Oxygenic photosynthesis can be described by the simple equation: 6𝐢𝑂$ + 6𝐻$𝑂 β†’ 𝐢(𝐻)$𝑂( + 6𝑂$ + 6$Β 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
β€˜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.