Blue biotechnology: Exploring and Exploiting the Reactions of Marine Photosynthesis: Exploring and exploiting the reactions of marine photosynthesis

Abstract

Oxygenic photosynthesis can be described by the simple equation: 6𝐶𝑂$ + 6𝐻$𝑂 → 𝐶(𝐻)$𝑂( + 6𝑂$ + 6$ This equation represents a set of reactions that are responsible for the light catalysed conversion of carbon dioxide and water into fixed carbon (carbohydrate) with the simultaneous release of oxygen. They are arguably the most important reactions to occur on the Earth. Oxygenic photosynthesis is thought to have evolved somewhere between 3.5-3 billion years ago in ancestral cyanobacteria. Photosynthesis is not only responsible for the oxygenation of the atmosphere but also provides the overwhelming majority of fixed carbon that is available for incorporation into organic molecules. It is responsible for fuelling life and driving biogeochemical cycles globally. It is estimated that photosynthesis in the marine environment accounts for half of the total carbon fixation that occurs each year. The majority of marine carbon fixation is carried out by a diverse array of eukaryotic and prokaryotic microalgae. In the context of the growing human population and climate change, marine photosynthesis becomes particularly relevant. A large proportion of the human population rely indirectly on marine photosynthetic organisms as a source of food and they form an important natural carbon capture/storage mechanism. There is also a growing interest in manipulating marine photosynthetic organisms towards increased biomass production and the sustainable production of biofuel and other high value molecules. Despite the fact that photosynthesis is one of the most relevant and most studied processes, there are still many aspects of it that are poorly understood. Photosynthesis is often considered as a linear process linking light capture with carbon reduction. There exist however, a diversity of photosynthetic pathways that control how light energy is captured and then converted into products. This thesis aims to investigate the ways in which microalgae change their photosynthetic strategy in response to different stimuli and the ways in which these strategies can be manipulated for biotechnological applications. The aim of Chapter 3 is to identify the diversity of photosynthetic strateg by natural phytoplankton communities. The metatranscriptomic response of Southern Ocean phytoplankton communities to nutrient addition was interrogated to identify genes relating different photosynthetic strategies that are more prevalent when nutrients are limiting. The results of this analysis show that some genes relating to alternative photosynthetic strategies are regulated by nutrient limitation. They also emphasise the importance of alternative photosynthetic strategies for energy acquisition in natural phytoplanktonic communities. The aim of Chapter 4 is to engineer a naturally occurring light driven proton pump ‘rhodopsin’ into a photosynthetic cell, to demonstrate an increase in chlorophyllbased photosynthesis. These results demonstrate that it is possible to improve the native chlorophyll-based photosynthetic rate of a host cell by introducing a heterologous light harvesting complex. The aim of Chapter 5 is to investigate the role of natural sinks of photosynthetic electrons in cyanobacteria. The results of this chapter show that the removal of natural electron sinks results in an increase in linear photosynthetic electron follow to a heterologous electron sink, demonstrating how photosynthetic strategy can be manipulated. Combined, this thesis demonstrates how diverse photosynthetic strategies identified from the environment can be applied to direct photosynthetic potential towards desired products.

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Identifiers

ORCID for Matthew Terry: orcid.org/0000-0001-5002-2708

ORCID for Ivo Tews: orcid.org/0000-0002-4704-1139

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