Date of Award

May 2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Biological Sciences

First Advisor

John A Berges

Committee Members

Erica B Young, Douglas Steeber, Harvey Bootsma, Christopher Klausmeier

Keywords

allelopathy, cell death, flow cytometry, grazing, phytoplankton, Sytox Green

Abstract

Phytoplankton are found in dynamic aquatic environments, subjected to variations in abiotic (i.e., light, temperature, nutrients) and biotic (i.e., grazing, species interactions) conditions. Phytoplankton responses to these environmental variations are typically observed as changes in chlorophyll a (a surrogate for biomass), but such measurements do not provide information about individual taxa and cannot provide information about cell condition, for example, whether phytoplankton are living or dead. Furthermore, in examining the dynamics of phytoplankton, emphasis is placed on growth, but consideration of loss processes is usually limited to grazing and sedimentation, and not cell death The present study examined cell death in phytoplankton at a taxon-specific level in response to abiotic and biotic factors. A combination of intensive field sampling in a local urban park pond and complementary laboratory experiments were used to correlate taxon-specific abundances of phytoplankton (living and dead cells, using a combination of flow cytometry and mortal staining) with temperature, light and nutrient concentrations. In addition, the effects of grazing by zooplankton and allelopathic interactions among species were explored in field manipulations and laboratory co-culture experiments, respectively. There were few strong, consistent correlations among abiotic parameters and abundances of living or dead phytoplankton cells. Changes in temperature and light were found to have some effect on phytoplankton cell death, with higher proportions of dead cells observed for some phytoplankton groups during periods of low temperatures or light. Other abiotic conditions, specifically nutrient concentrations, did not correlate with changes in phytoplankton abundance or cell death, consistent with previous studies. Thus, abiotic factors do not appear to be major drivers of cell death within the phytoplankton community of the local urban pond. In contrast, correlations were observed among taxa, especially among abundances of particular taxa and dead cells of another, suggesting that biotic interactions among phytoplankton may be influencing changes in the community. The effect of grazing on phytoplankton cell death was examined by using natural assemblages from the pond and varying concentrations across a 2-fold gradient. Little effect of grazing was found on any taxa in late summer/early autumn over two years, and grazing did not lead to the production of dead cells. Possible allelopathic relationships were studied using laboratory co-culturing experiments for the cyanobacterium Microcystis aeruginosa, the chlorophyte Chlamydomonas reinhardtii and the diatom Synedra, each belong to groups implicated in field studies. These experiments did not show evidence of allelopathic interactions. While a specific driver was not identified in promoting cell death or changing phytoplankton abundances, these studies did provide further evidence that abiotic factors seem to be less important to cell death than biotic ones. Grazing by zooplankton does not directly cause phytoplankton cell death, though there is indirect evidence of allelopathic interactions, but we have not been able to demonstrate this in the species tested so far.

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