Life in the Frozen Ocean.

Present seasonally or year-round in polar and subpolar seas, sea ice is one of the most complex and biologically rich ecosystems on Earth. Throughout the history of our planet, sea ice has periodically covered vast proportions of the world's oceans, and it may also serve as a plausible habitat on other ocean worlds. In this review, we provide a comprehensive overview of current knowledge on sea ice as a habitat, both on Earth and in extraterrestrial environments. We focus on bacteria, microalgae, and their associated viruses, describing the key physicochemical characteristics that shape this unique ecosystem. Additionally, we explore hypotheses on how microorganisms colonize sea ice, survive by protecting themselves and altering their environment, and ultimately proliferate and evolve. Finally, we consider the potential role of the sea-ice microbiome in the evolution of life on Earth and its possible existence beyond our planet.