EVIDENCE OF DENITRIFICATION AND NITRATE AMMONIFICATION IN THE RIVER-RHONE PLUME (NORTHWESTERN MEDITERRANEAN-SEA)

Water samples were collected from the turbid plume water of the River Rhone at 2 stations (near the river mouth and at the plume front) betw een November and June 1993. The 2 dissimilatory pathways of nitrate re duction, denitrification and nitrate ammonification, were measured usi ng a combination of the acetylene inhibition and N-15 techniques. Irre spective of the sampling site, denitrification and nitrate ammonificat ion rates, measured without nitrate and glucose amendment, ranged from 1.0 to 4.3 mu mol l(-1) d(-1) and 0.7 to 2.5 mu mol l(-1) d(-1), resp ectively. The percentage of nitrate reduced to ammonium varied between 12 and 33% of the total nitrate dissimilated. Both processes occurred simultaneously and competed for nitrate as an electron acceptor. The reduction of nitrate to ammonium was inhibited by oxygen, indicating t hat the mechanism of reduction was dissimilatory. The dissimilatory ch aracter of nitrate ammonification was unequivocally confirmed in exper iments using a bacterial community isolated from a natural water sampl e.