Pathways of autotrophic CO2 fixation and of dissimilatory nitrate reduction to N2O in Ferroglobus placidus

Abstract

The strictly anaerobic Archaeon Ferroglobus placidus was grown chemolithoautotrophically on H₂ and nitrate and analyzed for enzymes and coenzymes possibly involved in autotrophic CO₂ fixation. The following enzymes were found [values in parentheses = μmol min^–1 (mg protein)^–1]: formylmethanofuran dehydrogenase (0.2), formylmethanofuran:tetrahydromethanopterin formyltransferase (0.6), methenyltetrahydromethanopterin cyclohydrolase (10), F₄₂₀-dependent methylenetetrahydromethanopterin dehydrogenase (1.5), F₄₂₀-dependent methylenetetrahydromethanopterin reductase (0.4), and carbon monoxide dehydrogenase (0.1). The cells contained coenzyme F₄₂₀ (0.4 nmol/mg protein), tetrahydromethanopterin (0.9 nmol/ mg protein), and cytochrome b (4 nmol/mg membrane protein). From the enzyme and coenzyme composition of the cells, we deduced that autotrophic CO₂ fixation in F. placidus proceeds via the carbon monoxide dehydrogenase pathway as in autotrophically growing Archaeoglobus and Methanoarchaea species. Evidence is also presented that cell extracts of F. placidus catalyze the reduction of two molecules of nitrite to 1 N₂O with NO as intermediate (0.1 μmol N₂O formed per min and mg protein), showing that – at least in principle –F. placidus has a denitrifying capacity.