Global significance of light-driven proton pumps in eukaryotic marine phytoplankton

Sunlight is the ultimate source of energy on our planet and the efficient capture and use of light is an exquisitely evolved process across all kingdoms of life, ranging from unicellular microbes to multicellular organisms. In addition to use of light as an energy source for growth, it is also an important source of environmental information. Microbes have evolved particularly diverse systems to use light to generate energy, avoid hostile environments and identify suitable environments for nutrition and growth. The metabolic mode in which organisms convert light energy into chemical energy for growth is called phototrophy (from Greek [photo-], "light" and [trophe], "nourishment"). The most important biological process on earth to power phototrophy is oxygenic photosynthesis, which employs multisubunit protein complexes containing chlorophyll pigments known as photosystems and produces the oxygen we breathe. These photosystems are highly-efficient in capturing and using light, but heavily-depend on iron to function.

Additionally, a second mechanistically distinct process, which is independent from photosystems, can also power phototrophy and employs membrane-embedded photoreceptors called rhodopsins. Rhodopsins are molecules composed of opsin membrane proteins, which bind the pigment retinal but unlike photosystems do not need iron to function. Their operating principle is based on their unitary simple nature. Instead of employing complex photosystems, which are encoded by multiple genes, rhodopsins combine the tasks of light absorption and energy-conservation into a single protein encoded by a single gene. Upon absorption of light, the chemical structure of retinal pigment changes and triggers a cascade of structural changes within the molecule. Rhodopsin photoreceptors were first discovered in ancient prokaryotic (cells lacking a nucleus and membrane-bound organelles) archaebacteria, but later also in very distantly related bacteria. The high abundance of bacterial rhodopsins in marine environments has shown that rhodopsin-based phototrophy is a globally significant microbial process in the ocean. Surprisingly, rhodopsins have recently also been identified in unicellular eukaryotes (organisms with nucleus and nuclear envelope enclosing the genetic material) including photosynthetic marine phytoplankton. However, the function of eukaryotic rhodopsins in the presence of more energy-efficient photosystems remains puzzling.

In a preliminary study, we provided first experimental evidence, that genes encoding for rhodopsins are highly up-regulated in iron-limited phytoplankton. They were also more abundant in iron-limited oceans, which cover about one third of the global ocean surface. These findings provide first direct evidence for our research hypothesis that rhodopsins in marine phytoplankton provide a previously unknown backup mechanism for iron-dependent chlorophyll-based photosynthesis, to enhance production of chemical energy and growth when iron is lacking for iron-dependent photosystems. This new mechanism is of particular interest, because recent research has shown that ocean acidification due to increased dissolution of anthropogenic carbon dioxide can decrease the iron availability to phytoplankton, which probably will alter phytoplankton diversity in the oceans and favor species that have a competitive advantage (e.g. by rhodopsin-based phototrophy) under reduced iron concentrations. In our research project we will use new molecular genetic methods to test our research hypothesis and further explore the cellular role and environmental significance of rhodopsins in globally important marine phytoplankton.
Our results will provide fundamental new insights into how marine phytoplankton use rhodopsins. It will be of great interest to the scientific community, because phytoplankton are subject to many different disciplines, from marine and climate science to material science and renewable energy.

This project is primarily driven by a gap in our knowledge as to how light-driven proton pumps enable eukaryotic phytoplankton to cope with iron limitation. The global significance of these pumps that have been obtained from bacteria was revealed by the first eukaryotic metatranscriptome projects with phytoplankton on a global scale and the first complete genome sequences from phytoplankton adapted to low iron concentrations (e.g. Fragilariopsis cylindrus from the Southern Ocean). Knowledge from this project will provide new fundamental insights into how phytoplankton has evolved in iron-limited sea water that covers about 35% of the global ocean. Our previous work in the field of marine microalgae has led to 'Nature', 'Science' and PNAS papers including worldwide coverage on the internet, in news papers, magazines and radio (e.g. recent interview on 'Rundfunk Berlin Brandenburg', rrb). Thus, our work has impact and we are well versed in disseminating it to wide audiences and will continue to exploit this strong starting position.

In addition to conventional routes: journals, science meetings and the web, we will continue to use other local channels to bring our work to wider audiences. These routes will build on the "Beacon of Public Engagement" award "CueEast" (Community University Engagement East) to UEA and partners; see http://www.uea.ac.uk/ssf/cue-east. The Beacon Scheme is strongly supported by NERC and BBSRC. We will also be involved in "SAW" (Science Art and Writing - http://www.sawtrust.org/; PLoS Biology 2008 6; e211). Mock has just been awarded the 'Postgraduate Certificate in Higher Education Practice' and he was elected as a Fellow of the Higher Education Academy. His awarded teaching experience enables him to develop different kinds of learning modules to disseminate science to children and adults from various socioeconomic backgrounds.

We plan the following activities to deliver impact to various beneficiaries:

1) Public exhibitions are an ideal way to reach the local population and inspire engagement with science issues. In National Science Week, we participate in hands-on exhibitions of 'Norfolk Science-Past and Present' at the Norwich Castle Museum. We will also contribute to a showcase in the 'Norwich Forum' where PhD students and young PDRAs show their work in form of posters and presentations to the public.
2) Science, Art and Writing Projects (SAW) will be set up each year at local primary schools by Mock and Strauss. These will involve visits and talks about the importance of marine microalgae for life on our planet. Experiments will be carried out to isolate microalgae from sea water. For the diatoms, we will exemplify their extraordinary, diverse and sometimes unearthly appearance.
3) Mock is directly engaged with four different companies (Phycal, Synthetic Genomics, Supreme Biotechnology, Spicer Biotechnology) for the commercialization of algae and their products for several different end users. These engagements are based on discovering a gene in diatoms that enhances growth (patent pending in the US). Synthetic Genomics (US) (Imad Ajjawi, PhD) and Supreme Biotechnology Inc. (Mr. Tony Dowd) have approached Mock to setup collaborative projects. However, Phycal is the first company that have signed a license to commercially use Mock's products. This business relationship will be used to explore potential commercialization also for light-driven proton pumps, if this project will be able to show that those proteins enhance photosynthesis and growth in commercially used microalgae.

These examples show that Mock's group is fully engaged with industrial partners and that he has a proven track record of being able to commercialize the outcomes and discoveries of his research, which indicates significant impact of his work.