NSFGEO-NERC An unexpected requirement for silicon in coccolithophore calcification: ecological and evolutionary implications.

The oceans cover more than three quarters of the surface of the Earth and tiny algae in our seas are responsible for half of all photosynthesis on our planet. These single celled organisms, known as phytoplankton, form the basis of marine food webs and their activities can have an enormous impact on the geology of our planet. One group of phytoplankton known as the coccolithophores produce a covering of calcium carbonate plates (coccoliths) and can form vast blooms in the oceans. When the coccolithophores die, their coccoliths settle to the ocean floor, leading to the formation of sedimentary rocks, such as chalks and limestones.

In many parts of the ocean the low availability of nutrients (such as nitrogen and phosphorus) limits phytoplankton growth. Competition for nutrients plays an important role in determining which phytoplankton species can grow in different environments. One of the most successful phytoplankton groups in modern oceans is the diatoms, which are fast-growing, making it difficult for many other phytoplankton types to compete with them. However, diatoms need lots of dissolved silicon to make their silica cell walls. In some marine environments, the availability of silicon limits the growth of diatoms, allowing other phytoplankton (which do not need silicon) to grow in their place.

It is commonly thought that the calcifying coccolithophores have no requirement for silicon. However, we have recently discovered that some important coccolithophore species actually possess silicon transporters that are similar to those used by diatoms. Remarkably, we found that these coccolithophores use silicon to make their calcium carbonate coccoliths. Therefore the processes of silica formation in diatoms and calcite production in coccolithophores, which were previously believed to be distinct processes, show a completely unexpected link. These findings have important implications for the evolution of biomineralisation in phytoplankton and for the competitive interactions between coccolithophores and diatoms.

Not all coccolithophores show a requirement for silicon. We found that the species responsible for the massive coccolithophore blooms, Emiliania huxleyi, does not possess silicon transporters and exhibits no need for silicon in the calcification process. The absence of a requirement for silicon may have enabled bloom-forming species to grow better in areas where silicon is low (e.g. after a diatom bloom).
There is therefore a clear need to understand the role of silicon in coccolithophore biology.

In this proposal we will address this issue using a combination of laboratory experiments and computational modelling approaches. Firstly, we will use molecular genetic and laboratory experiments to determine which of the major coccolithophore species exhibit a requirement for silicon. We will then select species for detailed physiological analysis, to determine how silicon contributes to the formation of coccoliths and how coccolithophores take up silicon from the surrounding seawater. These studies will allow us to examine the evolutionary history of the requirement for silicon and determine when certain lineages appear to have lost this trait. Using parameters on Si uptake and usage derived from our experimental work, we will use computer simulations to model global coccolithophore distributions and identify environments where the requirement for Si appears to be playing an important role in coccolithophore ecology.

The research will provide novel insight into physiology, ecology and evolution of coccolithophores, including information on how and why coccoliths are produced, which is currently poorly understood. The research will also inform us on the evolution of coccolith formation, which will be vitally important if we are to understand how coccolithophores have been influenced by past changes in the Earth's climate and how they may respond to changes in the future.

Economic and societal beneficiaries

This proposal aims to deliver mechanistic understanding of a fundamental aspect of marine phytoplankton physiology that will provide insight into competitive interactions between phytoplankton. We therefore expect it to deliver a lasting impact, not only to scientists but more widely to a variety of interested parties. As the research is likely to influence our understanding of marine ecosystem dynamics, the findings will be of relevance to a wide variety of stakeholders with interests in the marine environment. The research will have a wide impact in areas that are underpinned by marine productivity and the stability of marine ecosystems, from food production and tourism through to environmental management.
The proposed work represents cutting-edge blue-skies research and it can be hard to predict the wider impact of research of this nature. The impact of the research on a range of stakeholders may be indirect. It should be noted that an important aspect of the research is to examine how advances at the cellular level can be used to understand processes at the global ecosystem level. The proposed research therefore aims to build pathways where diverse stakeholders at the ecosystem level can benefit from fundamental scientific advances at a very different scale. Thus, a wider benefit of the research will be to facilitate the integration of environmental scientists to further the dissemination of blue skies research. This approach and its implementation are discussed in more detail in the Pathways to Impact attachment.
Beyond environmental science, the research also has applications in industrial biotechnology as there is enormous interest in the biotechnological potential of biomineralised phytoplankton. Applications from drug delivery through to enzyme immobilisation and microphotonics have all been investigated for biomineralised phytoplankton or biomimetic structures derived from these organisms. However, the ability of materials scientists to generate and manipulate biominerals for industrial biotechnology remains limited. There is a strong drive to learn from natural systems, where the diversity and complexity of form and function of biominerals is enormous. The research will provide insight into how Si may be used to modulate the formation of calcium carbonate structures, which may have direct applications for industrial biotechnology.

End users

Non-academic stakeholders include policy forming bodies such as Governmental Environment and Climate Change Departments (e.g. the European Union, the UK Met Office, UK Government Departments, including DECC and DEFRA) as well as international bodies and NGOs (IPCC, environmental and fisheries charities, pressure groups). All of these groups have a potential interest in the factors driving marine primary productivity. Improved quantitative understanding of marine ecosystem function will allow these policy forming bodies to address issues such as ecosystem management protection, and prediction and mitigation against any undesirable changes.
A major aim of this proposal is to build processes that enable researchers working on fundamental aspects of environmental research to engage with and inform end users. This forms the basis of the 'Genes to Ecosystems' workshop described in Pathways to Impact.

Development of UK skill base

The staff involved in this project will be trained to high level in both experimental and computational techniques. These clearly are of benefit to the academic research sector, but if these researchers do not choose to follow an academic career path, these skills will contribute significantly to the UK skill base within both the private and public sectors. Recent alumini from our laboratory are using skills relating to this research area in both the public sector (e.g. Knowledge Exchange for a research institute), and the private sector (e.g. development of educational software, the microscopy industry).