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Title: A theoretical investigation of phytoplankton resource allocation, growth and photosynthesis in variable light environments
Author: Talmy, David
Awarding Body: University of Essex
Current Institution: University of Essex
Date of Award: 2013
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By fixing nutrients into biomass, marine phytoplankton influence ecosystem function, ocean biogeochemical cycles and the earth system. Thus, quantitative representations of phytoplankton activity are essential if we are to understand the current, and project the future, state of the earth . The last century has seen huge advances in the scale of observation, theory and experimentation concerning marine phytoplankton. Yet: large scale projections of earth system processes usually rely heavily on empirical parameterisations. The manner in which individuals, populations and communities adapt to climate change depends on a suite of biological mechanisms that may not be fully accounted for with purely empirical parameterisations. The overarching aim of this thesis is to develop a theoretical modelling framework for the estimation of phytoplankton growth and photosynthesis by accounting for organism adaptation to its environment, and mechanistic biological constraints. The thesis starts with an introduction to the science area (chapter 1) and a review of quantitative systems approaches to ecosystem modelling developed in the second half of the previous century (chapter 2) , with emphasis on representations of phytoplankton growth and photosynthesis. The third chapter presents a model that assumes organisms optimally partition resources to a suite of components involved in resource acquisition and growth, subject to a set of physiological constraints. The model is used to predict the manner in which resources are (re)distributed to suit an environment in which the abiotic conditions do not vary with time. The model is found to be a poor predictor of observed stoichiometry and rates of photosynthesis observed in diatom species thought to occupy more variable environments. In the fourth chapter, the theoretical environment in which hypothetical cells must balance resource allocation is refined to represent more dynamic ocean regimes. Results indicate that resource allocation can differ substantially when organisms are adapted to either a static, or a dynamic environment. Furthermore, comparison with observations from different ocean regimes indicates that the degree of environmental variability profoundly influences the ability of the model to predict plasticity of photosynthesis-irradiance relationships and cellular chlorophyll. The manner in which observed biological rates vary with organism size is of interest to earth systems scientists, in part because sin king of particles to the deep ocean depends on community size distribution. However, reliable represent at ions of the size dependence of growth and photosynthesis have long eluded ecosystem modellers. In the final theory chapter, the model is altered to account for a size related constraint on the capacity to store carbon. Using optimality arguments similar to early chapters, the model is used to generate predictions of growth and photosynthesis of phytoplankton size classes exposed to variable conditions. Results indicate that enhanced storage capacity can raise the daily average growth rate of large phytoplankton in environments that involve prolonged light limitation. Furthermore, optimal resource allocation to components of the cell responsible for carbon fixation can be higher in large organisms with greater storage capacity. Comparison of model predictions of photosynthetic rate in different size classes agree with observations from a eutrophic ecosystem. The overarching conclusions of this thesis are two fold. First, organism adapation to its environment is likely to depend on the degree of spatio-temporal variability in environmental variables, not just daily averaged values. To the best of my knowledge, this has not before been shown in a quantitative framework that links subcellular biological constraints with environmental variability and observations made both in 'vitro and in situ. Second, constraints on carbon storage that in some cases are related to organism size are likely to interact with changes in the environment in a manner that influences the size dependence of biological rates. This may be the first time that carbon storage limitations have been linked with organism growth and photosynthetic rates in variable light environments. The thesis finishes with a discussion of the significance of the results to large scale simulations of ocean biogeochemistry.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available