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) between November and June 1993. The 2 dissimilatory pathways of nitrate reduction, den~trification and nitrate ammonificat~on, were measured using a combination of the acetylene inhibition and 15N techniques. 11-respective of the sampling site, denitriflcation and nitrate dmmonification rates, measured wlthout nitrate and glucose amendment, ranged from 1.0 to 4.3 pm01 1. ' d-' and 0.7 to 2.5 pm01 1-' d-l, respectively. The percentage of nitrate reduced to ammomum vaned bet\\,een. 12 and 33% of the total nitrate dissimilated. Both processes occurred simultaneously and competed for nitrate as an electron acceptor The reduct~on of nitrate to ammonium was inhibited by oxygen, indicating that the mechanism of reduction was dissimilatory. The dissimilatory character of nitrate ammonificat~on was unequivocally confirmed in experiments uslng a bacterial community isolated from a natural water sample.