Date of Award


Document Type

Honors Thesis (Open Access)


Colby College. Biology Dept.


Peter D. Countway

Second Advisor

Denise A. Bruesewitz

Third Advisor

Christopher M. Moore


Pseudo-nitzschia is a pennate marine diatom that produces the neurotoxin domoic acid (DA). Domoic acid accumulates in the digestive tract of filter feeders and becomes concentrated at higher trophic levels. When DA concentrations reach critical levels, consumption of toxic shellfish can induce a condition known as Amnesic Shellfish Poisoning (ASP), which has been responsible for the deaths of marine mammals, seabirds and humans. This study worked to respond to a toxic Pseudo-nitzschia bloom in the Gulf of Maine during the fall of 2016, with the goal of quantifying and identifying Pseudo-nitzschia spp. in the bloom region and to determine the role of macronutrients (N, P, Si) in cell growth and toxin production. Cell densities were quantified using quantitative PCR targeting the 18S rDNA gene for samples collected from 29 sites along the coast of Maine. The average gene copy number per cell in Pseudo-nitzschia pungens was determined to be 79.4 ± 10 18S rDNA genes. Cell concentrations were compared to nutrient and domoic acid concentrations at each bloom site. The relationship between individual nutrient concentrations and toxin concentrations was inconclusive regarding the role of individual nutrients limiting Pseudo-nitzschia growth, but examining nutrient stoichiometry in relation to the Redfield ratio offered evidence that silica limitation was the primary driver of toxicity in the bloom. To further examine this theory, three nutrient limitation experiments were conducted, subjecting cultures of Pseudo-nitzschia pungens to phosphate and silicate limitation to examine how cell growth and domoic acid production were affected. While macronutrient limitation halted cell growth in all experiments, DA production was not detected in any lab-cultivated Pseudo-nitzschia pungens. Further experimentation should repeat nutrient limitation experiments with the confirmed toxin-producer P. australis, a species known to be present in high concentrations during the 2016 bloom event, but not obtained in culture.


Pseudo-nitzschia, harmful algal bloom (HAB), domoic acid, amnesic shellfish poisoning (ASP), nutrient limitation, macronutrient