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Fragilariopsis kerguelensis (O'Meara) Hustedt is considered the most abundant and widely distributed diatom species in the Southern Ocean, often reaching up to 90% of the total diatom assemblage (Hart 1942; Hasle 1968; Verity and Smetacek 1996). Like all diatoms, it is a unicellular microalgae that is encased in an intricate siliceaous cell wall, the frustule. The species is a pennate diatom of the order pennales, such that the cells possess bilateral symmetry down a mid line (Poulsen 1999). Generally found in ribbon-shaped chain colonies of 20 to 100 cells, this species often forms monospecific blooms in the Antarctic Polar Frontal Zone (Bathmann et al. 1994; Verity and Smetacek 1996). They are relatively large-celled, 10 - 76 µm long apically, and have heavily silicified frustules, with silica to nitrogen ratios of 2- 6 (Smetacek 1999). This pronounced silicification, along with a low density of well-marked striae on the theca, make it easily differentiated from related species occurring in the Southern Ocean (Hasle 1965, Hasle and Syvertsen 1997). The thick silica case is extremely strong, evolving as protection from crustacean zooplankton and ensuring the long-term persistence of this species in the surface layer of oceanic Antarctic waters (Hamm et al. 2003).

Its high abundance and unusually large need for silica have made F. kerguelensis a major constituent of the diatom ooze forming the Antarctic opal belt, the largest deposit of biogenic silica in the world ocean (Tréguer et al. 1995; Zielinski and Gersonde 1997). In fact around 75% of the world’s marine biogenic silicate sediments is produced by by F.
 kerguelensis, although these waters only account for 20% of global production (Tréguer et al. 1995; Verity and Smetacek 1996).  Accordingly, F. kerguelensis is arguably the most important diatom species in the global silicon cycle, and is an indicator species of a low-carbon high-silica exporting regimes in an otherwise iron-limited ecosystem of the Southern Ocean (Smetacek et al. 2004; Abelmann et al. 2006; Assmy et al. 2006).

Morphology


Like all diatoms, the cell is encased within a silicified cell wall case, called the frustule, which is made up of two interlocking halves, the valves or theca. One theca is larger than the other, with the smaller (the hypotheca) fitting inside the edge of the larger (the epitheca), much like a petri dish.  The theca are joined at this edge by a girdle band, which holds them together (Round et al. 1990; Hasle and Syvertsen 1997).

Fragilariopsis kerguelensis cells are solitary or attach at the theca surfaces of the frustule to form chain colonies up to 300µm long, which curve in a ribbon-like manner.  The individual cells are pennate in shape, with the cells rectangular but curving slightly towards the ends in girdle view, and narrowly elliptical in theca view. Frustule is heavily silicified, with apices generally rounded and often morphologically differing from one another. They are 10 -76 µm long apically, with the width between the apices ranging from 5 to 11 µm. Striae, regular ridges which cross the surface of the theca, are straight but curve slightly at the poles, with 4-7 striae per 10µm of theca length. They are relatively coarse, penetrating deep into the cell and being readily observable with light microscopy. Interstrial depressions are punctured with two rows of alternating conspicuous but comparatively sparse areolae, with 8-10 in 10µm. The theca of the frustule are more or less flat. The raphe are eccentric and not raised above the level of the theca. (Hasle and Syvertsen 1997; Scott and Thomas 2005; Assmy et al. 2006; Cefarelli et al. 2010). 

Figure 1: SEM micrographs of Fragilariopsis kerguelensis: (a) theca view of individual cell and (b) chain colony. (Micrographs taken by Rebecca Rowe).

Taxa



Empire Eukaryota


Kingdom Chromista


Phylum Ochrophyta


Class Bacillariophyceae


Order Pennales

Family Bacillariaceae


Genus Fragilariopsis

Species F. Kerguelensis

Distribution, abundance and habitat


Fragilariopsis kerguelensis is the most abundant and widely distributed species of the Fragilariopsis genus, if not diatom, found in the Southern Ocean (Hart 1942; Hasle 1968). With a circumpolar distribution, it is a characteristic species of Antarctic waters (Baker 1954; Van Der Spool et al. 1973). It is most common in oceanic areas, occurring less frequently inshore, with F. cylindrus and F. curta often outnumbering it close to the Antarctic continent (Hasle 1968). The optimal range of F. kerguelensis appears to be constrained by the Seasonal Ice Zone (SIZ) to the south and by low silica concentrations to the north (Zielinski and Gersonde 1997). However it does have a broad latitudinal range, being reported as having a sparse but continuous distribution up to 30oN (Burckle 1972; Van Der Spool et al. 1973). Peak abundance of F. kerguelensis occurs in the Permanent Open Ocean Zone (POOZ) of the Antarctic Circumpolar Current (ACC) south of the Polar Front. Its silica frustules are a major component of the diatom ooze that has accumulated in a vast area between the Polar Front Zone (PFZ) and the average winter sea ice edge. Known as the Antarctic opal belt, it is considered the largest marine deposit of biogenic silica in the world. (Zielinski and Gersonde 1997; De Baar et al. 1997; Abelmann et al. 2006).

