Monitoring the long-term stability of pelagic morphotypes in the model diatom Phaeodactylum tricornutum

Abstract

Phaeodactylum tricornutum Bohlin has long been considered as a representative model diatom, but critical uncertainties regarding its life cycle remain. It is characterized by a marked pleiomorphism with three basic morphotypes: triradiate, fusiform and oval. In liquid media, triradiate cells gradually convert into the fusiform morphotype, passing through well-known intermediate forms, whereas the reverse conversion has never been observed, but merely assumed. The triradiate morphotype was even claimed as teratological, with no apparent ecological significance, and was overlooked in many Phaeodactylum research experiments. The triradiate morphotype is far from being a merely abnormal development stage within its life cycle, but represents an important morphology within the Phaeodactylum population dynamics. Monoclonal triradiate and fusiform morphotype cultures and a mixed one containing triradiate and fusiform cells were monitored over a period of two years. Under selected growth conditions, a simulation model was constructed to extend the monitoring period to five years and to determine the main processes underlying our findings. The experimental design and model results demonstrated that the coexistence of both triradiate and fusiform morphotypes under constant temperature, light regime and agitation conditions over the long-term is feasible, despite most accessions maintained in culture collections are fusiforms or ovals. In addition, the fusiform-to-triradiate transformation does not follow the same intermediate forms as the well-known triradiate-to-fusiform reverse transformation. Conversely, the fusiform-to-triradiate transformation seems to be associated with the appearance of small triradiate cells in the culture, which represents a low frequency event, although the ecological meaning of such transformation remains unknown.