Limits of Growth of Some Simple Aquatic Plants

LIMITS OF GROWTH OF SOME SIMPLE AQUATIC PLANTS Michelle Low A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, Republic of South Africa, in fulfillment of the requirements for the degree of Doctor of Philosophy in Engineering. Johannesburg, 2016 Declaration I declare that this thesis is my own unaided work. It is being submitted for the degree Doctor of Philosophy in Engineering to the University of the Wit- watersrand, Johannesburg. It has not been submitted before for any degree or examination to any other University. ............................................................................ Michelle Low ....................... day of ........................... year ...................... i Abstract The process of photosynthesis is of great importance as it is the reaction of carbon dioxide (CO2) and water with the help of light, ’free’ energy from the sun, to form useful carbohydrates and oxygen. Photosynthesis is therefore useful both in carbon dioxide mitigation and growing bio-feedstocks towards making biofuel. This thesis aims to address two areas for analysing the photosynthesis process: 1. Looking at the physical limits of the growth; and 2. Improving the production rate of some aquatic plants, such as duckweed and microalgae. To address the first aim, the fundamental concepts of thermodynamics were used to analyse the photosynthetic process. It was found that the theoretical minimum number of moles of photons (NP) required (9–17) is less than the values reported by other researchers, suggesting that the photosynthesis process is highly irreversible and inefficient (operating at 35% efficiency orless). This is because the number of moles of photons will increase with greater process irreversibility (when the entropy generated is greater than zero). If the photosynthesis process is indeed that irreversible then the removal of heat (the heat not used by other cellular processes) by the plant becomes a major prob- lem. It is suggested that transpiration, and other cellular processes, are the processes by which that is done, and it is shown that the water needs of the plant for transpiration would dwarf those needed for photosynthesis. Knowing the fundamental limits to growth could also be of use because if an organism was growing at a rate close to this value there would be no advantage to try to do genetic modification to improve its rate. Following the ideas presented above a spectrophotometer was used not only to obtain the absorption spectrum of algae, but it was also used to grow small samples at specific light wavelengths. The algae species researched was Des- modesmus spp., which, for example, is used to remediate waste water or as a ii source of feedstock for biofuel production. It also tolerates high CO2 concentrations. This simple experimental method demonstrated that a specific light wavelength (in particular the Secomam Prim spectrophotometer) 440 nm was preferred for the algae growth. It was recommended that this specific light wavelength would be best for growth. It might also be useful to know this fact particularly when designing photobioreactors, as this could reduce the amount of heat released into the surroundings and thus make the process more energy efficient. Interestingly, the wavelength for maximum growth corresponded to one of the peaks in the absorption spectra but there was no increase in growth rate corresponding to any of the other peaks. To address the second aim, the author determined how well predictions on improving the growth of algae (Desmodesmus spp. for example), based on a theoretical model, would work when tested experimentally. What the re- searcher found was that the method improved algae production, using the same set of equipment. The production was improved by a factor of 1.28 and 1.26 (at product concentrations 1000 mg/L and 600 mg/L respectively) when retaining 40% of the algae suspension. The method may be particularly useful when large amounts of biomass are required as there is no extra cost of purchasing additional equipment. The same model was applied to a growth profile of duckweed (Spirodela polyrhiza 8483, which is convertible into biofuel or a source of food), and the author showed that the model could work if the duckweed was provided with an added carbon source. In order to find an economical and reliable alternative to bridge the scale gap between laboratory and industrial production, the author checked if duckweed species (Spirodela polyrhiza 8483, Spirodela polyrhiza 9509, Lemna gibba 8428, Lemna minor DWC 112, Wolffia cylindracea 7340 and Wolffia globosa 9527) could be cultivated in media less expensive than the basal laboratory medium (Schenk and Hildebrandt). The author found that duckweed can be cultivated more efficiently, and in a more cost-effective manner, in the