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Harmful algal blooms (HABs) and their associated phycotoxins pose a threat to both human and shellfish health around the world. Dinophysis spp., a causative organism of diarrhetic shellfish poisoning (DSP) in humans, and its two toxin classes: dinophysistoxins (DTXs) and pectenotoxins (PTXs), have been documented throughout the year in the Chesapeake Bay. While DTX concentrations currently remain below regulatory limits in regional seafood products, further research is needed to understand environmental drivers, both biotic and abiotic, that may be impacting Dinophysis spp. feeding on prey, growth, and toxin production. To characterize populations of Dinophysis in situ, an Imaging FlowCytobot (IFCB) was deployed off the Virginia Institute of Marine Science (VIMS) pier for five sampling seasons. The IFCB captures images of phytoplankton cells every ~20 minutes, generating large, continuous data sets that are then automatically classified using a machine learning algorithm, in this case, a convolutional neural network (CNN) framework. The IFCB-generated abundance data for the dinoflagellates Dinophysis acuminata, and Prorocentrum cordatum, as well as the ciliate Mesodinium rubrum, were then incorporated into an ecological predictive model along with abiotic variables such as sea surface temperature, salinity, turbidity, pH, and discharge. Given that Dinophysis is an obligate mixotroph, the relationship between bloom timing of the prey item M. rubrum and D. acuminata were explored by fitting models with no lag of M. rubrum, a 14-day lag, and a 75-day lag. Additionally, P. cordatum abundance was explored as a proxy for D. acuminata abundance. Results revealed that D. acuminata abundance was significantly linked with salinity, time of year, M. rubrum abundance with a 14-day lag, and P. cordatum abundance. The results of this study provide further insight into potential biotic and abiotic factors regulating Dinophysis populations in Chesapeake Bay.To test another possible abiotic factor, turbulence, a culturing study was undertaken in the laboratory with four isolates of three species of Dinophysis: D. acuminata, D. ovum, and D. caudata. These isolates, from the Gulf and East Coasts of the U.S., were exposed to two levels of turbulence for six days to determine its effect on feeding, growth, and toxin production. While an effect of turbulence on ingestion rates of M. rubrum by Dinophysis was not detected, exposure to high turbulence inhibited growth of both D. acuminata and D. ovum and stimulated growth of D. caudata. As a result of inhibited growth, both D. acuminata and D. ovum had reduced toxin production rates, but D. acuminata was shown to accumulate toxin inside cells, while D. ovum released toxin extracellularly into the media. Conversely, the stimulation of growth for D. caudata resulted in decreased intracellular toxin content, but no effect on toxin production rate. While understanding phytoplankton population dynamics in the natural environment is complex, the results of these studies highlight some important environmental factors impacting Dinophysis spp. feeding, growth, and toxin production in the lab and field in both Chesapeake Bay and the East and Gulf Coasts of the U.S. |
