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Recent evidence suggests that programmed cell death (PCD) can play a role in stress-induced decline and termination of harmful algal blooms. However, components of the PCD cascade, i.e. reactive oxygen species (ROS) and caspase-like activity, have also been observed in the absence of exogenous stress, where their activities and functions remain unclear. Here, we characterized the variability of prevalence of cell death, ROS, and caspase-like activity at different growth phases and diel cycles in cultures of dinoflagellate Karenia brevis. Results show that ROS percentages increased with culture age and fluctuated in a phasing diel pattern, while caspase-like activity was observed throughout growth. In actively growing K. brevis cells, PCD components may be involved in key metabolic processes, while in stationary phase they may relate to stress acclimation. The circadian diel pattern of ROS may be explained by the balance between the metabolic generation of ROS and circadian rhythmicity of antioxidant enzymes. Overall, this work highlights not only the involvement of PCD components in the growth of marine phytoplankton, but the importance of understanding mechanisms controlling their accumulation, which would help to better interpret their presence in the field.

Paralytic Shellfish Poisoning (PSP) caused by the dinoflagellate Alexandrium catenella is a well-known global syndrome that negatively impacts human health and fishery economies. Understanding the population dynamics and ecology of this species is thus important for identifying determinants of blooms and associated PSP toxicity. Given reports of extensive genetic heterogeneity in the toxicity and physiology of Alexandrium species, knowledge of genetic population structure in harmful algal species such as A. catenella can also facilitate the understanding of toxic bloom development and ecological adaptation. In this study we employed microsatellite markers to analyze multiple A. catenella strains isolated from several sub-regions in the Gulf of Maine (GoM) during summer blooms, to gain insights into the sources and dynamics of this economically important phytoplankton species. At least three genetically distinct clusters of A. catenella were identified in the GoM. Each cluster contained representatives from different sub-regions, highlighting the extent of connectivity and dispersal throughout the region. This shared diversity could result from cyst beds created by previous coastal blooms, thereby preserving the overall diversity of the regional A. catenella population. Rapid spatiotemporal genetic differentiation of A. catenella populations was observed in local blooms, likely driven by natural selection through environmental conditions such as silicate and nitrate/nitrite concentrations, emphasizing the role of short-term water mass intrusions and biotic processes in determining the diversity and dynamics of marine phytoplankton populations. Given the wide-spread intraspecific diversity of A. catenella in GoM and potentially elsewhere, harmful algal blooms will likely persist in many regions despite global warming and changing environmental conditions in the future. Selection of different genetic lineages through variable hydrological conditions might impact toxin production and profiles of future blooms, challenging HAB control and prediction of PSP risk in the future.

In vertebrates, the aromatase coded by the cyp19a1a gene can catalyze the conversion from androgens to estrogens. Thus, the regulatory mechanisms of cyp19a1a gene expression are a critical research field in reproductive endocrinology. In this study, we use zebrafish as a model to study the dynamic methylation levels of the cyp19a1a gene core promoter during zebrafish ovarian folliculogenesis. The results show that there is an apparent fluctuation of the methylation levels of zebrafish cyp19a1a core promoter. Moreover, the methylation levels are inversely correlated with the expression levels of cyp19a1a transcripts when the ovarian follicles develop from PV into the MV stage. Also, the CpG dinucleotides which are close to the transcriptional starting site may have provided a significant blocking effect on inhibiting the transcriptional function of RNA polymerase II. Taken together, the results from the present study strongly suggest that DNA methylation was one of mechanisms that are involved in the regulation of cyp19a1a gene expression during folliculogenesis. This methylation mechanism modifying transcriptional process accompanied with zebrafish ovarian folliculogenesis might also shed new light on the regulation of cyp19a1a expression during the ovarian developmental stage in other vertebrates.

Karenia brevis is responsible for annual harmful algal bloom (HAB) events in the Gulf of Mexico (GoM). Although the factors regulating the initiation of K. brevis blooms have been extensively investigated, the mechanisms leading to bloom decline and termination are not well known. Understanding the triggers of bloom decline could enhance our ability to forecast bloom dynamics and their impact on coastal areas. Microzooplankton grazing and programmed cell death (PCD) have been recognized as important pathways for the loss of phytoplankton biomass. However, neither of these two factors has been comprehensively examined in K. brevis. My dissertation work describes mechanisms of intrinsic cell death in response to environmental stress and develops methods to evaluate microzooplanton grazing pressure on K. brevis in the field. In Chapter 2, I confirmed the existence of PCD in K. brevis using oxidative stress as a model trigger. Acute susceptibility to oxidative stress increased as cultures aged, while the youngest cultures showed the fastest overall rate of cell loss within the first 24 hours. In untreated cells, the prevalence of components in the PCD cascade were not related to cell death, but fluctuated not only between growth phases but also over a diel cycle. Given that K. brevis likely experiences drastic environmental change during bloom season in the GoM, cell death processes in response to high light, hypo-osmotic, cooling and heat stress were described in Chapters 3 and 4. PCD in K. brevis only occurred at the largest light shift (50 to 1000 μmol m⁻² s⁻¹), along with induction of non-photochemical quenching which may enable cells to resist or delay PCD. Intensity-dependent induction of PCD cascade occurred when salinity decreased by 5, 10, or 15 psu, while cell death was only evident after 15 °C temperature change. Lastly, in Chapter 5 I describe the development of a quantitative real-time PCR method that can be used to measure species-specific grazing pressure on K. brevis. Overall, this work adds to our knowledge of cell loss processes in phytoplankton in general as well as insights into the effects of abiotic and biotic factors on the decline and termination of K. brevis blooms specifically