Papposphaera sagittifera, HET

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 : 36-38

P. sagittifera HET ( Figs 4–16 )

The Arctic material from West Greenland ( Fig. 7 ), North East Water Polynya ( Figs 4–5 , 13 ) and Svalbard ( Figs 6, 8–12, 14–16 ) basically confirms what is already known about P. sagittifera HET from this region ( Thomsen 1981). The rationale behind illustrating three complete cells from TEM is to document the consistent presence of flagellation. The flagella and the haptonema typically curl up densely which renders measurements of these structures impossible. A haptonema is visible in e.g. Fig. 4 (arrow) where it is differentiated from the flagella by being significantly thinner. The apparent absence of flagellation in the numerous SEM illustrations of complete cells ( Figs 10–12, 14 ) is bound to be an artefact somehow caused by the manipulation of the cells during filtration and subsequent drying. All complete cells illustrated from SEM clearly display organisms that have the typical cell polarity to be expected from a flagellated individual. Dimensional issues across the different regions are accounted for in Table 1 .

Central area calcification is invariable an axial cross in combination with a number of longitudinal bars evenly spread across the coccolith. The highest number of such bars observed is 7 ( Fig. 8 ) which creates a densely packed central area. Most coccoliths tend to have 3–5 of these longitudinal bars (see e.g. Fig. 14 ).

The calyx takes the shape of a four-winged rosette with the four wings positioned at right angles to each other ( Figs 5–6, 9 ). Each wing has parallel sides. The exterior edge is straight, whereas the interior edge has a variable number of steps that gradually decrease the width of the wing towards the distal end ( Fig. 3A ). There seems to be a significant variability with respect to both the width of the wing and the number of steps on the interior edge. The degree to which the four-winged rosette diverges from the stem is dependent on the angle between the exterior edge and the line that proximally connects the inner and exterior edges of the wing.

Papposphaera sagittifera HET is generally conceived as having monomorphic coccoliths yet with a tendency towards a varimorphic state in as much as there is variability among coccoliths in the length of the central process and with the longer ones prevailing in a circum-flagellar cluster. The SEM images ( Figs 10–12, 14 ) clearly and consistently show that the actual pattern of variability goes beyond trivial dimensional issues. The major part of the coccoliths covering the cell surface – with the exception of the apical and antapical poles of the cell – are in fact devoid of a central process. All that is left is a short stub-like feature ( Figs 8 , 14 ) or just a low mound created by the lifting up of the arms of the axial cross where they meet centrally ( Fig. 9 ). It is of course possible and also relevant to interpret these as being just very much reduced standard calicate coccoliths without a central appendage. However, it does anyway represent a differentiation among coccoliths that is almost equal to having dimorphic coccoliths. TEM images (e.g. Figs 4, 7 , 13 ) are less informative with respect to illustrating the distribution of calicate and non-calicate coccoliths within a coccosphere. The electron beam does not adequately penetrate the middle part of the cell leaving the massive appearance in this area of non-calicate coccoliths difficult to resolve. The two-dimensional TEM image and a certain repositioning of individual coccoliths caused by the handling of the cells during processing, further add to the interpretational challenge with respect to accounting for the actual positioning of calicate versus non-calicate coccoliths. Disregarding these reservations the cells illustrated in Figs 4 and 13 do appear to have a few calicate coccoliths positioned in the middle part of the coccosphere. This might indicate a possible regional or seasonal variability among specimens of P. sagittifera that we do not at present have material to possibly account for.

Figures

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.

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.