Polarella is a dinoflagellate, and, when described in 1999, was the only extant genus of the Suessiaceae family. Since then, multiple extant genera in the family have been described. The genus was described in 1999 by Marina Montresor, Gabriele Procaccini, and Diane K. Stoecker, and contains only one species, Polarella glacialis. Polarella inhabits channels within ice formations in both the Arctic and Antarctic polar regions, where it plays an important role as a primary producer. Polarella is a thecate dinoflagellate, wherein the cell has an outer covering of cellulose plates, which are arranged in nine latitudinal series. The general morphology of Polarella is similar to that of a typical dinoflagellate. and Polarella has a zygotic life history, wherein it alternates between a motile vegetative phase and a non-motile spiny cyst. While it is thought that the cysts of Polarella have lost their ability to form fossils, the cyst life cycle stage has acted as link to extinct members of the Suessiaceae family.
Etymology The genus name Polarella is derived from the polar environment, which the holotype inhabits, and the species name glacialis, meaning from the ice, is in reference to how the species inhabits the ice.
Taxonomy The genus Polarella was first described by Marina Montresor, Gabriele Procaccini, and Diane K. Stoecker as published in the Journal of Phycology in 1999. The dinoflagellate was initially identified from the CCMP 1383 cell culture, a culture that was collected from the McMurdo Sound in the Ross Sea, Antarctica. Montresor et al. (1999) utilized scanning and transmission electron microscopy to examine both the cyst and motile vegetative stages of the dinoflagellate, and identified the presence of a pattern of nine latitudinal plates covering the outside of the vegetative cell. Based on this particular pattern of thecal plates, as well as DNA evidence showing four different insertion or deletion events in the 18S rRNA SSU sequence, which is highly conserved and thus very specific to a given species, between Polarella and species within the closely related genus Symbiodinium, the new genus Polarella was created, and Polarella glacialis was placed within this genus as the only species. Since then, no other species have been added to the genus, however in 2002, Palliani and Riding placed Polarella into the subfamily Umbriadinioideae based on morphological similarities between Polarella and the genus Umbriadinium, a dinoflagellate cyst from the early Jurassic period.
Habitat and ecology During the formation of ice in the Arctic and Antarctic, channels containing brine are created within the ice, and as the temperature decreases, the salinity of this brine increases. Polarella are known to inhabit these brine-filled channels within Arctic and Antarctic sea-ice. Initially, the type specimen of P. glacialis was sampled from the McMurdo Sound area of the Ross Sea in Antarctica. Following this, P. glacialis was found to have a bipolar distribution, as its presence in the waters of the Canadian Arctic was discovered. A study by Thomson and Wright (2004) then further confirmed the distribution of P. glacialis across Antarctica in samples from Davis Station. Polarella is photoautotrophic, has been found to greatly contribute to the biomass and primary production within the sea-ice brine channels it inhabits, and is present at particularly high densities as it blooms in the austral spring (September to November). Following this period, P. glacialis encysts during the austral summer (December and January). As a phytoflagellate, P. glacialis is often consumed by zooplankton, and as result, P. glacialis plays a key role as a primary producer, creating biomass for use at higher trophic levels within the polar ecosystems.
Description
Morphology of vegetative cells Cells of Polarella were described by Montresor et al. (1999). Vegetative cells range from 10-15 μm in length and 6-9 μm in width. These cells have an elongated shape and are somewhat dorsoventrally flattened, with the epitheca being rounded, and the hypotheca being similar in shape to an abbreviated cone. The outside of the cell is smooth, and has a covering of thin polygonal thecal plates consisting of cellulose. Once the outer membrane that surrounds the cell is removed, sutures between the thecal plates become visible. The thecal plates are placed in nine latitudinal rows, with three latitudinal series in the epitheca, two series in the cingulum, and four series in the hypotheca. There is variability in the shape and number of plates across all regions of the cell. The cingulum is deep and wraps around the mid region of the cell, and the sulcus is a shallower indentation, present only through the hypotheca. The transverse flagellum wraps around the cell along the cingulum, and the longitudinal flagellum extends along the sulcus and trails behind the cell. The point where the two flagella meet at the intersection of the cingulum and sulcus is covered by an overhang of the right side of the epitheca.
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