Denitrification and the Denitrifier Community in Coastal 1 Microbial

18 Denitrification was measured in three structurally different coastal microbial mats by 19 using the stable isotope technique. The composition of the denitrifying community was 20 determined by analyzing the nitrite reductase (nirS and nirK) genes using clone libraries 21 and the GeoChip. The highest potential rate of denitrification (7.0±1.0 mmol N m d) 22 was observed during summer at station 1 (supra-littoral). The rates of denitrification were 23 much lower in the stations 2 (marine) and 3 (intermediate) (respectively 0.1 ±0.05 and 24 0.7±0.2 mmol N m d) and showed less seasonality when compared to station 1. The 25 denitrifying community at station 1 was also more diverse than that at station 2 and 3, 26 which were more similar to each other than either of these stations to station 1. In all 27 three stations, the diversity of both nirSand nirK-denitrifiers was higher in summer 28 when compared to winter. The location along the tidal gradient seems to determine the 29 composition, diversity and activity of the denitrifier community, which may be driven by 30 salinity, nitrate/nitrite and organic carbon. Both nirS and nirK denitrifiers are equally 31 present and therefore they are likely to play a role in the denitrification of the microbial 32 mats studied. 33 34 35