Mineral–microbe interactions in deep-sea hydrothermal systems: a challenge for Raman spectroscopy

In deep-sea hydrothermal environments, steep chemical and thermal gradients, rapid and turbulent mixing and biologic processes produce a multitude of diverse mineral phases and foster the growth of a variety of chemosynthetic micro-organisms. Many of these microbial species are associated with specific mineral phases, and the interaction of mineral and microbial processes are of only recently recognized importance in several areas of hydrothermal research. Many submarine hydrothermal mineral phases form during kinetically limited reactions and are either metastable or are only thermodynamically stable under in situ conditions. Laser Raman spectroscopy is well suited to mineral speciation measurements in the deep sea in many ways, and sea-going Raman systems have been built and used to make a variety of in situ measurements. However, the full potential of this technique for hydrothermal science has yet to be realized. In this focused review, we summarize both the need for in situ mineral speciation measurements in hydrothermal research and the development of sea-going Raman systems to date; we describe the rationale for further development of a small, low-cost sea-going Raman system optimized for mineral identification that incorporates a fluorescence-minimizing design; and we present three experimental applications that such a tool would enable.