Polar marine biology
This week the course focused on marine life. How diverse, abundant and adaptable it is. However, there is one region where life has to deal with extra challenges. A changing light system, from 24 hours of sunlight to complete darkness for months, harsh sub-zero conditions and changes to the composition of seawater from the freezing and thawing of ice. This is of course the Polar Regions (Arctic in the north and the Antarctic in the south). Many people may look at ice and think it couldn’t harbour life, they would be wrong. Many microbes are adapted to the cold, and are given the term psychrophiles, the average density of cells within sea ice ranges from 104 – 107 (10,000 – 10,000,000) per ml! Along with this we also find many types of plankton in the Polar Regions.
If we begin in spring, before the sea-ice has melted there is already plenty of growth going on. Phytoplankton (microscopic marine plants) begin to grow as soon as enough light penetrates through the ice. Some of these algae form substances which cause them to stick together and producing long “curtains”. These algae are already providing food for zooplankton, producing a complete ecosystem even before the ice has gone! You can see a movie of the algae here.
Once we move into summer it isn’t hard to imagine life flourishing here. Constant sunlight provides plenty of energy for photosynthesis, is always available and nutrients (at the beginning of the season) are plentiful. It is these conditions that can lead to some of the largest blooms of phytoplankton being seen within the Arctic Circle, as well as some very productive fisheries (e.g. the Barents Sea).
However, as the season begins to shift there are a variety of conditions to deal with. The freezing of water into ice, the cold temperatures and the changing light. Cells have a high composition of water within them, which when it freezes it will expand. This is not good for a single celled organism, as if the cell membrane is pierced by the ice they will die. Many cells have adapted to this by having mechanisms to protect the fluidity of the membrane (stop it from freezing) and producing substances known as extracellular polymeric substances which prevents ice from forming.
A beautiful feature known as frost flowers can arise during ice formation, these are crystallised moisture from the surface of new sea ice which has rapidly formed leading to a steep temperature gradient. Frost flowers have been found to contain up to 106 bacterial cells per ml. These bacteria have also been found to have intriguing abilities, such as the reduction of mercury and uptake of sulphurous compounds.
Finally, we move into the period of darkness known as the polar night. It was long thought
that life almost shut down during this time, zooplankton and phytoplankton produced spores or resting stages which would wait for more favourable conditions. Like a bear going into hibernation. However, recently it has been shown that not all is quiet during the polar night.
Phytoplankton switch their life style from autotrophy (produce their own energy from sunlight) to mixotrophy (get energy by ingesting bacteria) and zooplankton still actively move within the water column, rather than resting. In fact it was recently found that the light from a bright full moon can cause a response in zooplankton. Investigations into the stomachs of sea-birds and fish during the polar night have found a high proportion are
full, suggesting they are still actively foraging. What was thought of as a time of rest has been shown to be anything but! It can be difficult to get into Polar Regions during this time to carry out research, but a handful of projects are actively looking at this with some exciting results!
Eventually the sun will begin to return, and the cycle of the Arctic will begin again. The Arctic is warming 3 times faster than the global average and this is impacting the timing and magnitude of sea-ice melt as well as increasing water temperatures. To see the current and projects changes in Arctic sea ice, see this video. There are a number of other impacts, such as the declining size of zooplankton and changing phytoplankton species which may impact food webs within the region.
Zooplankton images: arcodiv.org
Ice algae: https://www.mpg.de/6949942/Arctic-ecosystem-climate
Phytoplankton image: http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=51765
Frost flowers: http://www.npr.org/sections/krulwich/2012/12/17/167469845/suddenly-theres-a-meadow-in-the-ocean-with-flowers-everywhere
Berge J., et al., 2015. In the dark: A review of ecosystem processes during the Arctic polar night. Prog. Ocean. 139.