Date of Award

Fall 11-10-2020

Semester of Degree

December

Document Type

Restricted Access Dissertation

Degree Name

Ph.D. in Chemistry

Department

Chemistry

Major Professor

David Kieber

Steering Committee Member

Theodore Dibble

Steering Committee Member

Huiting Mao

Steering Committee Member

Rafel Simo

Steering Committee Member

William Miller

Abstract

Acrylate is produced by many marine phytoplankton in equimolar quantities to dimethylsulfide (DMS) from the enzymatic cleavage of dimethylsulfoniopropionate (DMSP). However, while DMS and DMSP have been studied intensively for more than 30 years, very little is known about acrylate even though it should be prevalent in the oceans and its concentrations and fluxes should be high in coral reefs and blooms of DMSP-rich phytoplankton. To better understand the importance of acrylate in the oceans, I studied the photochemistry and biological consumption of acrylate in seawater.

Acrylate absorbssolar radiation at wavelengths longer than 290 nm, but it did not photolyze in seawater. Instead, acrylate was photochemically produced in seawater when exposed to sunlight, with production rates much faster in coastal waters compared to open-ocean waters. Photoproduction rates were strongly correlated to the UV absorbance of the seawater, suggesting the involvement of chromophoric dissolved organic matter. Apparent quantum yields (AQYs) for acrylate photoproduction decreased exponentially with increasing wavelength, and AQYs were temperature dependent with a mean (± std dev) activation energy, Ea, at 320 nm equal to 21.9 ± 4.0 kJ mol-1 . Acrylate photoproduction peaked at ~330 nm, with ~30% of the total production in the UV-B and ~70% in the UV-A. Using wavelength and temperature dependent AQY data, depth- integrated photoproduction rates were modeled yielding a global photochemical production of 18.0 ± 9.8 Tg y-1 for acrylate in the surface-mixed layer. This corresponded to 1.7% of the total annual production of biologically-labile carbon from the photolysis of marine dissolved organic carbon.

A detailed study was undertaken to examine the acrylate cycle in a coral reef in Mo’orea, French Polynesia. Dissolved acrylate concentrations were low in coral-reef waters and in offshore open-ocean Pacific waters (ca. 1–2 nM), but concentrations were substantially higher (up to ~ 50 nM) in waters close proximity to prevalent coral and macroalgae in the reef. Dissolved acrylate and DMSP were rapidly taken up by heterotrophs in the reef, with mean turnover times of 6 and 3 h, respectively. A large diurnal pattern was seen for the concentrations and the biological consumption of dissolved acrylate and DMSP in the reef, with higher concentrations and faster consumption observed in daytime samples compared to samples collected at night. Given the high cellular concentrations observed in cultures of several coral algal symbionts, these coral-derived substrates are proposed to be important in the ecology of coral reefs. Collectively, this work advances our understanding of the marine acrylate cycle with respect to acrylate photochemistry and its role as a biological substrate in coral reefs.

Available for download on Friday, November 10, 2023

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