- Title
- Modelling the starch, TAGs and functional biomass kinetics of green algae as a function of nitrogen concentration and light flux
- Creator
- Shamsuddin, Fahim Murshed
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Algae as a potentially sustainable source of fuel, feed and nutraceuticals has led to interest in the algae biorefinery concept for production of sustainable products. The cultivation of algae has several advantages as algae can grow in non-potable water and in non-arable land and can sequester CO₂ from flue gas. In addition, different components of the algae produce different value-added products such as biodiesel, which is obtained from TAGs, bioethanol from starch and animal feed from functional biomass. However, there is a lack of commercial viability for photosynthetic algae due to limitations in biomass productivity and the capacity to optimise the chemical composition of the algae, both of which are caused by suboptimal nutrient loading and low light conditions. Therefore, this thesis aims to understand how to optimise the production of the individual biochemical algae components, namely, starch, TAGs and functional biomass by modelling batch photosynthetic cultures. Special attention was given to the influence of external nitrogen and light conditions on composition kinetics. The primary species studied in this thesis is Chlorella vulgaris for varying nitrogen load and Scenedesmus obliquus for varying light flux. Biochemical measurements on S. Obliquus for different light conditions were reported and compared with the simulation model. Finally, trial experiments on the effect of glucose relative to a photosynthetic control on algal growth were also performed on Nannochloropsis oculata and Tetraselmis Chuii. A review of algal composition modelling was carried out to assess the current gap in modelling functional biomass, starch and TAGs/lipids as a function of light and extracellular nitrogen. It was found that batch and fed batch studies simulated the algae as being composed of only two fractions, lipids and functional biomass. However, the starch fraction was omitted in most published work, although it is a major component of algae and possesses different kinetic behaviour from lipids and functional biomass. Nevertheless, there were some studies of continuous cultures on starch, TAGs and functional biomass, but to date there are no batch mode studies on photosynthetic cultures that model starch, TAGs and functional biomass simultaneously. A mathematical model of starch, TAGs and functional biomass kinetics of C. vulgaris was developed to elucidate the primary mechanisms of photosynthetic kinetics. This was achieved by explicitly modelling starch, TAGs and functional biomass fractions for a batch photosynthetic reactor. In addition, TAGs production due to starch glycolysis was also considered in the model. This biochemical pathway has not been previously modelled in the literature. The model was composed of six coupled ordinary differential equations and 11 secondary equations and was coded and solved in Matlab. The parameters were tuned by minimising an objective function, which was the overall relative error of the model. There were 15-17 biological fitting parameters in the model depending on the limiting conditions affecting starch glycolysis. Four sets of limiting conditions were identified for modelling starch glycolysis and TAGs synthesis from photosynthesis. The first study carried out was the analysis of the influence of extracellular nitrogen on composition kinetics. The nitrogen quota and light dependant photosynthetic activity were found to be primary regulators for starch-TAG kinetics in C. Vulgaris. In contrast metabolite saturation, C-N uptake ratio and starch concentration did not have a significant impact on composition kinetics. Once the influence of nitrogen concentration was investigated, the following step was to test if starch glycolysis was dictating composition kinetics under the influence of moderate to high light intensities. In order to model the influence of carbon uptake for different light fluxes, an exponential decay function, in contrast to the Poisson’s single hit model used in the previous chapter for C. vulgaris, was utilised. The species that was modelled was S. obliquus and the light intensities tested were 200 µmols/m².s and 1000 µmols/m².s. The selection of these conditions was due to the existence of experimental literature values that allowed the validation of the model. The model predicted the experimental data with an average percentage error of 10 % for 200 µmols/m².s and 17 % for 1000 µmols/m².s. The only discrepancy between the model and experiment was for starch concentration for 1000 µmols/m².s light intensity. The origin of this discrepancy was attributed to not accounting for metabolite catabolism of TAGs. This occurred at high light intensities because the culture may have reached a point in its growth phase, where substrate catabolism began. Experimental work was carried out to investigate the composition of S. obliquus for different nitrogen concentration and light conditions. The starch, TAGs, protein and chlorophyll contents were in good agreement with literature values and batch culture model predictions. This agreement between modelling and experiments was arguably attributed to the fact that nitrogen quota, which dictates starch, TAGs and protein kinetics, is independent of reactor running mode, i.e., batch or continuous. Preliminary experiments for mixotrophic algae were conducted at The Port Stephens Fisheries Institute (PSFI) to assess whether mixotrophic cultivation of green algae could improve yields relative to a photosynthetic control. The species that were investigated included N. oculata and T. Chuii. During the first 7 days of growth, cell densities for mixotrophic and phototrophic algae were similar. After this initial period, cell densities of mixotrophic algae were half the density of the photosynthetic culture. Bacterial contamination was the primary reason for a lack of improvement in cell density. It was hypothesised in this thesis that starch glycolysis linked starch degradation to TAGs accumulation during nitrogen depletion. This hypothesis was tested for two different species, C. vulgaris and S. obliquus for different nitrogen and light levels. It was found that starch glycolysis could explain the kinetic behaviour of algae during nitrogen depletion and light intensities in the light limited and saturated region for batch photosynthetic studies. Currently, this has not been reported in literature models. Future studies may attempt to validate this model for species other than green algae or for different growth modes such as heterotrophy or mixotrophy using acetate as a carbon source due to its ability to inhibit bacterial growth.
- Subject
- green algae; fuel sources; biorefineries; algae cultivation
- Identifier
- http://hdl.handle.net/1959.13/1408704
- Identifier
- uon:35874
- Rights
- Copyright 2019 Fahim Murshed Shamsuddin
- Language
- eng
- Full Text
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