PATHWAYS OF AUTOTROPHIC CO2 FIXATION AND OF DISSIMILATORY NITRATE REDUCTION TO N2O IN FERROGLOBUS-PLACIDUS

The strictly anaerobic Archaeon Ferroglobus placidus was grown chemoli thoautotrophically on H-2 and nitrate and analyzed for enzymes and coe nzymes possibly involved in autotrophic CO2 fixation. The following en zymes were found [values in parentheses = mu mol min(-1) (mg protein)( -1)]: formylmethanofuran dehydrogenase (0.2), formylmethanofuran:tetra hydromethanopterin formyltransferase (0.6), methenyltetrahydromethanop terin cyclohydrolase (10), F-420-dependent methylenetetrahydromethanop terin dehydrogenase (1.5), F-420-dependent methylenetetrahydromethanop terin reductase (0.4), and carbon monoxide dehydrogenase (0.1). The ce lls contained coenzyme F-420 (0.4 nmol/mg protein), tetrahydromethanop terin (0.9 nmol/mg protein), and cytochrome b (4 nmol/mg membrane prot ein). From the enzyme and coenzyme composition of the cells, we deduce d that autotrophic CO2 fixation in F. placidus proceeds via the carbon monoxide dehydrogenase pathway as in autotrophically growing Archaeog lobus and Methanoarchaea species. Evidence is also presented that cell extracts of F. placidus catalyze the reduction of two molecules of ni trite to 1 N2O with NO as intermediate (0.1 mu mol N2O formed per min and mg protein), showing that - at least in principle - F. placidus ha s a denitrifying capacity.