Harmful Dinophysis species produce toxins which cause diarrhetic shellfish poisoning (DSP) in humans. In North American waters, the frequency and intensity of Dinophysis blooms has increased in recent decades. The first closure of shellfish harvesting in North America occurred in response to a bloom of Dinophysis ovum along the Texas coast in 2008. Since 2008, blooms of Dinophysis have been reported along every US coast. Despite being recognized as the main causative agent of DSP in humans, studies of Dinophysis were limited for many years due to difficulty sampling in situ populations and challenges establishing laboratory cultures of Dinophysis species. The unique mixotrophic physiology of Dinophysis species and their reliance on specific prey were the primary factors preventing establishment of laboratory cultures. The first successful culture of Dinophysis in 2005 by Park et al. (2006) and long-term monitoring of microplankton communities by programs like the Texas Observatory for Algal Succession Timeseries (TOAST) have opened new avenues for research regarding Dinophysis blooms. Here, the results of combined laboratory, field, and modeling efforts incorporating in situ monitoring data from TOAST are presented. Synthesis of these efforts offered new insights into the dynamics of harmful mixotrophic bloom-forming Dinophysis and their prey. A major result of the research presented here was the successful establishment in laboratory culture of a novel isolate of Dinophysis ovum from the Gulf of Mexico. Using this isolate, laboratory experiments were performed to test hypotheses regarding Dinophysis growth, bloom dynamics, and toxigenicity. The first series of experiments tested the impact of physical factors (temperature, salinity, irradiance) on Dinophysis growth. The results of these experiments are discussed in the context of observed Dinophysis blooms along the Texas coast. The second set of experiments tested the hypothesis that Dinophysis population density influences toxigenicity, and that cells regulate toxin production by a form of quorum sensing. Finally, data from laboratory experiments and TOAST field data were used to parameterize a mechanistic model of Dinophysis and prey interactions. This model was used to test the effect of prey size on Dinophysis bloom dynamics. Growth experiments revealed temperature had the strongest impact on Dinophysis growth and bloom dynamics, with an optimal growth range of ~15 - 25 ?C. Additionally, laboratory experiments supported the hypothesis that population density affects Dinophysis toxigenicity; cells in low-density (<100 cells mL-1) treatments were observed to be more toxic individually than high density (>1000 cells mL-1) treatments. Finally, modeling results revealed an impact of prey size on Dinophysis bloom dynamics. Larger prey were capable of producing larger Dinophysis blooms despite smaller prey populations in model simulations. These results will be valuable for Dinophysis bloom anticipation and mitigation efforts. In a broader sense, the work presented here has important implications for marine ecology, especially regarding marine mixotrophs, and should be considered in future studies of climate change and marine food webs.
