The life cycle of Gonyostomum semen (Raphidophyceae)

Abstract

G. Cronberg. 2005. The life cycle of Gonyostomum semen (Raphidophyceae). Phycologia 44: 285–293.

Gonyostomum semen (Raphidophyceae) is today one of the most common flagellates in humic lakes in Sweden and appears regularly in 30% of these lakes. The first mass development of this alga was recorded in 1948 in Lake Helgasjön, south Sweden. In this paper the life cycle of Gonyostomum is described for the first time. During the winter Gonyostomum survived as resting cysts on the surface of the bottom sediment. In spring (April to May) meiosis took place in the cysts, and on excystment two haploid gametes appeared. After a period of mobility, the gametes formed pairs. The gametes then fused, and a new diploid Gonyostomum cell developed. The vegetative stage of Gonyostomum, which was seen during the summer period (May–October), is thus the diploid phase. During this period normal, longitudinal cell division took place. Under unfavourable conditions, temporary cysts could be formed. Cyst formation started from the centre of the cell, which swelled and became spherical. This was a thin-walled, temporary cyst, which was mobile and, like ordinary cells, had one flagellum protruding forward and one trailing behind. The temporary cysts could accumulate and settle in large clusters where the flagella were lost or had become invisible. When favourable conditions reappeared, the temporary cysts divided and formed two new gametes. In autumn, when the water temperature decreased to below about 10°C, the temporary cysts went into dormancy and became resting cysts. The cyst wall thickened, and a brown body appeared within the cyst. Below 4–6°C no vegetative cells were found.