The biophysical basis of iron-light co-limitation of phytoplankton photosynthesis

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Photosynthesis is the conversion of light energy to chemical energy by plants, involving the use of carbon dioxide and the production of oxygen. Globally, photosynthesis is responsible for large transfers of oxygen and carbon between the atmosphere, land and oceans and is an important component of the planet's life support system. In the oceans photosynthesis is carried out by single-celled plants called phytoplankton. These organisms are responsible for half the photosynthesis that occurs on the planet, with the other half being performed by land plants. Phytoplankton are at the bottom of the marine food chain and provide the energy (food) for all the other marine organisms. In common with all plants, phytoplankton need resources to grow, including light, water and nutrients. Water is obviously in plentiful supply in the ocean, however the availability of light and nutrients can limit growth. Such limitation of phytoplankton growth can have important consequences for how the marine ecosystem works and how the biological processes in the ocean affect the removal of carbon dioxide from the atmosphere. One of the nutrients required by phytoplankton is iron. This element, which is very abundant in the crust of our planet, is required in small amounts by all living organisms. However, the amount of iron in the surface of the ocean can be extremely low. Although it has been suspected for some time, it is only within the last 16 years that marine scientists have proved that low iron concentrations limit the growth of phytoplankton in some areas of the oceans. Most of the iron needed by a phytoplankton cell is contained within the structures that plants use to harvest the light energy required for photosynthesis. These structures, called photosystems, convert light energy into electrochemical energy (electrons). Phytoplankton growing in low light environments need a greater number of photosystems in order to harvest enough light. Low light conditions can therefore increase the amount of iron needed by phytoplankton and growth can be limited by both iron and light at the same time. The aim of this project is to increase our understanding of how phytoplankton cope with growing in a low iron environment, frequently under conditions where light may also be low but highly variable. The work will be carried out at the University of Essex in the UK. Phytoplankton will be grown in the laboratory under conditions designed to simulate the ocean. New techniques will be used to measure how the number and activity of photosystems that make up the photosynthetic apparatus adjust to cope with low iron or light conditions. These techniques involve measuring the rate at which light is absorbed by the photosynthetic apparatus and how fast this light energy is passed through the apparatus as electrons. Research will also be performed on ships in the open ocean, working with scientists from Southampton Oceanography Centre. This combination of laboratory and fieldwork is often essential in studies of ocean biology. Work on ships at sea is challenging and the range of measurements made is less than can be achieved on shore. Therefore, information obtained in the laboratory helps us interpret what we observe in the real environment. The answers to questions posed in this project will be important for increasing our understanding of how iron availability effects the growth of phytoplankton, one of the key components of the important ocean ecosystem.