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The rifting zone of Bransfield Strait, Antarctica, is tectonically and geologically unique. It is a back-arc basin that was opened by extensional forces associated to roll-back subduction after cessation of spreading activity of the Phoenix Ridge, and the transtension of the westward ending of Scotia-Antarctica Plate boundary. The Bransfield Rift/ Ridge is still active generating volcanism or magma rise to force hydrothermal activity. During the ANT-XXV/4 cruise onboard R/V "Polarstern", standard CTD and beam transmission measurements were done to determine temperature anomaly and turbidity. Water sampling was performed to determine δ 3 He and to find thermophilic microorganisms to examine the Orca seamount hydrothermal activity. A temperature anomaly of ~0.08 °C, a pick of turbidity, and high value of δ 3 He (>10%) were found inside Orca seamount. Results are consistent with a hydrothermal flux coming from the seamount. The report of the first observation of thermophilic and hyperthermophilic microorganisms in cold deep Antarctic waters is part of this study. Inside Orca seamount these microorganisms were found at three different depth levels close to the bottom. We suggest that the fluid migration from the volcano resulted from recent magmatic activity and provided the required elemental nutrients for microbial growth. Besides some thermophiles were found outside the seamount in a small quantity close to the seafloor. These would probably be related to subsidiary structures of the Orca seamount, or were transported by currents from other active volcanic sites as Deception Island. The finding of these thermophilic and hyperthermophilic microorganisms raise questions about the dispersal and their resistance in these extreme environments.; ¿Existe un flujo hidrotermal activo desde el monte submarino Orca en el estrecho Bransfield, Antártica?. La zona de rifting del estrecho Bransfield, Antártica, es tectónica y geológicamente única. Es una cuenca de trasarco originada por fuerzas de extensión asociadas a roll-back después del cese de la subducción y expansión de la dorsal Fénix, y la transtensión relacionada con la actividad del límite oeste de la Placa Scotia-Antártica. El rift/ dorsal del estrecho de Bransfield, actualmente activo, provoca volcanismo o ascenso de magma al que se asocia actividad hidrotermal. Durante el crucero ANT-XXV/4 del R/V "Polarstern", se realizaron mediciones estándar de CTD y transmisión de luz (haz) para determinar anomalías de temperatura y turbidez, y también muestreo de agua para establecer su valor de δ 3 He y presencia de microorganismos termofílicos y así evaluar la potencial actividad hidrotermal en el monte submarino Orca. Una anomalía de temperatura de ~0.08 °C, un pico de turbidez y un alto valor de δ 3 He (>10%) se encontraron dentro del cráter de Orca. Los resultados son consistentes con un flujo hidrotermal proveniente del monte submarino. También se encontraron, por primera vez, microorganismos termófilos e hipertermófilos en aguas centers and intra-plate volcanoes (e.g., German et al., 2000). Every mentioned site has its own associated tectonics, geodynamics and geological characteristics that produce distinctive geophysical and geochemical signatures, which are reflected in the variability in composition, size or volume, spatial distribution, frequency and transience of the deep-sea hydrothermal vents (Lupton et al., 1998; Baker et al., 2001; Hey et al., 2004; Hannington et al., 2005). The rifting zone of the Bransfield Strait, located between the South Shetland Islands and the Antarctic Peninsula (Fig. 1) is tectonically and geologically unique (Lawyer et al., 1996). Subduction and extension processes occur simultaneously and there is currently no clarity in their geological evolution frías profundas de la Antártica. Dentro del cráter del monte submarino Orca, estos microorganismos se localizaron en tres niveles de profundidad diferentes cerca del fondo. Nosotros sugerimos que la circulación de fluidos hidrotermales, producto de la actividad magmática reciente, proporcionaron los nutrientes y el ambiente propicio para el crecimiento de estos microorganismos. Además, se encontraron fuera del volcán, cerca del lecho marino, microorganismos termófilos en una pequeña cantidad. Estos estarían relacionados, probablemente, con estructuras subsidiarias del monte submarino Orca o fueron transportadas por corrientes desde otros centros volcánicos activos como la isla Decepción. El descubrimiento de estos microorganismos termófilos e hipertermófilos genera nuevas preguntas sobre su dispersión y resistencia en estos ambientes extremos.