Molecular characterization and surface reactivity of cyanobacteria and their lysates
Creators: Swaren, Logan R
Publisher: name: University of Alberta Library
Year: 2022

<p>Cyanobacteria are among the most abundant primary producers in the oceans and are present in nearly every environment on the planet. They were one of the first organisms to evolve nearly 3.8 billion years ago, making them excellent analogs for the investigation of ancient ocean chemistry. The high density of proton-active functional groups on their surfaces makes them important in binding marine trace metals. Moreover, they are important components of the marine food web, as both intact cells and secondary organic matter. The distribution of cyanobacteria has a profound influence on trace metal speciation in modern and ancient oceans. This dissertation explores the impact of pH and ionic strength on the surface reactivity of cyanobacteria and investigates the reactivity and composition of organic matter derived from cell lysis. <br>
    In Chapter 1, I review our understanding of cyanobacterial surface reactivity, trace metal cycling in the modern marine environment, and the roles of particulate and dissolved organic matter (POM and DOM, respectively).<br>
    Chapter 2 focuses on the influence of solution ionic strength (IS) on the surface reactivity of the freshwater cyanobacteria Synechocystis. As particulates, including planktonic cyanobacteria, move from more terrestrial freshwater environments to more saline marine environments, they hold the potential to transport and release trace metals. The surface reactivity is investigated using common surface chemistry approaches, namely: (i) acid-base titrations, (ii) zeta potential measurements, and (iii) Cd adsorption experiments under simulated freshwater IS (0.01 M NaCl) and marine IS (0.56 M NaCl). The acid-base titrations and Cd adsorption experiment results are paired to produce a surface complexation model (SCM) for the cyanobacterium. Synechocystis demonstrated a more negative surface charge and a higher affinity towards Cd across the entire pH range studied. This suggests that Synechocystis, due to their ubiquitous distribution in aquatic environments, as well as other planktonic cyanobacteria could play an important part in trace metal cycling especially in environments with large salinity gradients such as estuarine and deltaic settings.<br>
    In Chapter 3, I measure the buffering capacity and reactivity of organic detritus produced from the mechanical lysis of the common marine cyanobacterium Synechococcus (referred to as cyPOM and cyDOM), through proton adsorption and release experiments using potentiometric acid-base titrations. The results are compared with existing literature on modern DOM samples from a variety of environments (e.g., terrestrial, marine, lab cultures). My results suggest that cyPOM is relatively unimportant in trace metal transport to the ocean floor even though it makes up an increasing portion of POM with depth relative to the intact cyanobacterial cells. When cyDOM is included in the system, even at much smaller amounts (e.g., 10:1 Synechococcus:cyDOM), the DOM dominates metal speciation as observed in nature.<br>
    In Chapter 4, I investigate the composition of cellular DOM derived from two extant cyanobacteria, the marine Synechococcus and euryhaline Synechocystis, using electrospray ionization (ESI) Fourier transform ion cyclotron resonance (FTICR) mass spectroscopy. Synechococcus- and Synechocystis-derived DOM contained 1548 and 1474 unique compounds, respectively, with &gt;80% of assignments constituting nitrogen-containing molecules with a significant portion of sulfur- and phosphorous-containing ligands, ~35% and ~37%, respectively. When compared to terrestrially-derived DOM, the cyanobacterial-derived DOM also has lower oxygen/carbon and higher hydrogen/carbon ratios. Interestingly, despite cyanobacteria comprising the major producers of marine DOM today, their contribution to the marine DOM pool appears less significant than DOM derived from land. However, in the Precambrian oceans – before the evolution of land plants – cyanobacterial-derived DOM was likely the primary source of the marine DOM pool. This means that the Precambrian DOM pool would consist chiefly of more reactive nutrient-rich (N, P, and S) molecules with a high affinity to bind trace metals<br>
    Chapter 5 summarizes our current understanding of marine dissolved organic matter and I propose future research directions. </p>