We present a psychotropic bacteria–based biosensor that can be used in low–temperature seawater for the
analysis of nitrate + nitrite (NOx
–). The sensor can be used to resolve concentrations below 1 µmol L–1 at low
temperature (<2.5°C) and high salinity (35‰), and in situ use in the deep sea was demonstrated by measuring
NOx
– profiles in sediment at 1500 m depth, where the temperature was 2.5°C. Analysis of marine sediment at
70 m depth showed very heterogeneous NOx
– profiles, most of which exhibited pronounced maxima due to
nitrification in the oxic surface layer. At this shallow water site, the in situ profiles were similar to profiles measured
in the laboratory, but generally laboratory profiles appeared more uniform, presumably as a result of
reduced fauna activity and disturbance of the microstructure of the sediment surface during core recovery.