Date of Award

Winter 2012

Document Type


Degree Name

Doctor of Philosophy (PhD)


Ocean/Earth/Atmos Sciences

Committee Director

Alexander B. Bochdansky

Committee Member

Fred C. Dobbs

Committee Member

Margaret R. Mulholland

Committee Member

David T. Gauthier


The dark ocean is vast, high in pressure, cold, and scarce in resources, but has been shown to support a diverse and active microbial community wherever it is studied. Such studies, however, are scarce due to the difficulty of sampling at such depths, and are difficult to interpret due to compounding effects of pressure and temperature on physiology. Protists, functionally defined as the microbial portion of the domain Eukarya, are particularly neglected in studies of deep-sea microbiology. Here, I present three studies on microbial eukaryotes in the deep sea: first, a study of the abundance of microbial eukaryotes in the deep sea, second, a quantitative approach to study broad-scale diversity in the deep sea; and last, a series of experiments to explore the effect of deep-sea conditions on surface-isolated flagellates. In the deep sea, I found that eukaryote abundances decrease much more sharply than prokaryote abundances with depth, though most of this decrease occurs in the upper 1000 m, below which eukaryote abundance is relatively constant. In water masses below 1000 m, 50-70% of total eukaryotes detected by CARD-FISH can be attributed to one of the seven groups (six taxonomic using CARD-FISH and one by morphology when stained with DAPI) . In the epipelagic 100 m samples, only 20% of total eukaryotes fall into one of these groups. This difference is driven largely by the morphotype I call the "split nucleus", which does not decrease in absolute abundance with depth, instead increasing in its proportion of the eukaryotic population in deeper waters. Lastly, I found that eukaryotic microbes, typified by two heterotrophic flagellate species which appear to be ubiquitous in the world's oceans, can survive and even grow despite long-term exposure to the cold, high-pressure conditions of the deep sea, indicating that protists transported to the deep sea by advection or on particles can seed populations there.