Sea ice: More than just a slab of frozen water?
Sea ice is mainly found in remote polar oceans such as the Southern and Arctic. On average it covers around 20 million square kilometres of the earth’s surface. To put this scale into perspective, sea ice covers at its maximum 13% of the total earth’s surface whereas tropical rain forests cover less than 7%.
Sea ice plays a crucial role in moderating the global climate as its surface reflects incoming sunlight preventing it from being absorbed – this is called the albedo effect (Figure 1). Ice has an albedo of 0.40 to 0.75. This means that sea ice reflects between 40 to 75% of the total sunlight that comes into contact with its surface, dampening the effects of climate change.
Therefore in terms of not only size but global importance, sea ice can take its rightful place as being one the world’s largest and most global influential ecosystems. So sea ice is globally important, but can it be regarded as being more than just a slab of frozen water when examined on a microscopic scale?
How does sea ice form?
Sea ice forms at -1.86oc unlike non-oceanic ice which forms at 0oc. This is because the salt content of sea water reduces the freezing point. This is called the colligative property of water, as any solute present will lower its freezing point in a predictable manner.
The development of sea ice can be broken down into two different development routes; congelation growth and pancake cycle (Figure 2). Each development stage begins with the same step; microscopic ice crystals form around small suspended particles called condensation nuculei, which then float to the surface where they clump into frazil ice (Figure 2).
The image to the left demonstrates that although the end product is sheet ice, it can look visually different on the underside. This difference depends on which developmental route is undertaken, as congelation growth results in smooth-bottomed ice while the pancake cycle results in rough-bottomed sheet ice. The slight difference in visual appearance highlights the microscopic differences between the two forms.
Congelation ice has a columnar ice crystal structure which consists of vertically well ordered ice crystals, due to the calmer conditions in which it forms. Whereas pancake ice has granular ice crystals, which are characterised by a slush of randomly orientated ice crystals, because turbulent wave action prevents the formation of well ordered crystals.
The micro-biology of sea ice:
Microorganisms called Protozoans become trapped in the sea ice under the same mechanisms by which sea ice forms; via turbulent wave action and via the development of ice crystals (Figure 3). As frazil ice begins to form the ice crystals that float to the surface act as a net and scoop up free floating microorganisms. While turbulent wave action acts as a hydraulic pump, pushing the microorganisms into the sea ice.
Along with the microorganisms, sea water also becomes integrated into the ice forming a 3 dimensional structure of interconnected hyper-saline (salinity as high as 173) brine channels. These pools have such a high salt concentration they remain liquid even under freezing conditions. This allows microorganisms to survive because it acts as a buffer, preventing them from freezing. Freezing would ultimately lead to their death, as they are freeze avoidance and not freeze tolerant organisms.
The brine channels are usually less than 200 micrometres in diameter, but can vary due to changes in size and orientations of the ice crystals. This paper states that microscopic differences in the orientation of the ice crystals can lead to differing amounts of brine per unit of volume. This suggests that a microscopic change to ice crystals within sea ice can lead to differing densities of microorganisms, which usually range from 103 to 104 cells per cm3. This is because brine channels have a large surface area, which if added together represents an internal surface area of 0.6 to 4 m2 kg-1. Potentially 6 to 41% of this area at -2°c is occupied by microorganisms.
One of the main environmental stress factors microorganisms living within sea ice have to face is low temperatures. Sea ice temperature ranges from -1 to -15°c, but can go as low as -35°c in winter. The microorganisms that can survive the rigours of those temperature are described as being psychrophiles, as they can survive temperature ranges of -3°c to 20°c.
Psychrophiles that are commonly found in sea ice are bacteria, fungi and diatoms (Figure 3). Living at low temperatues has numerous disadvantages such as reduced enzyme activity, rigid outer cell membranes, slower transport of nutrients and decreased rates of protein production. Psychrophiles combat the effects of low temperatures by increasing their membrane fluidity (how much the membrane can flex) by adding fats such as polyunsaturated fatty acids (PUFAs) to their membranes. They also have cold active enzymes such as amylase, that are specifically adapted for optimal function at low temperatures which reduces the effects low temperature have on reaction rates.
So, is sea ice more than just a slab of water?
Sea ices importance can be seen on a global scale, as it can mediate the complex changes in global temperatures by reflecting the suns radiation – even though sea ice is mainly only found at the southern and northern poles. Although the sea ice development starts off at the same point with frazil ice – by taking a closer look into the microscopic world of sea ice it soon becomes apparent that the complexity and importance doesn’t just end at a global scale. As it’s complex in the way that its ice crystals can have varying orientations, which ultimately lead to varying brine channel diameters that are inhabited by psychrophilic microorganisms. These psychrophilic organisms are also complex in the way that they have over come the decreased membrane fluidity (membrane flexibility) and cellular reactions that occur at low temperatures by the incorporation of fatty compounds into their membranes and the use of cold active enzymes.
So, in the literal sense sea ice is just a slab of frozen sea water, but once you delve a little deeper its true complexity and importance is revealed.