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The urea cycle is a metabolic pathway used in mammals to incorporate excess nitrogen into urea and remove it from the body. However, it appears to play a far more wide-ranging role in the group of algae known as diatoms. Scientists from the Max Planck Institute of Molecular Plant Physiology in Potsdam are part of an international team of researchers that has succeeded in identifying the urea cycle in diatoms as a distribution and recycling centre for inorganic carbon and nitrogen. The urea cycle plays a key role in the fixation of the two elements and also makes an important contribution to the fact that diatoms can recover from short-term nutrient withdrawal and respond immediately to the availability of a greater supply of food by increasing their metabolic and growth rates. Genes that have reached the diatom genome through lateral gene transfer contribute to this capacity.
Diatoms are the main component of phytoplankton and thus form the basis of the marine food chain. Because they carry out photosynthesis with their chloroplasts, they account for a large proportion of the oxygen production in the earth's atmosphere. The fact that diatoms have a urea cycle, something that was originally believed to exist only in multicellular organisms, could explain their success in colonising the oceans. Moreover, the diatom cell nucleus contains genes that migrated to the diatom genome from chloroplasts and bacteria. An international team of researchers including Alisdair Fernie from the Max Planck Institute of Molecular Plant Physiology in Potsdam set itself the task of identifying the contribution made by the urea cycle to the metabolism of diatoms.
In the laboratory the researchers reconstructed the upwelling phenomenon found in the ocean, which causes nutrient-rich water to rise from deeper areas to the surface and therefore to the diatom habitat. Diatoms immediately respond to this excess supply of nutrients following a period of nutrient deprivation by increasing their rates of growth and proliferation. The researchers compared the reaction of normal cells with cells that do not have a functioning urea cycle in the laboratory. It emerged that the growth rate in the cell lines without a functioning urea cycle was 15 to 30 percent lower than that in the normal cells. It may be deduced from this that the urea cycle in diatoms serves in the formation of carbonaceous and nitrogenous compounds. This observation is extremely surprising as animals mainly use the urea cycle for the disposal of excess nitrogen and for the regulation of their mineral household.
In evolutionary terms, it would appear that the animal urea cycle developed from an older metabolic pathway. This discovery thus throws a new light on the phylogenetic relationships between diatoms, plants and animals. Before diatoms developed the capacity to carry out photosynthesis, which positions them closer to plants and green algae in phylogenetic terms, they may have been more closely related to the evolutionary ancestors of animals.
The same experiments revealed that a defective urea cycle also has a negative impact on other processes, such as cell wall synthesis and the citric acid cycle in the diatoms. "Our findings indicate that the different metabolic paths in diatoms are extremely well connected," explains Fernie.
The number of metabolic paths branching off from the urea cycle in diatoms is particularly high. For example, arginine and ornithine, the intermediate products of urea synthesis, are used in the development of components of the cell wall. The diatoms obtained the enzymes necessary for this through lateral gene transfer from bacteria. The reason for the superiority of diatoms over other unicellular organisms found in the oceans lies in the good connectivity between the different metabolic pathways.
Andrew E. Allen, Christopher L. Dupont, Miroslav Oborník, Aleš Horák, Adriano Nunes-Nesi,, John P. McCrow, Hong Zheng, Hanhua Hu, Alisdair R. Fernie, Chris Bowler
Exosymbiont-derived urea cycle used for intracellular carbon and nitrogen recovery in diatoms
Nature, May 12th 2011, DOI: 10.1038/nature10074
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