Micrograph of the fusiform morphotype of the diatom Phaeodactylum tricornutum. Credit: Alessandra De Martino, Ecole Normale Superieure, Paris
have discovered that marine diatoms, tiny phytoplankton abundant in the
sea, have an animal-like urea cycle, and that this cycle enables the
diatoms to efficiently use carbon and nitrogen from their environment.
researchers, from the J. Craig Venter Institute (JCVI) and other
institutions, published their findings in this week’s issue of the
team, led by lead author Andrew Allen from JCVI and co-author Chris
Bowler, Institute of Biology, Ecole Normale Supérieure, Paris, believes
that the cycle could be a reason for the domination of diatoms in marine
environments, especially after upwelling events–the upward movement of
nutrient rich waters from the deep ocean to the surface.
response to ocean upwelling, diatoms are able to quickly recover from
prolonged periods of nutrient deprivation and rapidly proliferate.
study provides fascinating insights into how diatoms have evolved to
become the dominant primary producers in many ocean regions,” says David
Garrison, program director in the National Science Foundation’s (NSF)
Division of Ocean Sciences, which funded the research along with NSF’s
Division of Molecular and Cellular Biosciences.
have unique cell walls made of 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.
Diatom photosynthesis in ocean environments is also responsible for about one fifth of the oxygen in the atmosphere.
In previous research, Allen, Bowler and colleagues sequenced the genome of the first pennate diatom, Phaeodactylum tricornutum.
that research, they developed new methods for determining the origin of
diatom genes. They also looked at nutrient metabolism in diatoms,
beginning with iron metabolism.
on that work, Allen and colleagues explored the evolutionary history of
diatoms, specifically P. tricornutum, and cellular mechanisms for
nutrient utilization in the environment, leading to the finding that
diatoms have a functional urea cycle.
was a stunning discovery, says Allen, because it was thought that the
urea cycle originated with the metazoan (animal) branch of life.
There it has played an important role in facilitating a wide range of physiological innovations in vertebrates.
example, urea synthesis enables rapid 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
latter was likely a prerequisite for life on land, and subsequently
enabled the biochemical pathways necessary for processing a high-protein
Allen and others have now shown that the urea cycle originated hundreds of millions of years before the appearance of metazoans.
Micrograph of the marine diatom Phaeodactylum tricornutum overexpressing a urea cycle protein. Credit: Andew Allen, JCVI, et al.
The team used RNA interference techniques to partially silence a key urea cycle enzyme in diatoms.
co-author Alisdair Fernie of the Max-Planck Institute of Molecular
Plant Physiology evaluated the metabolite profile of diatoms with and
without an impaired urea cycle.
Then Allen analyzed the data and found that urea cycle metabolites are critical for cellular recycling of carbon and nitrogen.
metabolites are also important for facilitating the rapid onset of
exponential growth characteristic of diatom recovery from nutrient
appears that the animal urea cycle, critical for cellular export of
carbon and nitrogen wastes, was co-opted from an ancestral pathway that
originally evolved as a nitrogen and carbon recycling and recovery
mechanism,” says Allen.
is a very interesting finding we didn’t expect to see, and essentially
changes the way we view diatoms relative to animals and plants.”
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
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.
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.
with Allen, Bowler, Fernie and other colleagues from JCVI, Ecole
Normale Supérieure, 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 Viçosa,
Brazil; and the Institute of Hydrobiology, Chinese Academy of Sciences,
China, contributed to this work.
research was also funded by the JCVI, the European Commission on
Diatomics Project, the Agence Nationale de la Recherche in France, and
the Czech Science Foundation.