Nannotax3 - ntax_Farinacci - Pavlova mesolychnon

Original descriptions of taxa - also emended descriptions for a few taxa. For coccolithophores, and many calcispheres, these are pages from the Farinacci & Howe Catalog of Calcareous Nannofossils. In other cases (e.g. non-calcifying haptophytes) the data is directly compiled on this site. The "Catalogue of Calcareous Nannofossils" was originally compiled by Prof A. Farinacci 1969-1989, since 2000 it has been updated and extended by Richard Howe - see The Farinacci and Howe Catalog - an Introduction.
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Citation: Pavlova mesolychnon van der Veer 1969

Type specimens: strain 6755(Van der Veer)

Type age (chronostrat): Extant, collected 20th May 1967

Standardised type level: 160_HOLOCENE

Type locality: a saltmarsh alongside a tributary of the Lynher River, East of the village Polbathick, and South of the village St. Germans, Cornwall, UK

Current citation: Rebecca salina (Carter 1937) Green in Edvardsen et al. 2000

Cells solitary, motile, dorsoventrally compressed, not metabolic, ofvariable shape, often oval, obovate, ovate or pyriform, often slightly asymmetric in dorsal view, (5½-)6-9(-13) x 4-4½ x 2½ µ. Two unequal, heterodynamic flagella and a contractile filament arising ventrally, close together, ⅕ - ⅓ of the body length from the anterior end. The long flagellum thick (about 0,4 µ) convoluted, 14-17 µ long, covered with tiny cylindrical to club-shaped scales, the basal part with very delicate hairs. The short flagellum 3-4½ µ long, about 0,2 µ, thick. Contractile filament up to four times the length of the cell body when fully extended, thickness variable, 0,1-0,2 µ, basal part thicker, 0,3 µ, covered with very delicate hairs. The contractile filament displays in transverse section a cavity surrounding partially or complete about 8 fibres arranged in a ring.

Chromatophores two, large, parietal and lateral, yellow-green, extended over of the sides ofthe cell, oval. In each chromatophore a pyrenoid central on the inner side. Pyrenoids without apparent reserve material.

Two to four bright refractive bodies between the chromatophores. Nucleus anterior, next to the insertion of the flagella and contractile filament. One central golgi apparatus, composed of many cisternae. Cells surrounded by a theca composed of two unit membranes, continuous with the plasmalemma. Theca covered with tiny club-shaped scales.

Light microscopy
Living cells were studied with phase contrast optics. The following description can be given for healthy motile cells.Usually the long flagellum moves in front, dragging the cell body. Sometimes, in escape reactions for instance, the cell can swim backwards for a few moments, being pushed by the flagellum. In rapid swimming the cell rotates, slowly swimming cells do not rotate. In this type of movement the ventral side is kept downwards. The long flagellum is thick and has a blunt tip. In rest it has two or three turning points, and often forms a cricoid, almost in a flat plane. The short flagellum is thinner. In rest it is straight or forms a single arc. The contractile filament is mostly shorter than the cell, and rarely longer than 1½ times the length of the cell, pointing backwards and running close to the surface of the cell. It can, however, be extended up to four times the length of the cell body, trailing behind in slowly swimming individuals. It can attach the cell to the substrate. In this condition the insertion near the flagella may become apparent when the cell changes its orientation before swimming away.

The contractile filament is often straight or slightly curved, but in a few instances it was observed to be irregularly undulated, with irregular movements suggesting its flexibility. It was never seen to coil up. During prolonged obser-vation all filaments became invisible, apparently by their contraction.

Two bright refractive bodies are mostly present. Sometimes there are two big and two small ones. These bodies are supposed to be food reserves. They are stained pink with brilliant cresyl blue. Fractures can be seen in the greater ones, even if the cell is not put under pressure. The chromatophores have about the same shape as a splitted peanut. The little pyrenoids are just discernable in flattened or burst cells.

