Assessing how cell size constrains carbon uptake in diatoms using direct measurements of cell surface carbonate chemistry

The diatoms are a group of unicellular algae that represent some of the most important photosynthetic organisms on our planet. Diatoms are particularly abundant in nutrient rich coastal regions where they form the base of the food web, supporting fishing and seafood industries. It is estimated that diatoms contribute up to 20 % of global photosynthesis. It is therefore surprising that there are major uncertainties relating to the form of carbon taken up by diatoms and how these mechanisms are influenced by the size of the cell.

Diatoms range from very small to very large (5-200 micrometre diameter) and can even form colonies, in the form of chains of cells linked together. This huge diversity in size has a major influence on the ability of each species to acquire nutrients from its environment, with the supply to larger species potentially limited by their diffusive boundary layer. Understanding how cell size constrains nutrient acquisition is therefore central to our understanding of diatom ecology and the distribution of different species, although direct measurements of the diffusive boundary layer around cells are lacking.

Although seawater contains a plentiful supply of dissolved inorganic carbon, only a small proportion of this is present as carbon dioxide (CO2). The supply of CO2 to the cell by diffusion is therefore not sufficient to support the high rates of photosynthesis observed in diatoms. This problem is much greater in large species, due to the significant diffusive boundary layer around the cell surface. Diatoms, and other marine phytoplankton, therefore have to utilise the pool of bicarbonate (HCO3-), either by actively transporting it across the membrane or by using an enzyme (extracellular carbonic anhydrase) to catalyse its conversion to CO2, which can then diffuse across the membrane. However, it is technically difficult to measure the proportion of carbon taken up by these different mechanisms and different diatom species show considerable variability. Moreover, the role of the enzyme extracellular carbonic anhydrase has been much disputed. Because of this uncertainty, we do not have a mechanistic understanding of how changes in CO2 supply can influence the composition of diatom communities. With the concentration of CO2 in seawater predicted to change dramatically in the coming centuries, this uncertainty hampers our ability to predict how different species may respond to the changing availability of CO2.

Improved knowledge of the microenvironment around diatom cells is necessary if we are to understand how they acquire carbon from seawater. We have developed tiny ion-selective microelectrodes that can be placed at the surface of a single diatom cell. By measuring pH and carbonate (CO32-), we can calculate fluxes of carbon across the membrane and estimate to what extent the supply of CO2 to the cell surface may be limiting.

This project will use these new techniques to address some of the major questions relating to carbon acquisition by diatoms. We wish to examine the extent to which CO2 supply is limiting to cells of different sizes and examine how mechanisms of carbon uptake are adjusted to cope with changes in carbon supply (e.g. elevated CO2) or carbon demand (e.g. a greater rate of carbon fixation is needed at high light). We will also examine whether the supply of CO2 influences the ability of certain species to form chains, in order to understand the environments where we might expect these species to be successful. These studies using microelectrodes will be complemented by a molecular genetic approach to study to the role of the enzyme, extracellular carbonic anhydrase, which appears to play a critical role in the supply of CO2 to some diatoms. Finally, we will also examine natural populations of diatoms to see how they are influenced by changes in the availability of carbon dioxide throughout the progression of a typical diatom bloom.

Economic and societal beneficiaries
Diatoms underpin some of the most productive fisheries in coastal upwelling regions. This proposal aims to deliver mechanistic understanding of a fundamental aspect of marine diatom physiology that will provide insight into the structure of diatom communities. 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, such as food production and tourism through to environmental management.
The proposed work represents cutting-edge fundamental research, making it difficult to predict the full impact of the research and it is likely that the impact of the research on diverse stakeholders may be indirect. One of the key aspects of the research is to understand how studying processes at the microscale level can help us to understand processes at the ecosystem level. The proposed research therefore aims to identify mechanisms through which diverse stakeholders operating 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.

End users
The research will impact multiple non-academic stakeholders including 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.

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 outside of academic research, in both the public sector (e.g. Knowledge Exchange for a research institute) and the private sector (e.g. development of educational software, development of microscopy technology).