Rapid growth and concerted sexual transitions by a bloom of the harmful dinoflagellate <i>Alexandrium fundyense</i> (Dinophyceae)

Abstract

Transitions between life cycle stages by the harmful dinoflagellate <i>Alexandrium fundyense</i> are critical for the initiation and termination of its blooms. To quantify these transitions in a single population, an Imaging FlowCytobot (IFCB), was deployed in Salt Pond (Eastham, Massachusetts), a small, tidally flushed kettle pond that hosts near annual, localized <i>A. fundyense</i> blooms. Machine-based image classifiers differentiating <i>A. fundyense</i> life cycle stages were developed and results were compared to manually corrected IFCB samples, manual microscopy-based estimates of <i>A. fundyense</i> abundance, previously published data describing prevalence of the parasite <i>Amoebophrya</i>, and a continuous culture of <i>A. fundyense</i> infected with <i>Amoebophrya</i>. In Salt Pond, a development phase of sustained vegetative division lasted approximately 3 weeks and was followed by a rapid and near complete conversion to small, gamete cells. The gametic period (∼3 d) coincided with a spike in the frequency of fusing gametes (up to 5% of <i>A. fundyense</i> images) and was followed by a zygotic phase (∼4 d) during which cell sizes returned to their normal range but cell division and diel vertical migration ceased. Cell division during bloom development was strongly phased, enabling estimation of daily rates of division, which were more than twice those predicted from batch cultures grown at similar temperatures in replete medium. Data from the Salt Pond deployment provide the first continuous record of an <i>A. fundyense</i> population through its complete bloom cycle and demonstrate growth and sexual induction rates much higher than are typically observed in culture.