The extent, duration, and seasonality of sea ice and glacial discharge strongly influence Antarctic marine ecosystems. Most organisms' life cycles in this region are attuned to ice seasonality. The annual retreat and melting of sea ice in the austral spring stratifies the upper ocean, triggering large phytoplankton blooms. The magnitude of the blooms is proportional to the winter extent of ice cover, which can act as a barrier to wind mixing. Antarctic krill, one of the most abundant metazoan populations on Earth, consume phytoplankton blooms dominated by large diatoms. Krill, in turn, support a large biomass of predators, including penguins, seals, and whales. Human activity has altered even these remote ecosystems. The western Antarctic Peninsula region has warmed by 7°C over the past 50 years, and sea ice duration has declined by almost 100 days since 1978, causing a decrease in phytoplankton productivity in the northern peninsula region. Besides climate change, Antarctic marine systems have been greatly altered by harvesting of the great whales and now krill. It is unclear to what extent the ecosystems we observe today differ from the pristine state

Sponges are important components of marine benthic communities of Antarctica. Numbers of species are high, within the lower range for tropical latitudes, similar to those in the Arctic, and comparable or higher than those of temperate marine environments. Many have circumpolar distributions and in some habitats hexactinellids dominate benthic biomass. Antarctic sponge assemblages contribute considerable structural heterogeneity for colonizing epibionts. They also represent a significant source of nutrients to prospective predators, including a suite of spongivorous sea stars whose selective foraging behaviors have important ramifications upon community structure. The highly seasonal plankton blooms that typify the Antarctic continental shelf are paradoxical when considering the planktivorous diets of sponges. Throughout much of the year Antarctic sponges must either exploit alternate sources of nutrition such as dissolved organic carbon or be physiologically adapted to withstand resource constraints. In contrast to predictions that global patterns of predation should select for an inverse correlation between latitude and chemical defenses in marine sponges, such defenses are not uncommon in Antarctic sponges. Some species sequester their defensive metabolites in the outermost layers where they are optimally effective against sea star predation. Secondary metabolites have also been shown to short-circuit molting in sponge-feeding amphipods and prevent fouling by diatoms. Coloration in Antarctic sponges may be the result of relict pigments originally selected for aposematism or UV screens yet conserved because of their defensive properties. This hypothesis is supported by the bioactive properties of pigments examined to date in a suite of common Antarctic sponges.

Motility and chemotaxis allow cells to move away from stressful microenvironments. Motility of Escherichia coli in batch cultures, as measured by cell swimming speed, was low in early-exponential-phase cells, peaked as the cells entered post-exponential phase, and declined into early stationary phase. Transcription from the flhB operon and synthesis of flagellin protein similarly peaked in late exponential and early post-exponential phases, respectively. The increase in swimming speed between early-exponential and post-exponential phases was correlated with twofold increases in both flagellar length and flagellar density per cell volume. This increased investment in flagella probably reflects the increased adaptive value of motility in less favorable environments. The decrease in speed between post-exponential and stationary phases was correlated with a threefold decrease in torque produced by the flagellar motors and presumably reflects decreased proton motive force available to stationary-phase cells.