Life On Ice: Plankton Living In Pack Ice
The polar regions of our planet are two of the most extreme places on earth but they are also inhabited by many species of organisms which the public hold dear like penguins, polar bears and whales. Some of the organisms which are the most overlooked here are also some of the smallest and most important. These organisms are the plankton found in the seas around these two masses of ice but also within it.
Plankton specifically phytoplankton living in the arctic and Antarctic circle like their cousins from warmer climes are very important to the marine ecosystem. Being the main primary producers are the route all energy from the sun takes as it makes its way up the food chain. But the plankton here are special in another way as well, the plankton living here poses the ability to live in the pack ice when it forms overcoming the many challenging factors it creates.
A TED Ed video giving a background on plankton
How did they get there?
Pack ice is the ice that forms when sea water is frozen, the volume of pack ice present is ephemeral changing with the seasonal temperatures throughout the year with it trapping plankton when it forms.
Pack ice formation
Frazil ice forms on the sea surface and look like slicks this happens as the sea is well mixed by the high winds present in the arctic as ice crystals form in the upper mixed layer of the water column they float to the surface and gather together forming larger slicks of crystals. It’s is this stage when the ice becomes impregnated with the plankton as well as with bacteria. This happens as the ice crystals rise to the surface almost rising as a curtain through the water trapping any plankton above then as they rise leaving virtually none of the plankton behind.
Main Challenges They Face
The main challenges plankton face when living in the pack ice are those created by abiotic factors such as Low temperatures, High salinities and Low light intensity.
Low temperatures are harmful to the plankton as it reduces the speed at which cell processes occur, it can cause the water in the plankton to freeze forming ice crystals which damages cells ultimately killing the individual.
The low temperatures also bring much higher salinities within the brine in the ice plankton may experience salinities 3 times that of seawater . This high salinity presents a challenge as it interferes with the osmoregulation within the plankton as the high external salinity interferes with the maintenance of diffusion gradients.
Low light conditions occur in the ice due to to being blocked by the ice itself and any snow cover that may have fallen on the ices surface. When this occurs up to 99.9% of the sun light at the surface can be blocked out . This is an issue for phytoplankton which need light to carry out photosynthesis.
Also as the temperature decrease the space within the brine channels decreases making it harder for the planktotrophic plankton to move to capture and consume other trapped plankton in the ice.
Although the plankton face many challenges staying alive within the sea ice they possess a number of adaptations to cope with what they face. This section covers the adaptations against some of the main abiotic factors in the pack ice.
In order to survive the low temperatures, the many plankton are able to form tough, stress resistant cysts in response to entering the ice sensing this change by falling temperatures or limited inorganic nutrients . One of the most important factors is maintaining a fluid functional lipids membrane. In
order to do this many plankton, increase the proportion of polyunsaturated fatty acids (PUFAs) that have shorter chain lengths [3,4]. It has been found that the enzyme family polyketide synthases is present in bacteria found in the ice this enzyme family is required for the production of PUFAs .
In order to allow cell process to continue at the low temperatures within the ice the plankton possess enzyme which have high catalytic activity at low temperature. Antifreeze proteins such as Dimethylsulfoniopropionate (DSMP) which prevents the formation of ice crystals.
The high salinity encountered in the brine interferes with the plankton osmoregulation and can dehydrate the organism. In order to combat this plankton possess may salt tolerant enzymes  and regulate lipid (fat) packing when salinity is high .
Light is important to phytoplankton in order conduct photosynthesis, thus sea ice phytoplankton have many physiological adaptations to grow and live in low light conditions. Accessory pigments which are very good at absorbing parts of light which are capable of penetrating the sea ice are found in high concentrations . There has even been some evidence that some phytoplankton have been able to switch to feed on organic matter when levels of light are low .
Moving from open water to the confined spaces of brine channels presents many challenges to staying alive for plankton. But with a host of evolved adaptations it is possible for these tiny organisms to stay alive in one of the most extreme environments on the planet.