Date of Award

2026

Abstract

Phytoplankton are central to marine ecosystem functioning, driving primary production, nutrient cycling, and higher trophic level dynamics. In the spring and summer of 2023, the Gulf of Maine experienced an unprecedented bloom of the dinoflagellate Tripos muelleri, reaching densities exceeding 50,000 cells L-1, an order of magnitude higher than previously recorded, and disrupting the region’s typical spring diatom-dominated succession. This bloom provided a unique opportunity to examine the metabolic strategies, ecological interactions, and environmental drivers underlying the bloom persistence and decline. An integrative approach was used, combining 18S and 16S rRNA amplicon sequencing, pigment and image analysis, network modeling, functional prediction using PICRUSt2, and depth-related metatranscriptomics to characterize the bloom’s community structure, associated bacterial dynamics, and the metabolic strategies employed by the Tripos community. In addition, community composition was integrated with oceanographic data to identify the potential triggers of the bloom’s persistence and decline. The bloom was nearly monospecific and persisted until early August, with environmental factors explaining 53.3% of community variance. Associated bacterial communities displayed dynamic responses over the course of the bloom, with compositional and network shifts corresponding to the bloom trajectory. Functional predictions indicated enrichment of pathways for organism matter utilization and stress response during bloom peak, followed by increased activity in recycling and degradation pathways during bloom decline, highlighting the connection between Tripos bloom abundances and microbial ecosystem dynamics. Depth-dependent metatranscriptomic analyses revealed significant transcriptional partitioning across the mixed layer. Above-MLD populations were enriched in genes associated with stress response, protein turnover, membrane transport, cytoskeletal organization, and cellular signaling, suggesting active physiological responses to dynamic surface conditions. Below-MLD populations exhibited enrichment of domains associated with intracellular signaling, ion transport, metabolic regulation, and alternative energy acquisition pathways, including bacteriorhodopsin-like proteins. Overall, these three chapters create a cohesive story emphasizing the versatility and adaptable metabolic strategies of Tripos muelleri, the environmental factors driving bloom persistence and decline, as well as the bacterial community’s response to a near-monospecific, novel bloom. These finding provide a foundation for understanding the ecological interactions and community-level functional responses associated with a novel dinoflagellate bloom in the Gulf of Maine.

Document Type

Dissertation

First Advisor

Elizabeth L Harvey

Second Advisor

Ashley Bulseco

Third Advisor

David Plachetzki

Department or Program

Biological Sciences

Degree Name

Doctor of Philosophy

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