Deep Sea Research Part I: Oceanographic Research Papers
Instruments and methodsA new deep-sea probe for in situ pH measurement in the environment of hydrothermal vent biological communities
Introduction
Hydrothermal vent organisms are confined to the region where hydrothermal fluid interacts with seawater, providing both reduced and oxygenated compounds necessary to the chemoautotrophic bacterial primary producers (Fisher, 1990; Tunnicliffe, 1991; Childress and Fisher, 1992). In this region, heterogeneous venting and turbulent mixing of hot fluid with cold seawater creates large gradients in the environmental conditions. The distribution of vent organisms (e.g. tubeworms, polychaete annelids, mytilids, etc.) is highly discontinuous and directly related to these gradients (Tunnicliffe, 1991). To investigate the interactions between vent fauna and their environment, efforts have been directed towards the thermal and chemical characterisation of the fluid/seawater mixing zone using discrete sampling, in situ analysis, and temperature recording (Johnson et al (1986b), Johnson et al (1988a), Johnson et al (1988b), Johnson et al (1994) Sarradin et al (1998), Sarradin et al (1999); Sarrazin et al., 1999).
The most outstanding features of this environment are wide variations at small (centimetre to decimetre) spatial scale (Johnson et al (1988a), Johnson et al (1988b)) and fluctuations over time scales ranging from a few seconds to days (Johnson et al., 1988a; Chevaldonné et al., 1991). In situ measurement techniques, which greatly improve spatial and temporal resolution, are thus of particular interest. However, only a few parameters have been determined at depth with automated flow analysers (Si, ΣS, Mn, Fe, NO3−) (Johnson et al., 1986a; Massoth and Milburn, 1997; Le Bris et al., 2000) or probes (T, O2) (Johnson et al., 1986a), and there is still great need of instrumental development for this purpose.
To date, pH has not been measured in situ in the vicinity of vent organisms, although it is a key parameter of this medium. Indeed, hydrothermal fluids, with pH ranging from 2.5 to 4 (Von Damm (1988), Von Damm (1995); Charlou et al., 1996), are highly enriched in H+ as compared to seawater (pH close to 8). pH is thus a significant tracer of the vent fluid dilution and displays wide variations in the mixing zone. For example, pH values ranging from 5.7 to 7.9 (bottom seawater) were measured on samples collected around dominant vent organisms at the Mid-Atlantic Ridge (Sarradin et al., 1998) and East Pacific Rise (Sarradin et al., 1999).
Furthermore, pH is expected to play a significant role in biogeochemical processes. For instance, pH determines the equilibrium balance between the dominant protonated and non-protonated forms of sulphide (H2S, HS−) and carbon dioxide (CO2, HCO3−). This balance was shown to control the uptake of sulphur and carbon by the symbiotic host Riftia pachyptila (Toulmond et al., 1994; Goffredi et al., 1997; Scott et al., 1999). The bioavailability of sulphide and inorganic carbon to the symbionts is thus directly dependent on the pH of the medium. In addition, considering that the oxidation rate of sulphide by oxygen in seawater is four times faster at pH 8 than at pH 4 (Millero et al., 1987), the sulphide and oxygen contents in the mixing gradient could be related to this parameter. The pH may also influence the toxicity of heavy metals, which are highly concentrated in the hydrothermal environment. The reactivity of metals in the fluid/seawater-mixing zone is still poorly known but, as in other aquatic systems, the concentration and speciation of these species are likely to be pH-dependent. Furthermore, as shown for organisms from other marine ecosystems (Ho et al., 1999; Wong and Yang, 1997), the sensitivity of organisms to metals could be influenced by pH.
Oceanographic pH probes (Culberson, 1981) or pH microelectrodes for in situ sediment profiling (Gundersen et al., 1992) were developed for measurement at great depth. They are based on the glass electrode/reference electrode pair, adapted for pressure compensation of internal solutions. On the same principle, we developed a probe, derived from a standard medical pH electrode. This probe was readily adaptable to our operational conditions. Our objective was to obtain an in situ pH data series at the centimetre to decimetre scale over the different habitats of hydrothermal vent fauna. This paper presents the prototype design, along with the settings that are needed for operation from a submersible. This probe was implemented in association with a temperature probe and an in situ analyser. The determination of the pH values from both raw potential data and laboratory calibration is discussed. During the HOPE’99 cruise on the East Pacific Rise (EPR), this device provided the first in situ pH data within the habitat of hydrothermal organisms. The comparison of these results with the ranges determined from discrete samples demonstrates the advantages of in situ pH measurements.
