An ultrastructural and ecological study of Ceratium hirundinella (Dinophyceae)

Vegetative cells of Ceratium hirundinella taken from Esthwaite Water, a small productive lake in the English Lake District, were studied each month by scanning and transmission electron microscopy. Changes in size and abundance of chloroplasts and their lamellar spacing was shown to be related to their exposure to light within the lake. Changes in the quantities of starch and lipid present were mainly related to cyst formation or recent excystment. Excystment and encystment were studied by light and electron microscopy. Two cyst types were observed, one with an outer wall layer of silicon granules over a cellulose-like material and the other with only a smooth cellulose-like wall. The silicon coated cysts are formed each year within the lake and overwinter in the sediments, excysting after the required dormancy period to form the new spring increase in vegetative cells. These cysts may represent a zygotic stage in the life cycle or a hypnocyst. The smooth-walled cysts are formed in laboratory cultures and were observed in one lake. They do not become dormant and are susceptible to decay and attack by a chytrid fungus. An 'in-situ' fixation technique was used to study the ultra-structure of vegetative cells over a period of 24 hours. Formation of pyrenoid clusters and starch was associated with exposure to light, as was the spacing of thylakoid lamellae within chloroplasts. No evidence for phagotrophy was found by means of electron microscopy and the 'food-vacuoles' reported for this species were probably accumulation bodies, used for removing unwanted inclusions or cellular products. Two species of intracellular bacteria have been observed, probably forming symbiotic associations within C. hirundinella. 20% of cells examined in July 1979 had either a rod bacterium, a coccoid bacterium or both. One cell was observed to contain virus particles.