action identified as key component in carbon
By: Heidi Ledford
Viruses that infect microbes
in deep-sea sediments may be a key driver in
the world's largest ecosystem and integral to
the global carbon cycle, data reveal.
Lytic viruses, which cause
infected cells to burst, kill about 80% of the
single-celled organisms in the sediment and
sub-surface ocean layers, researchers calculate,
thereby releasing large amounts of dissolved
carbon into the deep seas1. Every year this
'viral shunt' releases up to 630 million tonnes
of the carbon sequestered by particles sinking
into these deep-sea benthic zones, suggesting
that viruses should be included in ocean carbon-management
The viruses provide a form
of population control that may date back to
the origin of life on Earth, says bio-oceanographer
Roberto Danovaro of the Polytechnic University
of Marche in Ancona, Italy, who led the analysis
of 232 deep-sea sediment samples with his colleagues.
"The viruses kill the microbes and stimulate
their growth as well," he says. "It's
almost a self-sustaining mechanism."
The viral data are the latest
in a series of revelations about the deep-sea
ecosystem, which covers 65% of the planet's
surface. Around one-tenth of Earth's living
biomass exists at the bottom of the ocean, despite
cold temperatures, impenetrable darkness and
intense pressure. "A few years ago we were
assuming the deep-sea ecosystems had no currents,
no movements," says Danovaro. "Now
we know a lot of material and sediment can be
brought up from the deep in a few days."
The nature of the microbes
living in and below the sea floor remains hotly
debated. It was thought that the majority consists
of bacteria. But a study published last week
concluded that most cells in the sediment are
archaea2, a similar-looking but distinct
form of life.
Previous ocean-floor surveys
turned up plenty of bacteria but relatively
few archaea. Some studies may have been biased
- several were based on DNA extraction and staining
methods that may have met with limited success
in penetrating the relatively impermeable archaeal
cell membrane. Similarly, lipid-profiling methods
based on important components of cell membranes
called phospholipid-based fatty acids are generally
held to be a good marker for living bacteria
because these lipids degrade rapidly after the
cell dies. Yet many archaea do not make these
particular fatty acids.
Kai-Uwe Hinrichs of the University
of Bremen in Germany and his colleagues, however,
took samples from more than 1 metre below the
sediment surface and milled the cells they collected
in liquid nitrogen to loosen up their membranes.
They also measured a different class of lipid,
one that is common in both archaea and bacteria.
The researchers discovered an abundance of archaea,
finding that at that depth, archaea make up
more of the microbial biomass than bacteria
However, results from a few
research sites do not reveal what lies on and
beneath the entire ocean floor. "We must
be careful not to assume that when we find something
in a system, it must be true across environments,
in different kinds of marine systems,"
says Mark Gessner, a microbial ecologist at
the Swiss Federal Institute of Aquatic Science
and Technology in Dübendorf. Indeed, another
study published last week surveyed deep-sea
hydrothermal vents and found that many of the
viruses found there were not lytic, instead
being of a type that rarely causes its hosts
Further technological improvements
are needed to get a higher-resolution image
of the microbial forest below the sea floor.
"We're dealing with very low signal and
all of our methods are at the limit of their
ability," says Hinrichs.
1. Danovaro, R. et al.
Nature 454, 1084-1087 (2008).
2. Lipp, J. S. , Morono, Y. , Inagaki, F. ,
and Hinrichs, K.-U. Nature 454, 991-994
3. Biddle, J. F. et al. Proc. Natl Acad.
Sci. USA 103, 3846-3851 (2008).
4. Williamson, S. J. et al. ISME J.
advance online publication doi: 10.1038/ismej.2008.73
(21 August 2008).
Published online 27 August 2008 | Nature
454, 1038 (2008)