The Black Sea as an Extreme Habitat of Earth's Ocean: Biogeochemical Functioning, Unique Ecosystems, and Astrobiological Relevance

The Black Sea, characterized by its unique oceanographic and biogeochemical gradients and oxygen-depleted (anoxic) waters, serves as a natural analogue of past planetary-scale geological events as well as more recent human-induced changes. In this review, based on a synthesis of the most recent research, we demonstrate how the extreme ecosystems of the Black Sea provide valuable insights into ecological resilience and adaptation in a changing global ocean. We also elaborate on how the Black Sea's biogeochemical oceanographic extremities parallel conditions found in some of the most extreme environments of our planet as well as newly discovered oceans of the Solar System, offering a crucial analogue for astrobiological and extreme environment research. As such, the Black Sea holds significant relevance not only for understanding Earth's past oceans and present ecological dynamics but also for advancing the exploration of life's potential beyond our planet. ▪ The Black Sea is shaped by oceanographic gradients with Earth's largest volume of oxygen-depleted and sulfide-rich waters. ▪ The habitats of the Black Sea host not only unique bacterial and archaeal lineages but also eukaryotic organisms adapted to extreme conditions. ▪ It is also an excellent laboratory environment to study the geological past of Earth's ocean and for future studies of the Solar System's ocean worlds. ▪ Managing the Black Sea's environmental challenges should be a priority as well to keep this unique natural laboratory stable and accessible for future generations.