A new chemical analyzer for in situ measurement of nitrate and total sulfide over hydrothermal vent biological communities

A new submersible chemical analyzer, ALCHIMIST (AnaLyseur CHIMique In SiTu), based on colorimetric detection and flow injection analysis (FIA), was adapted to allow in situ measurements of nitrate+nitrite (N+N) and total dissolved sulfide (∑S) in the deep sea hydrothermal environment. Before in situ trials, the influence of hydrostatic pressure and temperature on the analytical responses was examined under simulated conditions (1–300×105 Pa, 5–25°C). First trials were performed during dives of the Remotely Operated Vehicle (ROV), VICTOR 6000, over the Lucky Strike hydrothermal vent field (Mid-Atlantic Ridge). ALCHIMIST, installed on the ROV, enabled simultaneous N+N and ∑S calibration and measurements at 1650 m depth, at a rate of 22 analyses per hour. At depth, the precision of the in situ ∑S and N+N measurements is estimated to be, respectively, 1.1% and 0.8%. The detection limit is 0.8 μM for ∑S and 0.5 μM for N+N. At a vent site of the Lucky Strike area, ALCHIMIST enabled to resolve, at the decimeter scale, the chemical gradients which characterize the patchy distribution of hydrothermal fauna. Additionally, temperature–concentration relationships offered further information on the processes controlling the chemistry of the habitats. Like former in situ analyzers used in this field, this new instrument should be valuable to characterize the physico-chemical characteristics of vent fauna environment.