alternative media types, while maintaining growth rates, RGR ≈ 0.09 day-1, and starch contents, 5– 17%(w/w), comparable with that obtained with the conventional laboratory media. Thus, by looking at the photosynthesis process thermodynamically and experimentally, it is shown to be possible to improve the process by using concepts presented in this thesis. iii To my loving and supportive parents Lily and Yen Sun Low To David To Christine To Sushi-Sun To Lee Low Lok & Toy Ling and Chan Mhew-Ann & George Law iv Acknowledgements “PhD is an apprenticeship, one cannot teach one how to do research.” - Prof D. Glasser I have many people to thank from my PhD candidacy. Thank you for all of your help and support. I have learnt a lot from you. A special mention to the following people who had played a pivotal role in the construction of this thesis: • To my supervisors since day one: Prof David Glasser and Prof Diane Hildebrandt. Your advice, questions, critique, patience, support, ideas, time, politeness and kindness are invaluable. Thank you for your honesty and the opportunity for me to fail, to grow and to learn from the both of you. You are both truly amazing researchers, educators and mentors. • To Dr Tonderayi Matambo for all biotechnology related input, experimental expertise, advice and supervisory support. • To Dr Kevin Harding for his assistance and support. • To Pippa Lange. We had worked together in such a short and pressured time frame. I thank you for your time, honesty, your structure, guidance and words. Thank you especially for your editing abilities towards this thesis. Thank you to Norman Blight for the final check. • Thank you to my family, friends, colleagues and students for their un- derstanding, support, encouraging words and love. • I, the author, would like to thank the staff members from the following entities for their support: – University of the Witwatersrand School of Chemical and Metallur- gical Engineering; v – MaPS (Material and Process Synthesis) [previously known as Centre of Materials and Process Synthesis—COMPS] at the University of South Africa (UNISA); and – Rutgers University, NJ, USA: Prof Eric Lam and his research group; Prof Benjamin Glasser and Dr. Linda Anthony. • I would also like to thank the following bodies for their financial support: – National Research Foundation (NRF) Sabbatical grant to complete doctoral degree funding instrument, Thuthuka funding instrument (PhD Track) and NRF Scarce Skills Scholarship; * The NRF has financially supported the author. The views are those of the author and not of the NRF. This work is based on the research supported in part by the NRF of South Africa for the grant, 84344 and 92656. Any opinion, finding and conclusion or recommendation expressed in this material is that of the author(s) and the NRF does not accept any liability in this regard. – University of the Witwatersrand Postgraduate merit award and Staff bursary; and – Prof Eric Lam and Rutgers-NSF IGERT Project on Renewable and Sustainable Fuels Solutions (DGE-0903675) for Financial Support of International Travel and for daily stipend support. vi Contents Abstract . ii Dedication . iii Acknowledgments . v List of Figures . xvi List of Tables . xix Nomenclature . xix 1 Introduction 1 1.1 Background and motivation . 1 1.1.1 Climate change . 1 1.1.2 Carbon dioxide mitigation . 3 1.1.3 Types of aquatic plants . 5 1.1.4 Motivation for using an aquatic plant . 5 1.2 Research aims and objectives . 9 1.3 Layout of the thesis . 10 References . 11 2 A black-box thermodynamic analysis of the photosynthesis process: determining the operational limits 17 Abstract . 17 2.1 Introduction . 18 2.1.1 Photosynthesis process . 18 2.1.2 Thermodynamics . 21 2.1.2.1 First and Second Laws . 21 2.1.2.2 Energy balance: heat release from photosynthesis . 23 2.1.3 Options toward optimization in photosynthesis . 26 2.2 Approach . 26 2.2.1 Energy balance . 28 2.2.2 Entropy balance . 30 vii CONTENTS CONTENTS 2.2.3 Relating the energy and entropy balance . 32 2.3 Results and Discussion . 35 2.3.1 Reversible and adiabatic . 35 2.3.2 What about the pressure assumption? . 36 2.3.3 Reality is irreversible and non-adiabatic . 38 2.3.3.1 Contours of heat with the number of photons 38 2.3.3.2 Contours of irreversibility . 43 2.3.3.3 What do plants do with the excess heat? . 43 2.4 Conclusion . 45 References . 45 3 A black-box experimental analysis on the growth of microalgae Desmodesmus spp. at specific light wavelengths 52 Abstract . 52 3.1 Introduction . 53 3.1.1 Microalgae . 53 3.1.2 The effect of light . 55 3.1.3 Monochromatic light . 56 3.1.3.1 Early studies . 56 3.1.3.2 Filters . 57 3.1.3.3 Dyes . 58 3.1.3.4 Light emitting diodes (LED) . 59 3.1.3.5 Laser . 61 3.1.4 Effect of light on algal composition . 61 3.1.4.1 Growth and lipid content .

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