Abstract : To explain anaerobic nitrite/nitrate production at the expense of ammonium mediated by manganese oxide (Mn(IV)) in sediment, nitrate and manganese respirations were investigated in a strain (Shewanella algae C6G3) presenting these features. In contrast to S. oneidensis MR-1, a biotic transitory nitrite accumulation at the expense of ammonium was observed in S. algae during anaerobic growth with Mn(IV) under condition of limiting electron acceptor, concomitantly, with a higher electron donor stoichiometry than expected. This low and reproducible transitory accumulation is the result of production and consumption since the strain is able to dissimilative reduce nitrate into ammonium. Nitrite production in Mn(IV) condition is strengthened by comparative expression of the nitrate/nitrite reductase genes (napA, nrfA, nrfA-2), and rates of the nitrate/nitrite reductase activities under Mn(IV), nitrate or fumarate conditions. Compared with S. oneidensis MR-1, S. algae contains additional genes that encode nitrate and nitrite reductases (napA-α and nrfA-2) and an Outer Membrane Cytochrome (OMC)(mtrH). Different patterns of expression of the OMC genes (omcA, mtrF, mtrH and mtrC) were observed depending on the electron acceptor and growth phase. Only gene mtrF-2 (SO1659 homolog) was specifically expressed under the Mn(IV) condition. Nitrate and Mn(IV) respirations seem connected at the physiological and transcriptional levels.