The disappearance of stable-isotope-labeled formate in freshwater sedi ment from Lake Vechten was studied by gas chromatography-mass spectrom etry. Both deuterium- and C-13-labeled formate mere removed from pore water at a high rate (400 +/- 35 [standard deviation] and 1,700 +/- 60 0 mu M h(-1), respectively), with concomitant formation of nonlabeled formate. The removal rate of labeled and nonlabeled formate amounted t o 240 +/- 15 and 400 +/- 40 mu M h(-1), respectively. The formation of nonlabeled formate is explained by isotopic exchange due to the activ ity of formate dehydrogenase rather than by the turnover of formate.