Date of Graduation

5-2016

Document Type

Thesis

Degree Name

Bachelor of Science

Degree Level

Undergraduate

Department

Biological Sciences

Advisor

Alverson, Andrew

Reader

Jennings, Jackson

Second Reader

Kayser, Casey

Third Reader

Thallapuranam, Suresh

Abstract

Diatoms are single-celled micro-algae with cell walls composed of silica, that reproduce in a way that results in a decrease in cell size after each round of mitotic (asexual) division. A cell cannot continue to shrink indefinitely, so when the average size of the population reaches a critical threshold, diatoms reproduce sexually and restore their maximal size. It is unclear, however, how frequently diatoms undergo sexual reproduction in nature. As a proxy for sexual reproduction, I monitored changes in cell size of a population of the diatom Stephanodiscus niagarae from the plankton community of Lake Fayetteville (Arkansas, USA). Weekly sampling during the Fall 2015-Spring 2016 period revealed that S. niagarae is a winter species, occurring after a shift in the planktonic community of Lake Fayetteville, in which populations of warm-water green and blue-green algae decline and are replaced by a diatom-dominated community. S. niagarae first occurred in December, in clear colder water at 10oC or below. Within weeks, this population grew steadily from a few hundred to several thousand cells per milliliter, reaching a maximum density in late January. The population declined rapidly thereafter. Although this decline coincided with increased lake temperature and the proportion of cells infected by a chytrid parasite, it is not clear if these two factors are the proximal cause for S. niagarae declines. Asexual reproduction resulted in substantial variation in average cell size, and an increase in variance towards the end of the growth period. However, the modest variation in median cell diameter of about 5 μm indicated that a synchronous population-wide sexual reproduction event did not occur during the course of the study. These results suggest that the frequency of sexual reproduction in S. niagarae in Lake Fayetteville might be on the order of several to dozens of years. However, the possibility of asynchronous sexual reproduction and a population with overlapping generations cannot be ruled out.

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