Hydrobiologia 389: 169–182, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 169 Simultaneous occurrence of denitrification and nitrate ammonification in sediments of the French Mediterranean Coast Patricia Bonin 1∗ , Patrick Omnes 1 & Alain Chalamet 2 1 Laboratoire d’Oc´ eanologie et de biog´ eochimie CNRS-UMR6535, Centre d’Oc´ eanologie de Marseille, OSU, Campus de Luminy, 13288 Marseille, Cedex 9, France 2 Laboratoire d Ecologie Microbienne des Sols, UMR CNRS 5557, Lyon I, 43, Bd du 11 Novembre 1918, 69622 Villeurbanne, Cedex ∗ Author for correspondence: e-mail: pbonin@com.univ-mrs.fr Received 6 February 1998; in revised form 3 November 1998; accepted 25 November 1998 Key words: nitrate ammonification, denitrification, 15 N, acetylene, marine sediment Abstract Dissimilatory nitrate reductions in coastal marine sediment of Carteau Cove (French Mediterranean Coast) were studied between April 1993 and July 1994. Simultaneous determination of denitrification and dissimilatory nitrate reduction to ammonium was achieved by using a combination of acetylene blockage and 15 N techniques. After short incubations (maximum 5 h), a part of 15 N labelled nitrate added to the sediment was recovered as ammonium without incorporation in organic matter. The result indicate that a fraction of nitrate was reduced to ammonium by a dissimilatory mechanism instead of denitrifying. Denitrifying and nitrate ammonifying activities ranged from 0 to 19.8 μmol l −1 d −1 and from 2.3 to 83.2 μmol l −1 d −1 , respectively. Denitrification rates were highest in early spring whereas nitrate ammonification were highest in fall. The recovery of nitrate reduced as N 2 O–N plus ammonium was between 40 and 100%, the highest nitrogen losses were recorded in July. Depending on the station and time of year denitrification accounted for between 0 and 43% of the total nitrate reduction whereas dissimilatory nitrate reduction to ammonium (DNRA) accounted for between 18 and 100%. The reduction rate data suggest that the pathway of nitrate reduction to ammonium may be important in coastal sediments. Introduction Nutrients (N and P) are important in controlling pri- mary production in aquatic ecosystems. Nitrogen is the limiting nutrient in many estuaries and coastal marine systems (Ryther & Dunstan, 1971). However, microbial metabolisms play a key role in the regu- lation of dissolved inorganic nitrogen concentrations (Seitzinger, 1990). Denitrification, the bacterial dis- similatory reduction of nitrate to gaseous products (Payne, 1973), reduces the impact of the increased nitrate supply and thus may mitigate eutrophication of these environments (Knowles, 1982; Seitzinger, 1988). In addition to nitrate reduction by denitrifiers, bacteria in sediments may compete for nitrate in a second pathway leading to ammonium, termed dis- similatory nitrate reduction to ammonium (DNRA) or nitrate ammonification (Koike & Hattori, 1978; Sørensen, 1978; Buresh & Patrick, 1981; Kaspar, 1983; Enoksson & Samuelsson, 1987; Goyens et al., 1987). Denitrification and nitrate ammonification oc- cur under anaerobic conditions and the partitioning of nitrate between these processes appears to be a function of carbon to electron acceptor (nitrate) ratio (Tiedje et al., 1982). The simultaneous occurrence of both of these path- ways is of interest considering the nitrogen budget. Indeed reduction of nitrate to dinitrogen, resulting from denitrification, leads to the production of gaseous products rapidly lost from the ecosystem, whereas the alternative pathway (DNRA) conserves nitrogen in a readily useful form and thus may cause increasing nu- trient enrichment (Koike & Hattori, 1978; Sørensen, 1978). Thus, depending on the relative importance of