The time for sex: A biennial life cycle in a marine planktonic diatom

Whereas it is accepted that the population dynamics of higher organisms is strongly grounded in life history processes, such as the time taken to mature and reproduce, regulation in the abundance of aquatic protists is generally attributed to proximate properties of the environment in which they grow. We used 10 yr of data from the Gulf of Naples to determine the life history of a planktonic diatom, Pseudo-nitzschia multistriata, through the analysis of cell abundances and cell size patterns. Asexual and sexual phases recurred with remarkable regularity. Maximum abundance occurred annually, between the end of summer and the beginning of autumn, but cohorts of large cells, derived from sexual events, were found only every second year. We developed a model of population growth, based on parameters determined from laboratory cultures, with further tuning using information from natural populations. The model simulated the birth, maturation, and disappearance of age classes like those in natural populations, but only when we imposed seasonal variation in the rate of cell division and timed sexual reproduction during the stationary phase of a bloom. The model predicts that P. multistriata will become locally extinct if sexual reproduction does not occur within 4 yr. Our data and model show that coherent life cycle properties can emerge in natural populations of unicellular organisms, analogous to those in multicellular organisms, as a result of finely tuned regulation of cell division and sexual competence.