The cells can be flattened under the coverslip by sucking up as much medium as possible with filter paper, applied to opposite edges of the coverslip. Evaporation of the medium will lessen the space between object glass and coverslip still more. Pavlova mesolychnon flattens under pressure without forming pseudopodia. Even in thus flattened cells the nucleus is not easy to find, because its perimeter is obscured by other organelles. It is situated at the anteriorend of the cell, between and in front of the chromatophores.

In flattened cells some granules become visible in various positions. Using knowledge of electron microscopical origin, they can be interpreted as mitochondria or fat globules. Once a dark circular spot was observed, surrounded by small dots with a size near the limit ofvisibility in the microscope. This could be a golgi body. Neither an eyespot nor a contractile vacuole could be detected.

Electron microscopy
The electron microscope revealed the existence of a “theca”, composed of two unit membranes, one continuous with the plasmalemma of the flagella and the other one with the plasmalemma of the cell body. The theca is perforated (fig.4 and 11). It can be smooth or undulated.It is covered with tiny club-shaped scales, 800 A long and 200 A wide, attached to the theca by very thin hairs about 1200 A long (fig. 2, 10 and 18). The flagella and contractile filament are inserted in a papilla surrounded by a depression in the cell surface. This papilla forms a bridge between the main body of the cell and the theca (Fig. 10 and 11).

The long flagellum is covered by small cylindrical to club-shaped, hollow scales, 700 A long and 200 A wide. Its basal part is covered with very thin hairs, 2500 A long (Fig. 10 and 12). Its relative thickness is caused by the distance between the outer membrane and the core of microtubuli, which is greater than usual. The short flagellum seems to be smooth. There are roots running from theflagella bases to various regions of the cell, but the root system could not be analysed completely until now. Between the roots lies a sac which opens to the outer world through a canal traversing the papilla. The significance of this sac is not clear, it may be a reservoir of scales, as in Pyramimonas, or something like the Dinophycean pulsule.

The structure of the contractile filament is illustrated in fig. 13 to 19. The contractile filament contains a core of 7-8 microtubuli arranged in a ring. The core penetrates deep into the cell body. An electrodense “septum” traverses the contractile filament just outside the papilla. This septum is penetrated by a cavity system, which below the septum has developed into a cavity surrounding most of the core of microtubuli. A fingerlike process intrudes into the ring of microtubuli in this region. In more distal parts of the contractile filament the cavity system is represented by a tube which is crescent shaped in transverse section. About 3µ, from the insertion a transverse section shows a cavity completely surrounding the axial core of microtubuli. The basal part of the contractile filament is covered with very thin hairs as is the long flagellum.

The nucleus lies adjacent to the plasmalemma and the chromatophores. The nuclear membrane has only a few pores, which are about 700 A in diameter. A fold of endoplasmic reticulum continuous with the nuclear membrane lines the chromatophores (fig.6).

The structure of the chromatophores is illustrated in fig. 20. There is very little cytoplasm between the chromatophore and the plasmalemma. The lamellae usually consist of three thylakoids. On the inner surface of each chromatophore is a depression, occupied by a number of vesicles, which are probably an elaboration of the endoplasmic reticulum. Next to this depression lies the pyrenoid inside the chromatophore. The pyrenoid is not very conspicuous, but canals penetrate deep into this region of the chromatophore. In fig. 8 intrusions of cytoplasm inside the chromatophore can be seen.

The mitochondria are of the tubular type. They are oval, rod-shaped, up to a few µ, or fused into irregular complexes. The golgi apparatus consists of a stack of 8-15 cisternae{fig.8). The relations of the golgi apparatus to other structures need further investigation. The bright refractive bodies are confined each in a separate vacuole. They are crystallized, as a fine striation reveals {fig. 5). Droplets of lipids also occur in the cytoplasm. A group of small vesicles, perhaps paramural bodies (Marchant & Robards 1968), and a thin electro-dense layer in the cavity between theca and cell body are still forming a problem.

References:

van der Veer, J. (1969). Pavlova mesolychnon (Chrysophyta) a new species from the Tamar Estuary, Cornwall. Acta Botanica Neerlandica. 18: 496-510. gs

CATALOG - Pavlova mesolychnon

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