Diatoms, mighty microscopic algae, have profound influence on climate,
producing 20 percent of the oxygen we breathe by capturing atmospheric
carbon and in so doing, countering the greenhouse effect. Since their
evolutionary origins these photosynthetic wonders have come to acquire
advantageous genes from bacterial, animal and plant ancestors enabling
them to thrive in today’s oceans. These findings, based on the analysis
of the latest sequenced diatom genome, Phaeodactylum tricornutum, are
published in 15 October edition of the journal Nature by an
international team of researchers led by the U.S. Department of Energy
Joint Genome Institute (DOE JGI) and the Ecole Normale Supérieure of
The researchers compared Phaeodactylum with the diatom Thalassiosira pseudonana,
previously sequenced by DOE JGI, revealing a wealth of information
about diatom biology, particularly the rapid diversification among the
hundreds of thousands of diatom species that exist today. Phaeodactylum
was targeted for sequencing due to its value as a diatom model, given
the ease with which it can be grown in the lab and the availability of
tools to genetically transform it, and the comparisons with the
previously sequenced diatom genome of Thalassiosira pseudonana.
"These organisms represent a veritable melting pot of traits—a
hybrid of genetic mechanisms contributed by ancestral lineages of
plants, animals, and bacteria, and optimized over the relatively short
evolutionary timeframe of 180 million years since they first appeared,"
says first author Chris Bowler of the Ecole Normale Supérieure. "Our
findings show that gene transfer between diatoms and other organisms
has been extremely common, making diatoms ‘transgenic by nature’," he
The wholesale acquisition of genetic material has provided food for
thought to researchers bent on characterizing the diatom’s staying
power and ability to cope with environmental change.
"We believe this is the first time bacterial horizontal gene
transfer has been observed in eukaryotes at such scale," says senior
author Igor Grigoriev of DOE JGI. "This study gets us closer to
explaining the dramatic diversity across the genera of diatoms,
morphologically, behaviorally, but we still haven’t yet explained all
the differences conferred by the genes contributed by the other taxa."
From plants, the diatom inherited photosynthesis, and from animals
the production of urea. Bowler speculates that the diatom uses urea to
store nitrogen, not to eliminate it like animals do, because nitrogen
is a precious nutrient in the ocean. What’s more, the tiny alga draws
the best of both worlds—it can convert fat into sugar, as well as sugar
into fat—extremely useful in times of nutrient shortage.
The team documented more than 300 genes sourced from bacteria and
found in both types of diatoms, pointing to their ancient origin and
suggesting novel mechanisms of managing nutrients—for example
utilization of organic carbon and nitrogen—and detecting cues from
Diatoms, encapsulated by elaborate lacework-like shells made of
glass, are only about one-third of a strand of hair in diameter. "The
diatom genomes will help us to understand how they can make these
structures at ambient temperatures and pressures, something that humans
are not able to do. If we can learn how they do it, we could open up
all kinds of new nanotechnologies, like for building miniature silicon
chips or for biomedical applications," says Bowler.
Diatoms reside in fresh or salt water and can be divided into two camps, centrics and pennates. The centric Thalassiosira resemble a round "Camembert" cheese box (only much smaller) and pennates like Phaeodactylum
look more like a cross between a boomerang and a narrow three-cornered
hat—hence the species name, tricornutum. Not only is their shape and
habitat diverse, so too is their behavior; for instance, the former get
around by floating, the latter by gliding through the water or on
The lifestyle of diatoms can be characterized as "bloom or bust."
When light and nutrient conditions in the upper reaches of the ocean
are favorable, particularly at the onset of spring, diatoms gain an
edge and tend to dominate their phytoplankton brethren. When food is
scarce, they die and sink, carrying their complement of carbon dioxide
to the deeper recesses.
Bowler and his colleagues are also trying to understand the role that iron plays in the Phaeodactylum‘s
development. Iron is even more precious than nitrogen in the ocean and
its absence in the southern hemisphere is likely a major cause of
oceanic deserts of photosynthesis there. Bowler’s team has demonstrated
that when iron deficiency occurs processes such as photosynthesis and
nitrogen assimilation are suppressed. Other studies, which hail diatoms
as champions in capturing carbon dioxide, suggest a bold strategy of
using iron as a fertilizer to provoke massive diatom blooms. "Once they
have feasted, the weight of their silicon shells, which resemble glass,
causes the diatoms to sink to the bottom of the ocean when they die,
and the carbon that they assimilated is trapped there for millennia,"
says Bowler. "By sequestering carbon in this way we could reverse the
damage from the burning of fossil fuels."
Source : DOE/Joint Genome Institute. October 2008.