The eco-physiology of the colonial alga Phaeocystis Pouchetii

Field surveys carried out in the spring of 1984 in the eutrophicated coastal zone of the North Sea showed that the spring bloom of phytoplankton is characterized by two major events (chapter 2). In early spring a blooming of diatoms occurs which is followed by a vigorous outburst of the polymorphic alga Phaeocystis pouchetii. The highest cell numbers of Phaeocystis were observed in the near coastal zone with a seaward decreasing number over a transect perpendicular to the coast. This suggests a close relation between the initial nutrient concentration, which decreases also in a seaward direction, and the Phaeocystis biomass. Although the peak of Phaeocystis occurs after that of the diatoms, Phaeocystis is already present in small numbers in early spring and even in winter. During the peak of the bloom the inorganic nitrogen concentration is low, while in most cases inorganic phosphate was depleted. These low phosphate concentrations, and consequently the high N/P ratios, suggest that phytoplankton in the coastal waters of the North Sea is probably phosphate limited (chapter 2).
The present study aimed to investigate some of the ecophysiological properties of Phaeocystis enabling this
species success in its natural environment. Close examination of cultures and field samples showed the presence of two major growth forms: single flagellated cells, and immotile cells enclosed in spherical colony matrices. A third form, tiny flagellated cells ("microflagellates") was observed only by the end of the growth period (chapters 2 and 5). It was shown that the transition of one growth form (single cell) into the other (colonies) was affected by the nutrient concentration (chapter 5). Low phosphate levels significantly increased the number of single cells which formed a colony membrane. Subsequently cell division took place within this colony envelope, thereby developing into a colony. ...

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