Papposphaera sagittifera

Thomsen, Helge A., Østergaard, Jette B. & Heldal, Mikal, 2016, Coccolithophores in Polar Waters: HOL Revisited Papposphaera sagittifera HET and, Acta Protozoologica 55 (1), pp. 33-50 : 39-41

P. sagittifera combination coccospheres ( Figs 17–22 )

Two new occurrences of combination coccospheres from West Greenland are shown in Figs 17, 19 . The cell shown in Fig. 17 is particularly appealing because here the two groups of coccoliths have become partly separated to clearly show the morphological characteristics of each of them.

With reference to the combination coccospheres of P. sagittifera and T. borealis there is one issue of particular concern. It has become apparent that in each of the combination coccospheres illustrated by Thomsen et al. (1991) and also in the two new images reproduced here, the P. sagittifera HET calyx is in fact different from what is typically found in P. sagittifera HET when not forming part of a combination coccosphere. In the P. sagittifera combination coccosphere the four-winged rosette comprises wings that are fairly narrow, hardly ever stepwise elaborated on the inside, and with an angle proximally between the exterior edge of the wing and the connecting line between the two edges of the wing that leads to a much less flaring rosette in comparison with that generally found in P. sagittifera HET ( Fig. 3A ). The line distally connecting the exterior and interior edges of the wing deviates a few degrees from being perpendicular ( Figs 18, 20 ). In Fig. 22 the wings of the individual rosette have been pushed apart only to confirm the description of the individual wing provided above. A stepwise elaboration of the distal inner edge of the individual wing is occasionally observed ( Figs 18, 20, 22 ; arrows). The design of the individual wing and the reduced overall flaring of the rosette cause the individual rosette to appear with an overall triangular outline when viewed in a TEM ( Figs 18, 20–21 ). The very same features as described above are evident when consulting Thomsen et al. 1991 (loc. cit. Figs 2–4, 9 ).

A search among images of P. sagittifera HET has fortunately unveiled rare occurrences of this particular type of calyx scattered among others with the traditional stepwise and flared rosette design. First and foremost it should be mentioned that in the illustrations accompanying the type description ( Manton et al. 1976a, loc. cit. Fig. 3 ) there is in fact a single calyx among more than 15 ‘normal’ ones that fairly closely resembles those that occur in combination coccospheres. However, this feature was not specifically commented on in the type description. Also the SEM image of a Svalbard cell ( Fig. 14 with enlargements in Figs 15–16 ) displays at least one calyx that resembles those that occur in combination coccospheres ( Fig. 16 ) in between more typical P. sagittifera HET rosettes ( Fig. 15 ). It must also be noticed that there is typically a size difference between the two types of rosettes. The ‘triangular’ rosette ( Fig. 16 ) is thus close to 1 µm in length, whereas the ‘flaring’ rosette ( Fig. 15 ) is only 0.6 µm in length which is typical for this kind of rosette (ca. 0.65 µm in e.g. Fig. 13 ). The size range among rosettes in Fig. 17 is 0.8–0.9 µm which equals measurements from previously published combination coccospheres ( Thomsen et al. 1991; mean value 0.75 ± 0.09 µm / range: 0.65–1.00 µm / n = 12).

We have at present no explanation to offer with respect to these observations. Based in particular on the few findings referred to above of a combination coccosphere rosette type among ‘normal’ ones in genuine heterococcolithophore stages, but also on the fact that we have never observed a heterococcolithophore with a P. sagittifera central area calcification pattern and exclusively ‘triangular’ rosettes, we strongly believe that the heterococcolithophore forming part of combination coccospheres, such as those illustrated here in Figs 17, 19 , is in fact P. sagittifera sensu stricto. Somehow the explanation behind these anomalies must be an integrated part of events taking place during a phase transition. However, it is very obvious that we are lacking in understanding the phenomenon of phase shift to the extent that this remains unexplainable for the time being. In our minds it makes most sense to think of combination coccospheres with these differing rosette features as representing stages in the haploid to diploid phase shift and that the ‘triangular’ rosette is thus a de novo feature of a ‘new-born’ diploid individual.

The holococcoliths forming part of a combination coccosphere appear to be closely similar to those described from Turrisphaera borealis ( Manton et al. 1976b) . Holococcoliths in Fig. 17 measure 3.0–3.5 µm in length and ca. 0.7 µm in width at the most narrow part of the tower. There are 5–6 plates of crystallites spanning the width of the tower and they each have an edge-to-edge distance of ca. 0.09 µm (type material: 0.15 µm).

Figures

Figs 17–22. Papposphaera sagittifera combination coccospheres (TEM whole mounts from West Greenland). 17 – separated halves of a combination coccosphere; 18 – detail of calyces from Fig. 17 showing the regular triangular outline; the arrow points to a rosette wing that has a distal stepwise notching; 19 – combination coccosphere with the two halves united; 20–21 – detail of the calyces from Fig. 19; the arrow (Fig. 20) points to a rosette wing with a distal step-wise notching; 22 – the four rosette wings are spread out in a fan-like manner clearly showing the shape of the individual wing: the arrow points to a rosette wing with a distal step-wise notching.

Fig. 3. Interpretational drawings of coccolith structure within species of Papposphaera. Intraspecific variation is shown where relevant. A – P. sagittifera; B – P. sagittifera cfr.; C – P. sarion; D – P. arctica; E – P. iugifera; F – P. heldalii; a – body coccolith central area calcification; b – calyx design; c – calicate coccolith central area calcification; d – holococcolith contour (flagellar pole coccolith); e – holococcolith contour (body coccolith). Notice the inclusion of schematic drawings of a rosette wing from each of the P. sagittifera types of calyces.

Figs 1–2. Maps of collection sites. 1 – northern hemisphere; 2 – southern hemisphere. Showing also the AMERIEZ sampling grid (Thomsen et al. 1988).

Figs 4–9. Papposphaera sagittifera HET (TEM / Figs 4–5, 7; SEM / Figs 6, 8–9) from the Arctic (West Greenland / Fig. 7; NEW / Figs 4–5; Svalbard / Figs 6, 8, 9). 4 – whole cell with curled up flagella and haptonema (arrow); 5 – detail of calyx; 6 – lateral view of coccolith displaying the two cycles of elements in the rim and details of the stem and the calyx; 7 – complete cell; notice the difference in length of coccolith processes from one end of the cell to the other; 8 – details of central area calcification; notice the short stub-like central proboscis and also possibly tilted pentagonal elements from the distal rim circle (arrows); 9 – detail of two coccoliths one with a short central process and the other with just a central upheaval of the arms of the crossbar.

Figs 10–16. Papposphaera sagittifera HET (TEM / Fig. 13; SEM / Figs 10–12, 14–16) from the Arctic (Svalbard / Figs 10–12, 14–16; NEW / Fig. 13). 10–12 – whole cells selected to show the limited occurrence of coccoliths with central processes and the obvious irregularities in size and placement of the body coccoliths; 13 – whole cell showing flagella and haptonema; 14 – cell selected to illustrate the variability in coccolith size, orientation and central area calcification; notice the presence of two types of calyces (encircled); 15 – calyx of the standard ‘flaring’ P. sagittifera HET type (enlargement from Fig. 14); 16 – ‘triangular’ calyx (enlargement from Fig. 14) similar to those found in P. sagittifera HET when forming part of a combination coccosphere.