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

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© Springer-Verlag 1997

O R I G I N A L PA P E R

Julia A. Vorholt · Doris Hafenbradl · Karl O. Stetter · Rudolf K. Thauer

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

Received: 23 August 1996 / Accepted: 6 November 1996

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. Key words Autotrophic CO2 fixation · Dissimilatory nitrate reduction · Archaeoglobus species · Methanogenic Archaea · Methanofuran · Tetrahydromethanopterin · Coenzyme F420 · Cytochromes

J. A. Vorholt (Y) · R. K. Thauer Max-Planck-Institut für terrestrische Mikrobiologie und Laboratorium für Mikrobiologie des Fachbereichs Biologie, Philipps-Universität, Karl-von-Frisch-Strasse, D-35043 Marburg, Germany Tel. +49-6421-178-200; Fax +49-6421-178-209 D. Hafenbradl · K. O. Stetter Universität Regensburg, Lehrstuhl für Mikrobiologie, Universitätsstrasse 31, D-93053 Regensburg, Germany

Abbreviations F420 Coenzyme F420 · MFR Methanofuran · H4MPT 5,6,7,8-Tetrahydromethanopterin · td Doubling time

Introduction Ferroglobus placidus is a strictly anaerobic hyperthermophilic Archaeon that grows chemolithoautotrophically on H2 and nitrate as sole energy source. Nitrite, sulfate, sulfite, and O2 cannot function as terminal electron acceptors, and ferrous iron and sulfide can be used as alternative electron donors. Organic material is not essential for growth. However, casamino acids, yeast extract, acetate, and pyruvate increase cell yields about threefold when added in addition to CO2 (Hafenbradl et al. 1996). F. placidus belongs to the Archaeoglobales (Hafenbradl et al. 1996). Investigated members of this order are Archaeoglobus fulgidus (Achenbach-Richter et al. 1987; Stetter et al. 1987; Stetter 1988), Archaeoglobus profundus (Burggraf et al. 1990), and Archaeoglobus lithotrophicus (St