The Mechanisms of Calcification in Coccolithophores the Molecular Basis of Calcium and Inorganic Carbon Transport in Emiliania Huxleyi
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The Mechanisms of Calcification in Coccolithophores The molecular basis of calcium and inorganic carbon transport in Emiliania huxleyi Dissertation in fulfilment of the requirements for the degree “Dr. rer. nat.” of the Faculty of Mathematics and Natural Sciences at Kiel University submitted by Luke Colin Martin Mackinder Kiel March 2012 2 First referee: Prof. Dr. Ulf Riebesell Second referee: Prof. Colin Brownlee Date of the oral examination: 23rd May 2012 Approved for publication: Signed (Title, first and surnames), Dean: TABLE OF CONTENTS 3 Table of Contents Summary ........................................................................................................................................ 4 Zusammenfassung ......................................................................................................................... 6 CHAPTER 1. General Introduction ........................................................................................... 9 1.1 Coccolithophores – the rise of Emiliania huxleyi .............................................................. 9 1.2 Coccolithophores and the marine carbon cycle in a changing ocean ........................... 10 1.3 Calcification in coccolithophores ..................................................................................... 13 1.4 Ca2+ transport in coccolithophores .................................................................................. 16 1.5 Inorganic carbon uptake and usage in coccolithophores .............................................. 17 1.6 pH homeostasis in coccolithophores ................................................................................ 19 1.7 Thesis Outline .................................................................................................................... 20 1.8 List of publications ............................................................................................................ 21 References ................................................................................................................................ 23 CHAPTER 2. Molecular Mechanisms Underlying Calcification in Coccolithophores ........ 29 CHAPTER 3. Expression of biomineralization-related ion transport genes in Emiliania huxleyi .......................................................................................................................................... 41 CHAPTER 4. Calcification and photosynthesis in Emiliania huxleyi: Physiological and genetic responses to individual carbonate system parameters ............................................... 59 CHAPTER 5. Ion transport associated with calcification and inorganic carbon acquisition in coccolithophores: Insights from the Emiliania huxleyi genome ....................................... 103 CHAPTER 6. The cloning and partial characterization of two calcification related genes, - 2+ + AEL1 a putative HCO3 transporter and CAX3 a putative Ca /H exchanger, from the coccolithophore Emiliania huxleyi ........................................................................................... 135 CHAPTER 7. Synthesis and future perspectives ................................................................... 169 7.1 Calcification ..................................................................................................................... 169 7.2 Inorganic carbon transport ............................................................................................ 171 7.3 Coccolithophores in a changing ocean .......................................................................... 173 7.4 Future directions ............................................................................................................. 175 References .............................................................................................................................. 176 Acknowledgements ................................................................................................................... 178 Declaration of work .................................................................................................................. 179 4 SUMMARY Summary Coccolithophores are calcifying marine phytoplankton that through the fixation of inorganic carbon into calcite and particulate organic carbon play a fundamental role in global carbon cycles. As the CO2 concentration of the surface ocean increases through the anthropogenic release of CO2 by burning fossil fuels both a decrease in pH (ocean acidification) and a increase in dissolved inorganic carbon (ocean carbonation) are taking place. To understand the impact of these ocean changes on coccolithophores it is essential that we rapidly increase our knowledge of the cellular processes underlying coccolithophore physiology. This doctoral thesis focuses on the cellular and molecular processes involved in the transport of Ca2+, inorganic carbon and H+ in relation to calcification and photosynthesis in the coccolithophore species Emiliania huxleyi. The thesis comprises 7 chapters: Chapter 1 is a general introduction to coccolithophore cellular biology and global carbon cycling; Chapters 2-6 are a combination of publications and data chapters; Chapter 7 provides a synthesis of the results placing the presented data into context of coccolithophores in a changing ocean, highlighting future research areas. Chapter 2 is a published review of the current literature on the molecular aspects of calcification in coccolithophores. It identifies key gaps in our knowledge of coccolithophore cellular biology and presents new hypotheses for the transport of substrates to the site of calcification. Chapter 3 investigates some of the proposed hypotheses by examining the role of several candidate Ca2+, H+ and inorganic carbon transport genes in calcifying and non-calcifying cells of E. huxleyi, using quantitative reverse transcriptase PCR. The data provides strong - 2+ + + evidence that a putative HCO3 transporter (AEL1), a Ca /H exchanger (CAX3), a vacuolar H - ATPase pump (ATPVc/c’) and a gene encoding for a coccolith-associated protein, GPA, play key roles in E. huxleyi biomineralization. CAX3 and AEL1 were chosen for further analysis and were successfully cloned and expressed in Saccharomyces cerevisiae and Human Embryonic Kidney cells (HEK293) respectively (Chapter 6). However complete characterization of CAX3 and AEL1 2+ was unsuccessful. CAX3 failed to complement the Ca sensitive phenotype of a S. cerevisiae mutant, with further expression in a Ca2+ sensitive Escherichia coli mutant resulting in a lethal - phenotype. The investigation of HCO3 transport in HEK293 cells expressing AEL1 gave negative results, potentially due to the poor localization of AEL1 to the plasma membrane. The data highlights the importance of developing genetic transformation techniques in coccolithophores to reduce the dependency of using foreign expression systems for the characterization of genes. - 2- The influence of the individual carbonate system components (CO2, HCO3 , CO3 and H+) on coccolithophore physiology and genetic response is relatively unknown. Chapter 4 SUMMARY 5 disentangles the individual carbonate system components investigating their influence on calcification, particulate organic carbon fixation and gene expression in E. huxleyi. It identifies, for the first time, the genetic basis of a carbon concentrating mechanism (CCM) in coccolithophores, with the transcription of multiple CCM associated genes up-regulated at low - concentrations of HCO3 and CO2. Physiological data combined with expression data indicates that calcification does not function as a CCM under carbon limitation and is instead reduced to allow the redistribution of inorganic carbon from calcification to photosynthesis. Furthermore, - the data confirms previous studies that the substrate for calcification is HCO3 and that growth and organic carbon fixation rates are primarily influenced by CO2. The recent sequencing of the E. huxleyi genome has provided vast quantities of genetic data that requires detailed analysis to realise its full potential. Chapter 5 analyses the genome for calcification and photosynthesis related transport genes discovering that E. huxleyi has a diverse range of inorganic carbon, Ca2+ and H+ transporters, from classical plant, animal and bacterial families. The presence of multiple Na+/Ca2+ exchangers, a family of almost exclusively animal transporters indicates that coccolithophores have the potential to use both H+ and Na+ electrochemical gradients to drive secondary transport. Furthermore the identification of green algal CCM genes may provide a strong basis for investigating the evolution of CCMs in eukaryotic algae. The data presented in this thesis provides a significant step in our understanding of coccolithophore physiology at a cellular and molecular level. It offers a solid platform for future research in coccolithophore cell biology an area of research that is essential to comprehend the role of coccolithophores in gobal carbon cycling and how they will respond and adapt to future ocean changes. 6 ZUSAMMENFASSUNG Zusammenfassung Coccolithophoriden sind kalkbildende marine Phytoplankton. Sie sind von außerordentlicher Bedeutung für den globalen Kohlenstoffkreislauf, da sie inorganischen Kohlenstoff sowohl in Kalzit als auch in organischem Kohlenstoff binden können. Die zunehmende Verbrennung von fossilen Brennträgern führt zu einem Anstieg der Kohlenstoffdioxid (CO2) Konzentration im Oberflächenwasser der Ozeane. Das wiederum