Biomineralization processes in low-temperature, shallow-water hydrothermal vent at Tagoro submarine volcano, El Hierro Island (Central East Atlantic)

A novel hydrothermal system was discovered at the summit of the underwater Tagoro volcano at 89–120 m depth after the 2011–2012 eruption, characterized by the low-temperature venting of Fe-rich fluids that produced a seafloor draped by extensive Fe-flocculate deposits. The basanite-hornitos are capped by mm- to cm-thick hydrothermally derived Fe-oxyhydroxide sediment and contain micro-cracks and degasification vesicles filled by sulfides (mostly pyrite) and covered by sulfur-oxidizing bacterial mats. Electron microprobe studies on Fe-oxyhydroxide crusts show the presence of various organomineral structures, mainly twisted stalks and sheaths covered by iron-silica deposits, reflecting microbial iron-oxidation from the hydrothermal fluids. Sequencing of 16S rRNA genes also reveals the presence of other microorganisms involved in sulfur and methane cycles. Samples collected from hornito chimneys contain silicified microorganisms coated by Fe-rich precipitates. The rapid silicification may have been indirectly promoted by microorganisms acting as nucleation sites. We suggest that this type of hydrothermal deposits might be more frequent than presently reported to occur in submarine volcanoes. The discovery of this mineralization system and associated microbiota identifies a potential Fe-based chemosynthetic ecosystem, which typically have been studied at spreading centers and arc volcanoes. This underscores the importance of geomicrobiological interactions in shaping mineral deposits on Earth today, and in the geological past. This hydrothermal system provides an excellent laboratory to study the formation and evolution of newly formed hydrothermal deposits and their association with microbiota at an intraplate hot-spot volcanic edifice under low-temperature, shallow-water conditions.