Functional roles of biominerals during virus infection of Emiliania huxleyi
Creators: name: Johns, Christopher Thomas
Publisher: name: No Publisher Supplied
Year: 2023

description: A defining characteristic of marine coccolithophores are their production of calcium carbonate coccoliths, which impact both global carbon flux and our ability to view these planktonic protists from Earth-observing satellites. While the production and shedding of these coccoliths has garnered significant attention over the years, the actual mechanism, function, and purpose of calcification remains somewhat uncertain. Several hypotheses have been proposed to explain the ecophysiological roles of coccoliths, such as deterring predators (i.e. viruses and grazers) and enhancing photosynthesis. Emiliania huxleyi is the most abundant globally distributed coccolithophore, which can form massive mesoscale blooms in the North Atlantic. These blooms are routinely terminated by viruses, triggering a massive release of coccoliths. Currently, we have a limited understanding of the interplay between calcification on virus infection, as well as the ecosystem impacts of free coccoliths. The aim of my PhD thesis work is to better characterize the role of calcification in cellular and ecosystem processes, especially the interactions of free coccoliths with both cells and viruses. My main objectives are: 1. to contextualize and quantify the microscale interactive dynamics between free coccoliths, cells, and viruses; and to elucidate cellular responses to free coccoliths. 2. to identify host-derived infochemicals that trigger coccolith release.My dissertation will be divided into three chapters, which will individually characterize specific components of these objectives. After a brief introduction:Chapter 1 demonstrates the protective role calcification plays against viral infection. This work utilizes various calcified strains isolated from coastal fjords near Bergen, Norway, grown at different concentrations of calcium chloride (10 mM and 0.1 mM). I characterized the effects of calcium availability on cellular calcification using established flow cytometric methods, PIC analysis, scanning electron microscopy (SEM), and lipid analysis. Following these analyses, I tested the infectivity of both non-calcified phenotypes of the same strain to determine the protective potential of calcification. Chapter 2 elucidates the ecosystem consequences of free coccoliths and their implications for cell-coccolith and virus-coccolith interactions. We show observational and experimental evidence, supported by biophysical modeling, that free coccoliths are highly adsorptive biominerals that readily interact with cells to form chimeric coccospheres and with viruses to form ‘viroliths’ which facilitate infection. Adsorption to cells is mediated by organic matter associated with the coccolith base plate and varies with biomineral morphology. Biomineral hitchhiking increases host-virus encounters by nearly an order of magnitude and can be the dominant mode of infection under stormy conditions, fundamentally altering how we view biomineral-cell-virus interactions in the environment. Chapter 3 characterizes host-derived extracellular proteases induced by the presence of viruses and their impact on cell physiology. Results from Chapter 2 demonstrated that uncharacterized dissolved infochemicals are produced by the host after sensing the presence of viruses. This infochemical causes massive coccolith shedding, rendering cells susceptible to infection. We identified that this infochemical is an extracellular subtilisin-like protease, which can induce the production of extracellular vesicles (EVs). These EVs are subsequently adsorbed by cells, which induces cell stress responses, ultimately triggering program cell death. The cell stress response is the root cause of coccolith shedding. Furthermore, coccoliths can adsorb extracellular serine proteases and help accelerate cell death.