A study of the marine phytoflagellate Pyramimonas pseudoparkeae Pienaar et Aken (Prasinophyceae)

Abstract

The morphology and structure of P. pseudoparkeae is described in detail. The alga resembles other species in the genus but is most closely related to P. parkeae being separated from it by differences in scale structure. Important taxonomic (phylogenetic) characteristics of P. pseudoparkeae include the possession of four flagella, trichocysts, a complete covering of three scale types on the cell body, and a 3-over-1 arrangement of the basal bodies. The alga was grown successfully in a number of enriched and artificial seawater media. The alga grew well in a salinity of 35%0 but it is euryhaline and tolerated salinity levels ranging from 10 - 70%[0]. The relative growth rate (k') of the alga was significantly increased by raising the light intensity from 50 to 100 or 150 μ Em ⁻² s¯¹. At higher light intensities (200 and 300 μ Em⁻² , s¯¹ ) k' was reduced, probably through photo inhibition. The alga grew well at 20 and 25°C but could not tolerate a temperature of 30°C. The growth studies indicated that optimal growth (determined by the highest relative growth rate) was achieved in PES medium at a salinity of 35%0, a light intensity of 100 - 150 μ Em⁻² s¯¹ , and a temperature of 20 - 25°C. Under these conditions the mean doubling time (G) of the cells was 26 h. Scale structure in P. pseudoparkeae remained constant in the different seawater media used and under a range of salinity, temperature and light intensity. P. pseudoparkeae could not be grown axenically and was shown to have an absolute requirement for bacteria in culture. This bacteria/algal relationship is believed to be mutualistic because the alga also promoted the growth of the bacteria. The nature of the growth promoting factors involved are not known. Cell division In P. pseudoparkeae was similar to that described for other species in the genus. The cells remained motile throughout the cell division cycle and they divided preferentially during the dark. Cultures of the alga could at best be partially synchronized under optimal growth conditions because the shortest mean doubling time obtained was 26 h; i.e. two hours longer than the 24 h period in a 16h:8h synchrony induction photoregime. of the flagellar basal bodies, dictyosomes and nucleus {in that order}. Mitosis is characterized by an open spindle. Spindle microtubules, which are derived from the rhizoplast, are absent at telophase and no phycoplast develops. Cell division is completed within 90 min. All scale types covering the alga were produced continuously by the two dictyosomes within the cell. Scale morphogenesis was shown to be a rapid process with scales being completely formed within 10,5 min. This is the time taken for a single cisterna to pass through the dictyosome (comprising 20 cisternae). Flagellar scales were stored in a scale reservoir which was always connected with the flagellar pit via a duct. These scales were released when four new flagella developed from the replicated basal bodies. A compound microtubular rootlet was always associated with the duct of the scale reservoir. Body scales moved in vesicles from the dictyosomes directly to the plasmalemrna at the base of the flagellar pit where they were released by reverse pinocytosis. The scales of P. pseudoparkeae were shown to be pectinaceous In nature being predominantly composed of polysaccharide and containing a small amount (4%) of protein. TLC separation of sugar residues in acid hydrolysates of scales showed that the latter were composed of neutral sugars galactose, arabinose, xylose, rhamnose and a trace of fructose. Galacturonic acid is also thought to be a major constituent of the scales because they were digested with pectinase. The scale polysaccharide is sulphated. Aspartic and glutamic acid were major amino acid residues detected on scale hydrolysates analysed on an automatic amino acid analyser. The polyanionic nature of the scales is thought to underly the mechanism of external self-assembly of the scaleboundary, and to contribute to the maintenance of water and salt balance in the cell. P. pseudoparkeae reproduces asexually by binary fission or by producing non-motile, thick-walled cysts. Cysts developed spontaneously in cultures so that the stimuli causing encystment are not known. In fresh medium mature cysts released four motile cells which regenerated the motile phase. The cyst wall is bilayered. The outlet layer of the cyst wall has the properties of sporopollenin while the inner layer of the cyst wall has the same staining properties as scales and is possibly of similar composition. EDX analyses of the cyst wall showed that it is Tich in calcium and sulphur.