The type locality, from where F. kerguelensis was first identified and was named after, is the surface waters between Kerguelen and Heard Island (O’Meara 1877; Kopczyńska et al. 1998). Other specific locations of reported occurrence in the Southern Ocean include offshore of Wilkes Land (57o35’S 105o46’E), Sea-Ice near Davis Station (Archer et al. 1996) and the Weddell-Scotia Confluence (Garrison et al. 1987). 

Figure 2: Distribution of F. kerguelensis and relative abundances, constructed from the core top of 228 sediment cores taken from the Atlantic and Indian sectors of the Southern Ocean. Greatest abundances of F. kerguelensis are found near the Antarctic Polar Frontal Zone (Taken from Crosta et al. 2005).

Diet 


Fragilariopsis kerguelensis are phototrophic organisms, possessing two chloroplasts, one on each side of the transapical plane of the bilateral organism (Cefarelli et al. 2010). As diatoms, the major pigments are chlorophylls a and c, beta-carotene, fucoxanthin, diatoxanthin and diadinoxanthin (Hasle and Syvertsen 1997). Due to the production of an especially thick silica frustule, they have a greater requirement for silicic acid than most diatoms, with Si:N ratios of 2-6 (Smetacek 1999). Their iron requirement, however, has been found to be at least half that generally reported for oceanic algal species (Strzepek et al. 2011). In addition, studies suggest that F. kerguenlensis are able to access a wide range of iron species in seawater, and possibly store excess iron (Verity and Smetacek 1996).  As iron limitation is the main factor driving the high-nutrient, low-chlorophyll (HNLC) phenomenon suppressing productivity in the Southern Ocean, this is likely to contribute to F. kerguelensis often outcompeting other diatom species (Strzepek et al. 2011).  

Migration, movements


Diatoms are mostly non-motile, lacking flagella, however some pennate diatoms are able to move across a substrate using an actin-myosin motility system. This system utilizes the raphe, an elongated slit in the frustule, which also allows these diatoms to attach to a substrate. Locomotion is through channeling secretion through the raphe slit, producing a gliding motility (Poulsen et al. 1999). The maximum speed for any diatom using such locomotion is reported as 20µm per second (Van Den Hoek et al. 1995). Most raphid pennates are benthic, with this form of motility being ideal, but a few, including Fragillariopsis and Pseudonitzschia, have secondarily acquired a planktonic lifestyle where this system would be ineffective (Not et al. 2012). Furthermore, it has been proposed that the relatively simple raphe of Fragilariopsis is functionally fully reduced, suggesting that any form of manipulation of position within the water column is improbable for these organisms (Mann 1986; Hasle and Syvertsen 1997; Lundholm et al. 2002). 

Life history 


Fragilariopsis kerguelensis has a life cycle typical of diatoms, whereby long periods of vegetative reproduction during which diploid cells divide by binary fission, are interrupted by a brief period of sexual reproduction (Not et al. 2012). The sexual phase occurs when cells reach a critical size, as each consecutive mitotic event produces cells smaller than the parent cell. This is as each daughter cell receives one of the theca of the parent cell as the epitheca, and then grows the smaller theca (hypotheca) within it. Consequently, the daughter cell which receives the smaller valve will be smaller than the parent cell, resulting in a net decrease in the average size of diatom cells in a population each division cycle (Assmy et al. 2006). When the critical size is reached meiosis occurs, with the gametes, fusing to form a zygote, which inflates to generate an auxospore (Not et al. 2012). For pennate diatoms, gametes are isomorphous, with neither being flagellated, and are referred to as +/- matiing types (Hasle and Syvertsen 1997). From the auxospore, the new vegetative cell forms as the largest size for the species. Auxospores can also be produced from a vegetative cell, such that the frequency of sexual reproduction does not always match the frequency of reaching the critical size (Van Den Hoek et al. 1995). The frequency of the sexual phase in F. kerguelensis seems to vary from 2 up to possibly 40 years (Assmy et al.  2006).

Auxospores may also be produced to act as resting spores for when conditions are unfavourable to growth, such as during nutrient depletion and winter, with germination occurring when the conditions improve (Van Den Hoek et al. 1995; Horner 2002). When conditions are favourable, such as iron availability increasing from ice melting in early summer, F. kerguelensis respond by forming a bloom (Bathmann et al., 1997; Boyd, 2002). Fragilariopsis kerguelensis has been considered as a slow growing species under natural conditions (Assmy et al. 2006; 2007). Maximum growth rate, under Iron replete conditions, has been reported as 0.26 cells per day (Timmermans and Van Der Wagt 2010). 

Figure 3: Diagram of typical diatom life cycle. Note that in F. kerguelensis gametes are isomorphous, with neither being flagellated. (Taken from Kooijman 2000).

People


 

References


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