The Role of the Silica Frustule in Diatom Carbon Acquisition and Photosynthesis

Author: Tamsyne Smith-Harding

Smith-Harding, Tamsyne, 2018 The Role of the Silica Frustule in Diatom Carbon Acquisition and Photosynthesis, Flinders University, College of Science and Engineering

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Diatoms are arguably the most ecologically successful group of eukaryotic microalgae in aquatic systems. Fixing 25-50% of the 50 billion tonnes of organic carbon generated in the oceans annually, they dominate marine primary productivity. Although inorganic carbon is plentiful in the oceans, 90% is present as HCO3- and <1% present as CO2, the substrate for carbon fixation by ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBISCO). RUBISCO has a low affinity for CO2 and all known RUBISCOs are sub-saturated at present day levels. Carbon concentrating mechanisms (CCMs) act to increase the concentration of CO2 at the active site of RUBISCO. In microalgae, including some diatoms, one such mechanism involves the dehydration of HCO3- at the cell surface by external carbonic anhydrase (CAext), maintain a high equilibrium of CO2 available for uptake at the plasmamembrane. Since H+ exchange with water is slow, a pH buffer is necessary to maintain high catalytic rates. It has been suggested that the silica frustule may act as a buffer for CAext in diatoms and that CAext activity is modulated by the silica content of the frustule. The overall objective of this thesis was to determine the role of the frustule in CAext activity, overall CCM function and photosynthesis in the cosmopolitan marine diatom Chaetoceros muelleri. Chapter 1 investigated the role of CAext in inorganic carbon (Ci) acquisition and photosynthesis over the course of growth in C. muelleri. Whilst CAext activity increased over time in response to CO2 depletion, the role that it played in Ci acquisition for photosynthesis was variable. Specifically, CAext-mediated Ci supply increased between the first two sampling points, but was negligible later in the growth phase, where Ci acquisition was likely by direct uptake of HCO3- by anion exchange transporters at the cell surface. Chapter 2 explored the impacts of silica limitation, which produces poorly silicified frustules, on CAext activity, overall CCM function and photosynthesis. Silica-limited C. muelleri cells had less heavily silicified frustules and were twice the size of their silica-replete counterparts. Although CAext activity did not differ between silica treatments when the difference in cell size was accounted for, overall CCM function was greater in silica cells. Since larger cells are more prone to CO2 limitation the increase in cell size, rather than frustule silica content, likely regulates CAext activity in C. muelleri. Since the operation of a CCM may act as a sink for excess light energy, Chapter 3 explored the effects of silica limitation on photoprotective mechanisms and the production of harmful reactive oxygen species. Silica-limited C. muelleri cells were found to be more prone to photoinhibition than silica-replete cells, likely as a consequence of a downregulation of the CCM. Photoprotective mechanisms were upregulated in silica-limited cells. The upregulation of these mechanisms likely protected silica-limited cells from the production of harmful reactive oxygen species. In general, the physiological flexibility displayed by C. muelleri in response to Ci and silica availability likely contributes to the observed dominance of bloom-forming diatoms in coastal environments and may give them a competitive advantage under future climate scenarios.

Keywords: Diatoms, silica, carbonic anhydrase, carbon concentrating mechanisms, Chaetoceros muelleri

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2018
School: College of Science and Engineering
Supervisor: Professor Jim Mitchell