Date of Award

Spring 5-13-2017

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Oceanography

Advisor

Lee Karp-Boss

Second Committee Member

Peter Jumars

Third Committee Member

Emmanuel Boss

Abstract

Cell density (mass per volume) is a fundamental physical property of a phytoplankton cell, because it influences sinking, dispersal, and flow-cell interaction. Marine phytoplankton can alter their cell densities, likely through changes in ionic composition in the case of diatoms, and hence control their sinking and floatation rates (Smayda, 1970; Villareal, 1988). Surprisingly, there are no direct measurements of the densities of marine phytoplankton, and, thus, we know little about the relationships between cell density, cell size, and physiological factors, such as growth phase and nutrient and light availability.

Using density-gradient centrifugation with sodium polytungstate-seawater gradients, I measured the cell density of diatom cells and chains. To investigate relationships between cell density, and cell size, I focused on diatoms of the same genus, Coscinodiscus radiatus (two clones with average cell diameters (d) of 55 and 100 mm) and Coscinodiscus wailesii (d = 165 mm). Cell density of the three sizes or species was also modeled as a function of changes in the densities and volume fractions of three cell structural components: the cytoplasm, the cell wall (frustule), and the cell vacuole. Assuming no changes in physiology or frustule thickness (using literature values), I expected that larger cells would be less dense than small cells. Within a single genus (Coscinodiscus), median cell density decreased with increasing cell diameter. Observations and model showed the same general trend, but the range of cell densities predicted by the model was narrower than the range measured for living cells.

Measurements from previous studies have shown enhanced sinking during the stationary phase of growth and under nutrient and light depletion. Thus, I expected that these cells would be denser than exponentially growing, nutrient- and light-replete cells. I studied both species of Coscinodiscus and included a chain-forming diatom, Stephanopyxis turris, to include other species of different morphologies. Contrary to expectation, cell density of C. radiatus decreased during stationary phase, but C. wailesii density did not change significantly. To specifically test for the effect of nutrient depletion on cell density, cells were inoculated into media that represents nutrient conditions in the Gulf of Maine during spring. Nutrients in the media were depleted about 5 days after inoculation. Cell density of C. wailesii increased slightly after nutrient depletion (Mann-Whitney U test, p=0.02; rnr = 1.09 and rnd =1.11 g cm-3 for nutrient-deplete and nutrient-replete cells, respectively). The same trend was observed for S. turris (Mann-Whitney U test, pnr =1.06 and rnd = 1.10 g cm-3). Cell density of C. radiatus, on the other hand, decreased significantly during nutrient depletion (Mann-Whitney U test, pnr =1.21 and rnd =1.19 g cm-3 for nutrient-deplete and nutrient-replete cells, respectively) and during light depletion (Mann-Whitney U test, plr = 1.16 and rld =1.12 g cm-3 for light limited and light-replete cells, respectively).

Within all species, cell densities ranged widely. Results indicate that there are complicated relationships between cell density and physiology, which may vary with cell size and shape. Future studies should include other species and genera to determine if cell density decreases more generally with increasing cell volume, and should seek to determine how nutrient depletion and light limitation can decrease cell density.

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