Strategies of Resource Allocation by Diatoms under Dynamic Light

University of Technology Sydney. Faculty of Science.Diatoms are the evolutionarily youngest phytoplankton group and considered to be the most productive across diverse ocean, coastal and freshwater environments. Based on their evolutionary history in diverse environments, diatoms have acquired unique diverse mechanisms to cope with fluctuating availability of resources required for cellular maintenance and growth. Yet how these mechanisms actually operate to moderate metabolic functioning by the energetic tracking of light energy to carbon capture – commonly measured as “emergent signatures” or photosynthesis rates via fluorescence, O2 evolution and/or CO2 uptake –remains somewhat of a black box. This thesis addresses the response of diatoms to light, with particular emphasis on the gaps in current energy budgets that quantify trade-offs in O2 evolution and carbon-assimilation. An initial assessment of a variety of diatom species revealed distinct categories of photo-protective capacities (i.e. nonphotochemical quenching) that correlated with ecological niche, i.e. taxa originating from estuarine, coastal and open ocean environments. Low capacity to dissipate light energy via nonphotochemical quenching by open ocean diatoms was compensated for by an upregulation of midstream oxidase activity highlighting a key trade-off between light harvesting and light utilization strategies. Diurnal monitoring of diatoms with divergent photo-protective capacities further revealed species-specific dynamic respiratory trends, whereby diatoms with high nonphotochemical quenching capacity exhibited more dynamic RDARK while diatoms with low nonphotochemical quenching capacity exhibited more dynamic light-dependent respiration (LDR). Fluorescence-derived measures of photoacclimation (Ek,YII) were found to be significantly correlated to oxygen cycling and carbon retained as biomass. Subsequent metabolomic profiling provided deeper insight into these processes via the underlying light-driven metabolite reorganisation. Using the model coastal diatom (T. pseudonana), high light metabolic profiles were reflective of pathways that support higher growth rate (e.g. glycolysis and TCA cycle) compared to low light metabolic profiles associated with carbon conserving pathways (e.g. gluconeogenesis and glyoxylate cycle). Together these outcomes uncovered previously hidden dynamics of energy processing by diatoms –including dynamic respiration rates between taxa and with time of day, which also mapped differences in inherent metabolic pathways as well as “emergent” metabolic signatures (e.g. fluorescence, O2 and CO2 measures of primary productivity). Combining information from cellular currencies (fluorescence, oxygen and carbon) thus provides a more robust mechanistic understanding of metabolic processes. This thesis has created a foundation for future research to compile more comprehensive energy budgets and a framework for improved estimates of primary productivity models