Diatoms – Adapt or Die!
Sea Ice Diatoms are super cool. In a freezing, low light habitat they are the primary producers – the hothouse flowers – of the Polar Regions. In fact without diatoms a vital food source would be lost for many polar organisms starting with the herbivorous copepod Calanus glacialis who, during the spring melt, relies on the fatty nourishment diatoms provide.
Diatoms, (there are 30,000 – 100,000 extant species world wide) are scavenged from a freezing water column, to become encased in a sea ice crystal matrix, replete with the brine channels they will occupy (a relief of which is depicted below).
The greatest accumulation of these marine alga are in the ecotone porous region at the bottom or edges of sea ice condensing in volume as seen by the brown banded stain in the ice core photograph below.
Within these cramped conditions the photosynthesising autotrophs have developed a range of super cool adaptations to help them mitigate the damage potentially caused by severe abiotic gradients of temperature, light and chemical disparity.
Receiving light and utilizing it quickly is the key to success for polar diatoms. Reduced light is due to the tilt of the Earth away from the sun creating a low angle of light. Furthermore, the polar night can occur over 28 days of darkness, only punctuated by star and moon light. Snowfall can block light. Light attenuation also occurs travelling through the ice. Most significantly the albedo effect can reduce the potential light availability by up to 80% being reflected skyward. However, light radiances, measured down to 0.01% incident radiance, have been utilized by sea ice diatoms for the purpose of photosynthesis. Shockingly, in a low light environment, diatoms need to react fast to high incidences of radiance that might cause photolysis. To counter a sudden burst of sunshine the xanthophyll cycle is implemented with the carotenoid pigment diatoxanthin synthesised (shading) and heat dissipation (energy release) used to avoid cell denaturation.
Transitioning from summer to winter and from (relatively) high temperatures and light to cooler, darker times triggers a change in the production of intracellular carbon storage. Fat, rather than carbohydrates, is preferentially stored in the form of cytoplasmic lipid drops. The reduced temperature and light also trigger a reduction in cellular ATP production, growth and metabolism. It has also been suggested that sea ice diatoms can switch from photo-autotrophy to heterotrophy and utilise extra cellular glucose, 0.3% of their total carbon requirements but considered viable to sustain reduced metabolism over winter.
Ice formation outside or within the diatom frustule is problematic as crystal growth may puncture the silica walls of the algae, killing it. The cold temperatures can reduce metabolic rate. Diatoms use ice binding proteins (IBPs), extracellular polymeric substances (EPS’s) and Polyunsaturated fatty acids (PUFAs) as internal and external defences against the effects of cold temperatures.
ICE BINDING PROTEINS (IBPs),
Fragilariopsis cylindrus a sea ice diatom was shown to have specialist proteins which restricted ice growth suggesting diatoms were habitat sculptors. F. cylindrus proteins added to brine and slowly frozen show a hexagonal ice crystal growth (below). The ability to change crystal shape and direction of growth is likely to confer advantage in the space limited icy habitat.
A drop in temperature means an increase in ice crystal production as fresh water is taken up from the briny solution in the channels. More ice means more volume and pressure as the sea ice restructures. More ice also means an increase in brine salinity and the potential for salt crystals to form suggesting an increase in habitable space.
Polyunsaturated fatty acids (PUFAs)
Diatoms synthesis PUFA’s and other macromolecules. This fat production is why diatoms are so valuable to the nutrition of the polar marine web. Phospholipids – PUFA’s with a fatty acid exchanged for a phosphate group – are used in transport across cell membranes, i.e. for osmoregulation, waste disposal and nutrient flow. F. cylindrusi in trials with more temperate diatom species showed not only greater production of PUFA’s and other molecules under reduced temperature and salt stress but also greater adaptability when removed to more temperate less saline regimes.
EPS’s are polysaccharide molecules with a range of chemical ‘add on’s’ including proteins, acids and sulphates. Diatoms are said to produce the highest quantity of EPS in comparison to the output of other sea ice inhabitants, and do so in response to decreased temperatures and increased salinities. EPS’s have been seen to reduce the freezing temperature in water to below -4 °C at salinities between 34 and 52 psu. EPS’s can act as plugs in brine channels to stem the flow of escaping water and thereby increasing available moisture in a water limited environment.
Osmotic stress caused by high external salinities are countered against externally by the use of IBP’s and EPS’s and within the cell structure by regulation of its compatible solutes which may include DMSP, sugars and polyols. DMSP is used for osmoregulation and is thought to have antifreeze properties.
In conclusion, diatoms have a full compliment of tools to withstand the rigours of a polar season in the ice. What is the pay off for living in icy isolation? It is, in part, the security of a habitat with restrictive brine channels too small for (most) marine predators to penetrate.
Global warming and the reduction, through melting, of habitable sea ice territory does mean that a shift in floral and faunal assemblages is likely with food webs continuing to alter accordingly.