Date of Award

May 2023

Degree Type

Thesis

Degree Name

Master of Science

Department

Biological Sciences

First Advisor

John A Berges

Committee Members

Erica B Young, Rebecca D Klaper

Keywords

Adsorption, Combined toxicity, Freshwater microalgae, Glyphosate, Microplastics, Photosynthesis

Abstract

Microplastics (plastic particles 0.1 μm - 5 mm) are a major ecological problem in terrestrial and aquatic environments. While research on the effects of plastics on organisms has steadily increased, much less is known about the effects of microplastics that have been exposed to a secondary harmful contaminant, such as the herbicide, glyphosate (here referred to as “treated microplastics”). Microplastics that have not been so exposed (“pristine microplastics”) have typically been used in microplastic toxicity studies. A wide range of organisms have been used in studies of the effects of microplastics, but microalgae, which lie at the base of aquatic food webs, continue to be understudied. To understand the interaction between microplastics and contaminants with respect to their effects on microalgae, batch cultures of the chlorophyte alga Chlamydomonas reinhardtii were grown in saturating irradiance (100 µmol quanta m-2 s-1, 16:8 L:D cycle, 18 °C) under nutrient-replete conditions and exposed to treatments of pristine polyethylene microspheres (45-53 µm diameter, 0.1 g L-1 or 1 g L-1), dissolved glyphosate (220 µM or 44 µM), or polyethylene microspheres treated with glyphosate (220 µM or 44 µM), over a 14-day exposure. Growth was monitored using in-vivo Chlorophyll a fluorescence and flow cytometry, while measurements of photosynthetic quantum yield (“Fv/Fm,” using DCMU) and the proportion of dead cells (“mortality,” using the stain SYTOX-Green) were also determined. The main effect of glyphosate was significant and resulted in treated cultures to have significantly lower growth rates and quantum yield than cultures compared to control cultures without glyphosate exposure (µ of 0.99 d-1 versus 1.01 d-1 respectively and Fv/Fm of 0.47 versus 0.58 respectively, p<0.05 in both cases). The main effect of microplastics was also significant and also resulted in decreased quantum yield compared to cultures not exposed to plastics (Fv/Fm of 0.45 versus 0.62 respectively, p<0.05). However, exposure to microplastics actually increased growth rate and reduced mortality compared to cultures not exposed to plastics (µ of 1.09 d-1 versus 0.94 d-1 respectively and dead cells mean of 1.23 % versus 3.01 % respectively, p<0.05). The maximum biomass (“plateau,” measured as chl a fluorescence or cell numbers) achieved by cultures were not different. Glyphosate and microplastics did not interact in terms of significantly modifying each other’s effects. The findings contrast with others in the literature and show that the combined impact of microplastics and other contaminants on an organism is complex and dependent on various factors such as the concentration and characteristics of the contaminants as well as the type of test organism and the parameters being studied. Future work should focus on assessing the effects of different types and forms of plastics in combination with other contaminants.

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