Date of Award


Degree Type


Degree Name

Doctor of Philosophy in Oceanography


Biological Oceanography



First Advisor

Tatiana Rynearson


Phytoplankton are important primary producers at the base of the marine food web. Diatoms are one of the most diverse groups of phytoplankton. Compared to other phytoplankton, diatoms have higher nutrient requirements due to their larger size, and as such, their distribution is limited by nutrients. Because of this, diatoms thrive in high-nutrient ocean regions, such as coastal and high-latitude waters, where the bulk of prior diatom research has occurred. However, diatoms are also found in low-nutrient oligotrophic waters, where they can form blooms and contribute up to 30% of total annual carbon sequestration. Many studies have documented the metabolic capabilities of diatoms that allow them to persist in low-nutrient oligotrophic regions. One example, a diatom diazotroph association (DDA), has been studied from the perspective of the symbiont, but the role of the diatom host has yet to be explored. While diatom metabolic capabilities are documented, little is known about their diversity and distribution in these regions. This dissertation uses a variety of sequencing, culturing, and lab-based methods to investigate the presence, diversity, and associations of diatoms with varying nutrients to reveal the role of diatoms in oligotrophic waters.

Diatom diversity and distribution along a 1250 km transect from the low-nutrient offshore Sargasso Sea and Gulf Stream to the high-nutrient coastal waters of the U.S. Northeast Shelf were analyzed via metabarcoding in Chapter 2. Here, diatom-specific primers were used to amplify the diatom community by denoising sequences into amplicon sequencing variants (ASVs), akin to species-level differences. Several genera were exclusively found in low-nutrient waters including Bacteriastrum, Haslea, Hemiaulus, Pseudo-nitszchia and Nitzschia, and 70% of ASVs were exclusively found in only the two offshore regions. Diatom richness was similar within the offshore and the nearshore regions, but diatom community composition differed by region and was correlated with phosphate and temperature. These findings suggest offshore oligotrophic regions harbor diverse assemblages of diatoms, different from those of high-nutrient coastal waters. The metabolic capabilities of diatoms residing in only the low-nutrient offshore regions require further investigation.

One diatom species, found only in the offshore region in Chapter 2 and isolated from the oligotrophic Sargasso Sea, was part of an association with a diazotroph symbiont. This Hemiaulus hauckii-Richelia intracellularis DDA was grown long-term in media without a source of nitrogen, like that of the low-nutrient waters of the Sargasso Sea and used to probe the limits of the symbiosis and the role of the diatom host in the symbiosis. In Chapter 3, the DDA was grown with added nitrogen in the form of either nitrate (10 µM) or ammonium (10 µM), and compared to a control (no added nitrogen) seeking to potentially bypass the role of the nitrogen-fixing symbiont in the DDA. Cell counts and fluorescence were measured daily. At the final timepoint, stable isotopes were used to measure nitrogen and carbon fixation, cell counts were used to calculate growth rates, and dissolved and particulate nutrient samples were collected. Nitrogen fixation rate was higher in added ammonium compared to the control. Growth and rates of the DDA were also higher in the presence of both nitrate and ammonium, despite no significant uptake of nitrate from the media. The response of DDAs to added nitrogen indicates an obligate symbiosis in which the role of the nitrogen-fixing symbiont is not evaded, but rather stimulated by an outside source of nitrogen.

To investigate the metabolic role of the diatom host with added nitrate or ammonium, we analyzed the gene expression of Hemiaulus hauckii in Chapter 4. To do this, RNA samples were extracted and sequenced on an Illumina NovaSeq 6000 (2 x 100 bp) from the same sample flasks used for the DDA physiology experiment in Chapter 3. From these sequences,high-quality (95% BUSCO completeness) de-novo transcriptome of the diatom host (Hemiaulus hauckii) in a DDA was assembled, which represents the first assembled transcriptome of this organism. The Hemiaulus hauckii de-novo assembly was used to investigate the role of the diatom host cell via differential gene expression in the same added nitrogen treatments as Chapter 3. Although the DDA had higher growth, gene expression in the nitrate treatment was very similar to the control, with few genes differentially expressed. Contrastingly, with added ammonium, a source of added nitrogen readily taken up by the DDA, gene expression was dissimilar from the control, and many more genes were differentially expressed. In both added nitrogen treatments, many differentially expressed genes were related to carbohydrate metabolism, suggesting increased growth and carbon sources for the symbiont. Gene regulation analysis of the host cell points to a potential exchange of glutamate from the host cell, hypothesized to be exchanged for glutamine from the symbiont. Discerning the response of the DDA host to added nitrogen reveals the exchange of metabolites with and regulation of the symbiont, in a symbiosis important for global carbon and nitrogen cycling. Collectively, these studies investigate the ecological role of diatoms across nutrient regimes, with a case study of the response of an oligotrophic diatom-diazotroph association to added nutrients.

Available for download on Sunday, January 19, 2025