Enduring Antarctic nights – How an isolated ocean relies upon phytoplankton physiology.
The Antarctic ocean hosts a unique range of species, all physiologically adapted to be comfortable in challenging polar conditions. Perhaps the most crucial adaptations in this ecosystem belong to the organisms supporting most food webs, phytoplankton.
A polar night is the extended period of darkness which occurs when the axis of the earth tilts one of the poles, and surrounding area, outside of the path of the sun’s light. Occurring during the winter at the poles, these nights may extend up to 179 days of uninterrupted darkness. In the absence of sunlight, there is no photosynthesis occurring in these months, limiting food resources and the growth of organisms in these waters.
Antarctica is surrounded by the Antarctic Circumpolar Current (ACC), originating back to the formation of the continent. This current is strong enough to prevent the transport of plankton and small invertebrates in or out of the Antarctic Ocean, creating a barrier effect. The combination of polar nights and the ACC resulted in the historical view in which the Antarctic was innately unproductive, with little primary production, and high mortality in phytoplankton.
However, with satellite chlorophyll mapping, and a more detailed understanding of the biology of the continent and its seas, Antarctica has been proven to be highly productive; with a diverse and unique cast of biota ranging from invertebrate curiosities, such as the many endemic species of sea spider, to a broad range of top predators, including much beloved marine mammals such as Orca.
The base of the chain.
Phytoplankton are unicellular algae, generally suspended in the surface waters of the ocean; regarded as the foundation of a majority of marine food webs (see video 1). Much like any other shelf sea, coastal, or surface water marine habitat, the food webs of the Antarctic Ocean are supported by the primary production of phytoplankton.
The video above (video 1) details the importance of phytoplankton in supporting Antarctic food webs.
The Antarctic Ocean is highly productive, being responsible for 40% of atmospheric carbon sequestration of the world’s oceans, despite spending an average of four months in constant darkness per year. High carbon uptake is indicative of large biomasses of phytoplankton, which utilise CO2 for primary production (photosynthesis). This productivity is reflected by large biomasses of grazers, such as popularly marketable Antarctic krill, and consequentially, a high diversity of predators (see video 2), which makes this ocean home to several fisheries.
The video above (video 2) details the complex and commercially important food webs of Antarctica.
This raises the question; if phytoplankton are not being imported from outside the Antarctic, yet there is such a significant extent of primary productivity in these waters, how do the phytoplankton survive the polar night?
Physiological adaptations – Life beyond the light.
The mechanisms used by phytoplankton to survive polar nights are not yet fully understood, with ongoing research efforts targeting the physiological coping mechanisms of marine primary producers to survive extended periods of darkness. One such major research effort is the ANR Phytopol project.
During early winter, most plankton becomes trapped in forming ice. Fresh ice crystals rise to the surface of the water column, securing phytoplankton such as diatoms and dinoflagellates within a crystal matrix (Image 1), a labyrinth like structure of solid ice and channels and pores filled with brine. This, in conjunction with the shortening days, are thought to be an early trigger for the physiological processes, which allow the phytoplankton to survive the upcoming darkness.
Dinoflagellates can endure long lengths of time spent in a dormant state; these microscopic, unicellular organisms, averaging 30 microns in diameter, have been successfully revived from dormant states of over a century. This state of near-suspended animation is achieved by forming cysts, or spores, a seed-like resting stage, which reanimates in a process known as germination.
While microalgae are widely regarded as solely primary producers this may not always be the case. Some microalgal taxa demonstrate the capacity to absorb dissolved organic matter for sustenance, such as amino acids, from the surrounding water. These abilities have been shown to upregulate in low light, a feature that is relied on heavily by diatoms in polar and benthic habitats. This proves beneficial in semi-sealed brine channels within sea ice where there are elevated concentrations of nutrients from the constant digestion of diatoms by bacteria and zooplankton. Bacterivory has also been observed in small (<5 μm) flagellates, which may also be trapped within sea ice, or within the sediments during the polar nights.
For those species that are unable to become dormant there are also a range of long term physiological changes, which can be made to conserve energy during the dark months. Reduction in photosynthetic pigment production and regeneration has been observed in phytoplankton when placed into dark environments, this reduction can be measured by the narrowing of ranges of usable light wavelength by experimental populations. Recent work in the Arctic has revealed that very slight levels of photosynthesis are occurring during the polar nights; this has led to the development of a new theory in which microalgae are utilising photosynthetic pigments which are activated by wavelengths of light not visible to the human eye. Lastly, the low temperatures within the ice allow for the significant downregulation of metabolism, which can be maintained with the use of energy stored within the algal cell.
A dark future?
Experiments suggest that both Arctic and Antarctic phytoplankton have low dark mortality when grown in temperatures up to 6°C greater than natural, however, mortality begins to be generally significant and all species experience inhibition or limitation beyond this threshold.
These physiological adaptations allow phytoplankton to survive dark periods, allowing them to bloom in the spring, suporting a diverse and largely unique food web, and commercially significant fishery. However, if ocean warming continues, Antarctic communities may face yet another threat to their survival; the loss of primary producers which support a majority of the food webs found in these seas.