Life history stages of Pyramimonas tychotreta (Prasinophyceae, Chlorophyta), a marine flagellate from the Ross Sea, Antarctica

Abstract

SUMMARY During a summer cruise to the Ross Sea (Antarctica) areas of snow-covered sea ice were red-coloured due to high concentrations of the recently described Pyramimonas tychotret aDaugbjerg. Light microscopy of living material revealed that the population was comprised of quadriflagellate motile cells and thick-walled cysts. The red colour was due to large numbers of secondary carotenoid-containing granules, positioned in the periphery of motile cells and cysts. Mature cysts also contained numerous starch grains and lipid droplets. Cells from a red-coloured field sample turned green overnight as the secondary carotenoids disappeared when cells were placed in low light conditions. The sample then exhibited the typical grass-green colour of motile cells observed in water samples from the area. Under reduced light motile cells showed strong positive phototaxis. The encystment process involved the asexual transformation of quadriflagellate cells into cysts. A single type of square cyst scale, with perforated floors and walls, replaced the body scales of motile cells. A marked extension, often ending in a hook was at each corner of the cyst scales. Germinating cysts produced four motile cells. Electron microscopy showed the cyst wall to be tri-layered, with a thin, electrondense inner layer, a thick middle layer and a thin outer layer. Sea ice samples with dense populations of motile cells and cyst stages also contained elongate uniflagellate cells. These cells were covered with box scales, foot-print scales, an underlayer of pentagonal scales, limuloid scales and flagellar hair scales identical to those present on the quadriflagellate stage. We tentatively suggest that the uniflagellate stage represents a gamete and its presence implies the occurrence of sexual reproduction. Although, fusion of gametes was not observed, a biflagellate cell with a larger volume was seen which may have been a zygote. How this stage fits into of the life history remains to be explained.