Section snippets
The deep-sea pH probe
The probe is based on a miniature medical pH-combined electrode (o.d. 3 mm) (stomach pH probe, Medical Instruments Corporation™, Switzerland). Bodies of the glass electrode and Ag/AgCl/KCl reference electrode are made of 50 cm long flexible tubes. The glass electrode tube is inserted in the reference electrode tube (Fig. 1). As this allows the internal solutions to equilibrate with the external pressure, the electrode sustains high pressure without further modification. The pH-meter and
Influence of temperature and pressure on the probe response
The relationship between the potential measured by the combined-electrode (Ex) (i.e. the electromotive force of the glass electrode–reference electrode couple), and the pH of the medium in which it is dipped (pHx) at temperature Tx, is defined as a linear function (Bates, 1973).where S(Tx) is the slope of the electrode's response at Tx. is the standard potential of the electrode. These intercept and slope are characteristic of the probe and are expected to depend
Conclusion
In conclusion these first results obtained with a prototype pH probe underline the potentiality of this low-cost electrode for the study of the environmental pattern at the level of hydrothermal vent communities.
In comparison to discrete sampling, the probe provides larger data sets with better spatial resolution, leading to more representative values for the extreme and mean pH of the habitat. As an example, when compared to discrete sampling, in situ data depicted a significantly more acidic
Acknowledgements
This work was achieved with the financial support of IFREMER, URM 7 and the Dorsales programme. We greatly acknowledge F. Lallier, chief scientist of the HOPE diving cruise, for allowing this work to be performed. We particularly thank Ph. Rodier for technical assistance in the deployment of the pH probe. We are grateful to the N/O Atalante captain and crew and the Nautile team for their helpful collaboration. Thank to F. Gaill, D. Desbruyères and B. Shillito for their revisions of the
References (28)
- et al.
Time-series of temperature from three deep-sea hydrothermal vent sites
Deep-Sea Research I
(1991) - et al.
A submersible flow analysis system
Analytica Chimica Acta
(1986) - et al.
Short time temperature variability in the Rose Garden hydrothermal vent fieldan unstable deep sea environment
Deep-Sea Research I
(1988) - et al.
Chemical and biological interactions in the Rose Garden hydrothermal vent field, Galapagos spreading center
Deep-Sea Research I
(1988) - et al.
Biogeochemistry of hydrothermal vent mussel communitiesthe deep sea analogue to the intertidal zone
Deep Sea Research I
(1994) - et al.
A new chemical analyzer for in situ measurement of nitrate and total sulfide over hydrothermal vent biological communities
Marine Chemistry
(2000) The thermodynamics of the carbonate system in seawater
Geochimica et Cosmochimica Acta
(1979)- et al.
Unusual carbon dioxide combining properties of body fluids in the hydrothermal vent tubeworm Riftia pachyptila
Deep-Sea Research I
(1994) - Bates, G.R., 1973. Determination of pH. Theory and Practice. Wiley-Interscience, New York, p....
- et al.
An improved in situ pH sensor for oceanographic and limnological applications
Limnology and Oceanography
(1974)
Mineral and gas chemistry of hydrothermal fluids on an ultrafast spreading ridgeEast Pacific Rise, 17° to 19°S (Naudur cruise, 1993) Phase separation processes controlled by volcanic and tectonic activity
Journal of Geophysical Research
The biology of hydrothermal vent animalsphysiology, biochemistry and autotrophic symbioses
Direct potentiometry
Chemoautotrophic and methanotrophic symbioses in marine invertebrates
Aquatic Sciences
Cited by (61)
Fabrication, improved performance, and response mechanism of binary Ag–Sb alloy pH electrodes
2020, Electrochimica ActaIn situ UV–VIS–NIR spectrophotometric detection system as a research tool for environment-friendly chemical processes
2019, Environmental Technology and InnovationBathymodiolus growth dynamics in relation to environmental fluctuations in vent habitats
2015, Deep-Sea Research Part I: Oceanographic Research PapersCharacterisation and deployment of an immobilised pH sensor spot towards surface ocean pH measurements
2015, Analytica Chimica Acta