J. Craig Venter Institute Scientists, Along with International Team, Uncover New Insights into Evolution of Diatoms and Reveal Evidence for a Urea Cycle used to Metabolize Carbon and Nitrogen

SAN DIEGO, May 11, 2011 /PRNewswire-USNewswire/ -- Researchers from the J. Craig Venter Institute (JCVI) along with an international team of scientists have published a paper detailing new findings about marine diatoms showing that they utilize a urea cycle and that this cycle enables these organisms to efficiently utilize carbon and nitrogen from their environment. The paper is being published in the May 12th, 2011 issue of the journal Nature.  

The research team, led by Andrew Allen from JCVI, and co-author Chris Bowler, Institute of Biology, Ecole Normale Superieure, Paris, theorize that this could be a reason for the domination of diatoms in marine environments, particularly following upwelling events (the upward movement of nutrient rich waters from the deep ocean to the surface). In response to ocean upwelling, diatoms are able to quickly recover from prolonged periods of nutrient deprivation and rapidly proliferate.

Diatoms are eukaryotes that have a unique cell wall made from silica. They are key organisms for understanding the environmental health of marine ecosystems and are responsible for much of the carbon and oxygen production in the ocean. Specifically, diatom photosynthesis in ocean environments is responsible for about one fifth of the oxygen in the atmosphere.

In 2008 Allen, Bowler and colleagues sequenced the genome of the first pennate diatom, Phaeodactylum tricornutum. In that paper Allen and team developed new methods for determining the origins of diatom genes and found that hundreds were bacterial. The team also began to explore nutrient metabolism in diatoms beginning with iron metabolism.

Building on that work, Allen et al continued to explore the evolutionary history of diatoms, specifically P. tricornutum, and cellular mechanisms for nutrient utilization in the environment. This led to new findings presented in this paper showing that diatoms have a functional urea cycle. This was a stunning discovery because previously it was thought that the urea cycle originated with the metazoan (animal) branch of life where it has played an important role in facilitating a wide range of physiological innovations in vertebrates. For example, urea synthesis enables rapid osmoregulation (control of minerals and salts in the blood) in animals such as sharks, skates, rays and bony fish; and ammonia detoxification associated with water retention in amphibians and mammals. The later was likely a prerequisite for life on land and subsequently enabled the biochemical pathways necessary for processing a high protein diet. Allen et al have now shown that the urea cycle actually originated hundreds of millions of years before the appearance of metazoans. The JCVI team used RNA interference techniques to partially silence a key urea cycle enzyme in diatoms. Co-author Alisdair Fernie from the Max-Planck Institute of Molecular Plant Physiology evaluated the metabolite profile of diatoms with and without an impaired urea cycle. Allen et al analyzed the data and found that urea cycle metabolites are critical for cellular recycling of carbon and nitrogen and important for facilitating the rapid onset of exponential growth characteristic of diatom recovery from nutrient starvation.

Allen summarized by saying, "It appears that the animal urea cycle, which is critical for cellular export of carbon and nitrogen waste, was co-opted from an ancestral pathway that originally evolved as a nitrogen and carbon recycling and recovery mechanism. This is a very interesting finding that we didn't' expect to see and essentially changes the way we view diatoms relative to animals and plants."  

This work also suggests that diatoms have followed a fundamentally different evolutionary path from plants (the dominant oxygen producers in terrestrial environments), green algae, and other closely related organisms.  Rather, prior to evolutionary acquisition of photosynthetic machinery, the ancestors of diatoms were possibly more closely related to the ancestors of animals than to plants. This relatedness has resulted in diatoms and animals sharing some similar biochemical pathways such as the urea cycle. Although it appears that animals and diatoms ultimately use the urea cycle for different purposes, they are evolutionarily linked in a way that animals and plants are not.

Along with Allen, Bowler, Fernie and other colleagues from JCVI, Ecole Normale Superieure, and Max-Planck Institute, Germany, researchers from the Biology Centre ASCR, the Institute of Parasitology and University of South Bohemia, Czech Republic; the University Federal de Vicosa, Brazil; and the Institute of Hydrobiology, Chinese Academy of Sciences, China, contributed to this work. The research was funded by the National Science Foundation, internal JCVI funding, the European Commission on Diatomics project, the Agence Nationale de la Recherche in France and the Czech Science Foundation.

About the J. Craig Venter Institute (JCVI)

The JCVI is a not-for-profit research institute in Rockville, MD and San Diego, CA dedicated to the advancement of the science of genomics; the understanding of its implications for society; and communication of those results to the scientific community, the public, and policymakers. Founded by J. Craig Venter, Ph.D., the JCVI is home to approximately 350 scientists and staff with expertise in human and evolutionary biology, genetics, bioinformatics/informatics, information technology, high-throughput DNA sequencing, genomic and environmental policy research, and public education in science and science policy. The legacy organizations of the JCVI are: The Institute for Genomic Research (TIGR), The Center for the Advancement of Genomics (TCAG), the Institute for Biological Energy Alternatives (IBEA), the Joint Technology Center (JTC), and the J. Craig Venter Science Foundation. The JCVI is a 501 (c)(3) organization. For additional information, please visit http://www.JCVI.org.

SOURCE J. Craig Venter Institute