Name: So Hyun (Sophia) Ahn
Date: 10/10/2023
Time (EST/EDT): 10:00 am
Location: AREL lecture hall
Remote Access: email mees@umd.edu


Committee Chair: Dr. Patricia M. Glibert
Committee Members: Dr. Cynthia Heil, Dr. Diane Stoecker, Dr. Ming Li, Dr. Yantao Li
Dean’s Representative: Dr. Sujay Kaushal

Title: Interactions between nitrogen and temperature on the metabolism of the red-tide mixotrophic dinoflagellate Karenia spp. in support of predictive models: Implications for bloom dynamics on the West Florida Shelf

Abstract: The toxic mixotrophic dinoflagellate Karenia spp. forms blooms almost annually in the Gulf of Mexico, especially on the West Florida Shelf (WFS), causing negative effects on aquatic ecosystems and human health. They typically initiate blooms in late summer to early fall but can persist from months to years. In addition, on a daily basis, Karenia vertically migrates to surface water during the day, possibly experiencing temperature changes with co-change in light, nitrogen forms and availability, as well as prey availability. Therefore, this dissertation aimed to examine the interplay between Karenia’s photoautotrophic and mixotrophic metabolism and the fluctuations in environmental conditions to understand how these factors may relate to the conditions under which Karenia spp. are found in the WFS and to provide new physiological data for developing a mechanistic model of K. brevis on the WFS. The cultured K. mikimotoi cells were dynamically balancing the “push” due to photon flux pressure and reductant generation with consumption in overall metabolism (“pull” due to demand) when pulsed with 15N-NO3-, 15N-NH4+ or 15N-urea and incubated at 15, 20, 25°C. However, at 30°C, cells were significantly stressed, but the cells enriched with urea exhibited a smaller decline in fluorescence parameters than other N substrates, implying that urea might induce a photoprotective mechanism by increasing metabolic “pull.” However, in the winter and summer field experiments, the thermal history of the cells was more critical, which regulates the thermal preference of photosynthetic performance. Although bloom biomass was remarkably higher in summer blooms, cells were stressed photosynthetically (e.g., lower rETRmax) and nutritionally (e.g., lower 15N uptake rates). However, 15N-urea enriched summer cells exhibited relatively higher N uptake rates and total C and N cell-1 than with the other N substrates, especially in warmer waters, showing differential thermal responses depending on N forms. The cultured K. brevis did not selectively feed on Synechococcus of different qualities but ingestion rates were a function of prey-to-grazer ratios (R2=0.76). K. brevis could graze on Rhodomonas, implying that the spectrum of K. brevis’s prey could be broader. Furthermore, the growth and density-promoting effects of grazing under higher inorganic nutrient availability conditions indicate the synergistic effects of phago-mixotrophy as a nutrient source. In the field experiment, ingestion rates by natural Karenia brevis on Synechococcus were significantly regulated by the prey-to-grazer ratios and by temperatures to a lesser degree (R2= 0.75) when incubated at ambient (24°C) and ambient temperature ± 5°C (19, 29°C). The grazer effects on the photosynthetic performance of prey were higher in warmer temperatures. These results suggest that grazing on Synechococcus could directly regulate the density of prey populations but also indirectly reduce the photosynthetic performance of prey, especially at warmer temperatures.