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The genomes of two strains of marine cyanobacteria,
Prochlorococcus SS120 (1) and Synechococcus WH8102(2) ,
two photosynthetic marine microorganisms, were analyzed by Frédéric
Partensky, CNRS research director at the Centre d’Etudes d’Océanographie
et de Biologie Marine de Roscoff (Roscoff Marine Biology and Oceanography
Research Center, CNRS-Université Paris 6) and by other teams from
France and abroad. This research gives us an insight into the biology
of these tiny microorganisms that thrive in the poorest regions of the
ocean. This study shows that despite an extremely minimal genome that
sets them apart from fresh water cyanobacteria, these microorganisms are
perfectly well adapted to their underwater environment. The results of
this research are published in two articles that appeared simultaneously
on August 13, 2003, on the Internet site of Proceedings of the National
Academy of Sciences of the U.S.A (PNAS) and on the Nature site.
The centers of our oceans, considered to be biological
deserts until recently, are actually populated by tiny organisms known
as picoplankton, which do not grow over 2 to 3 micrometers in size. Many
of these organisms are photosynthetic. In other words, they are able to
capture carbon dioxide from the atmosphere and make oxygen by using the
energy of the sun. The most abundant of these organisms by far (over 95%
of cells) are known as "cyanobacteria," bacteria sometimes mistakenly
referred to as "blue-green algae." Two major genera of cyanobacteria
exist in marine picoplankton: Prochlorococcus and Synechococcus.
With an average size of 0.6 micrometers, Prochlorococcus is the
smallest photosynthetic microorganism known today and the most abundant
as well. It is characterized by a particularly small genome (1.75 million
base pairs or approximately 1/ 2,000th of the human genome) and an extraordinary
compaction of genetic data (approximately 1,900 genes, compared to the
30,000 or so genes of the human genome). Several elements lead us to believe
that the small size of its genome is not an ancestral characteristic but
is instead linked to its "recent" evolution with a trend towards
a reduction of the genomic size of the cells. Why and how did this phenomenon
come about? A reduced cell size is conducive to survival in areas with
little available light (limited light diffusion) and makes it possible
to optimize the cell exchange surface with the surrounding environment
and, as a result, its nutrients absorption capacity. But a cell must reduce
the size of its genome in order to reduce its size. In order to this,
Prochlorococcus had to make drastic cuts in its genetic information and,
contrary to other cyanobacteria, it has very few genes with multiple copies.
Still more surprising, some of the genes coding for apparently essential
functions such as the assimilation of nitrates, nitrites and urea have
totally disappeared from its genome. Using another conservation measure
made possible because of the relative stability of the marine environment,
Prochlorococcus (and to a lesser extent, its cousin, Synechococcus)
has only a very small number of the genes that are necessary for perceiving
environmental changes, genes that are usually very numerous and diversified
in fresh water cyanobacteria.
Much less abundant than Prochlorococcus in the middle of our oceans,
Synechococcus appears to be more of a "generalist" and,
therefore, more versatile. It cuts back on its consumption of iron, a
rare and precious element in the marine environment, by replacing it with
other metals such as nickel in some of the key enzymes necessary for its
metabolism. On the other hand, its system for capturing sunlight is much
more complex (and, therefore, much less energy efficient) than that of
Prochlorococcus. It also has a unique system of locomotion, totally
different from flagella or other locomotor organelles normally observed
in bacteria. This also distinguishes if from Prochlorococcus, which
is incapable of locomotion.
Marine cyanobacteria genomes are still far from revealing all of their
secrets. More in-depth analyses of comparative genomics and, especially,
the study of the function of many still unknown genes (over 30% of the
total) making up these genomes should make it possible for us to eventually
solve the mystery of how these surprising microorganisms, to whom we owe
a large part of the oxygen that we breathe, survive in the poorest areas
of our oceans.
1 - Sequencing was performed by the Genoscope d’Evry (CNRS-Centre
National de Séquençage-Université d’Evry), under
the direction of Marcel Salanoubat, CNRS research director. The annotation
of Prochlorococcus SS120 was supervised by Frédéric Partensky
and Alexis Dufresne of the Centre d’Etudes d’Océanographie
et de Biologie Marine de Roscoff (CNRS-Université Paris 6) with
the participation of a French team directed by Nicole Tandeau of Marsac
(Unité des Cyanobactéries: CNRS-Institut Pasteur), an English
team (University of Warwick), a German team (Humboldt University, Berlin)
and an American team (National Center for Biotechnology Information, Bethesda).
2 - Frédéric Partensky and Alexis Dufresne also contributed
to the annotation of the genome of a strain of Synechococcus WH8102, by
describing photosynthetic genes in general. This genome was sequenced
by the Joint Genome Institute (Walnut Creek, California). Work on the
Synechococcus WH8102 strain was supervised by Brian Palenik (Scripps Institution
of Oceanography, San Diego, California) and involved several American
teams (Woods Hole Oceanographic Institution and the Institute for Genomic
Research).
References:
Dufresne, A., Salanoubat, M., Partensky, F., Artiguenave, I. M., Barbe,
V., Duprat, S., Galperin, M. Y., Koonin, E. V., Legall, F., Makarova,
K. S., Ostrowski, M., Oztas, S., Robert, C., Rogozin, I. B., Scanlan,
D. J., Tandeau de Marsac, N., Weissenbach, J., Wincker, P., Wolf, Y. I.
et Hess, W. R. (2003) Genome sequence of the cyanobacterium Prochlorococcus
marinus SS120, a nearly minimal oxyphototrophic genome. Proceedings
of the National Academy of Sciences of the U.S.A. Forthcoming.
Palenik, B., Brahamsha, B., McCarren, J., Larimer, F., Land, M., Hauser,
L., Chain, P., Lamerdin, J., Regala, R., Allen, R. E., McCarren, J., Paulsen,
I., Dufresne, A., Partensky, F., Webb, E. & Waterbury, J. The genome
of a motile marine Synechococcus. Nature. Forthcoming.
Researcher
contact:
Frédéric Partensky,
Centre d’Etudes d’Océanographie et de Biologie Marine
de Roscoff (CNRS-Université Paris 6)
Tel: +33 2 98 29 23 14
Fax: +33 2 98 29 23 24
E-mail: partensky@sb-roscoff.fr
Press contact:
Laetitia Louis,
Tel: +33 1 44 96 49 88
E-mail: Laetitia.Louis@cnrs-dir.fr
Contact – Life Sciences Department:
Françoise Tristani,
Tel: +33 1 44 96 40 26
Fax: +33 1 44 96 49 19
E-mail: francoise.tristani@cnrs-dir.fr
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