Microbes entrained into new Arctic sea ice experience an extraordinary range of environmental conditions (wide-ranging temperatures, salinities, physical constraints, and inorganic and organic chemistries) as the ice grows through the dark cold winter, receives spring sunlight to drive photosynthesis and begins its summer demise during the melting period. Only the smallest of microbes, the bacteria (particularly the bacterial heterotrophs dependent on organic matter), inhabit all portions of the ocean's sea-ice cover, from its sea-water-flushed interface to its most environmentally extreme surface expressions of frost flowers. These delicate ice-crystal structures, subject to wind-borne dispersal (or collapse under snow), contain higher bacterial densities than the sea ice on which they grow, regardless of how cold and briny they may be in winter, raising both atmospheric and astrobiological implications. The Archaea, equally small microbes but famous for inhabiting extreme environments, are so far known only from cold winter sea ice; their functions in the ice remain mysterious. As communities within the ice, these diverse bacteria and Archaea undergo seasonal succession, thought to be limited in winter by the generic protective functions of extracellular polysaccharides (EPS) that fill the ice-pore spaces and accelerated in later seasons by warming temperatures (enabling EPS breakdown) and newly released products of photosynthesizing ice algae. Contributing to the population dynamics (via lethal attacks) and ice adaptability (via horizontal gene transfer) of these microbes are their viruses, present in higher densities in sea-ice brines than in any other marine environment. Selected field, experimental and model results will be used to demonstrate how these remarkable microbes and their extracellular products may be influencing ecosystems and key elemental cycles, sinks and sources in the sea ice and beyond, following their dispersal into air and sea.