Research in our laboratory focuses on microorganisms in the sea, from their diversity in biology to their roles in controlling the fluxes of elements and nutrients in the sea. In particular, our laboratory focuses on the metabolism and transformations of nitrogen by marine microorganisms.
Introduction for the nonspecialist
Life as we know it is composed of a limited number of elements. The major elements in organisms are carbon, nitrogen, phosphorus, sulfur, hydrogen and oxygen. Minor elements include metals such as molybdenum, iron, cobalt, cadmium and others. Microorganisms play important roles in making these elements available, even in some cases from rock, and also recycling them during decomposition.
The growth of organisms and the productivity of the biosphere is dependent on adequate supply and recycling of all the elements needed for life. Photosynthetic (phototrophic) organisms "fix" carbon from gaseous carbon dioxide in the atmosphere into organic carbon compounds (e.g. protein) that are consumed by herbivores and supports all other organisms. Nitrogen is one of the most important elements, next to carbon, as it is present in nucleic acids (DNA and RNA) and in protein. Not all nitrogen is available to organisms, which is why agricultural fields are fertilized (with nitrogen and phosphorus). In the case of nitrogen, a large fraction of the Earth's atmosphere is composed of nitrogen as a gas (almost 80% of the atmosphere). Some microorganisms have the ability to "fix" this atmospheric gas (in the form of N2) into compounds that can be consumed by other organisms, such as ammonium or amino acids. These N2-fixing microorganisms often form symbiotic associations with land plants (such as legumes) and have been used in agriculture to fertilize soils. Similarly, N2-fixing microorganisms in many habitats, including the oceans, are important in making the important nutrient nitrogen available for organisms to grow.
Nitrogen fixation is only part of the "nitrogen cycle". Nitrogen is a component of many chemical compounds, but more importantly exists in different oxidation states: since oxidation-reduction reactions serve to fuel biochemical reactions, the different oxidation states of nitrogen have been exploited by different microorganisms. The collective oxidation-reduction reactions catalyzed mostly by microorganisms (but some by higher organisms and some by abiological processes such as lightning) form the nitrogen cycle. One component of the nitrogen cycle is called denitrification. Denitrification results in the formation of N2 gas that is liberated to the atmosphere and balances the fixation of N2by N2-fixing microorganisms.
Components of the nitrogen cycle include nitrification, denitrification, nitrogen fixation, decomposition, anaerobic ammonium oxidation.