Download Ecology deep sea

The Ecology of Deep-sea Protists and their Potential
Role in Understanding Organic Carbon Input into the
Deep Ocean.
Alexander B. Bochdansky, Old Dominion University, Norfolk, Virginia
[email protected]
Little is known about the distribution and
ecology of eukaryotic microbes of the deep
sea. Most of these microbes are small
heterotrophic flagellates that feed on
bacteria, whose biomass in turn is fueled
by the input of dissolved and particulate
organic material from the surface. This
project is a research collaboration between
Old Dominion University and Gerhard J.
Herndl at the Royal Netherlands Institute
of Sea Research. It is part of a larger
research program (ARCHIMEDES) funded
by the Dutch Science Foundation, with a
focus on microbial processes in the deep
sea. Within ARCHIMEDES, the dynamics
of
microbial
communities
(viruses,
prokaryotes [Bacteria and Archaea] and
protists) in the major deep North Atlantic
water masses are studied and related to
biogeochemical characteristics of the
individual
water
masses.
Particular
emphasis is put on microbial activity
measurements under in situ pressure.
My research focuses on understanding the
distribution and activity of eukaryotic
microbes (generally referred to as protists)
in the context of large, basin-scale
variations of hydrographic and chemical
properties. Because of the remoteness of
the habitat there are many challenges to
studying the microbial ecology of the deep
sea. For instance, enumeration of protists
in the deep sea is not as trivial as it seems.
First, we do not know how many cells
make it to the surface intact after
undergoing pressure changes of 600 bar.
It is possible that gas bubble formation
inside the cells destroys their integrity.
Secondly, many bacteria and archaea
possess nucleomorphs that look identical
in size and shape to the nuclei of protists.
The protists can therefore only be reliably
enumerated with the aid of fluorescent
molecular probes and epifluorescence
microscopy (Fig. 8). Cell densities of
protists turn out to be so low that large
volumes of water need to be filtered
through membrane filters of very small pore
size (usually 0.8 µm) to obtain accurate
counts.
Another challenge is that physiological and
biochemical processes are certainly
influenced by differences in pressure and
temperature. Therefore, samples for live
incubation must be retrieved while retaining
both in situ pressure and temperature. With
seed funds from the U.S. National Science
Foundation, we designed and built a
pressure culture system which allows us to
incubate samples from the deep sea at the
same pressure and temperature that the
cells were exposed to at depth (Fig. 9).
Engineers at the Netherlands Institute of
Sea Research built high-pressure samplers
that keep temperature and pressure
constant while retrieving samples from
down to 6000 meters. This system will help
us to grow representatives of the deep-sea
microbial community and to understand the
extent to which deep-sea protists are able
to respond to sudden prey pulses such as
those caused by the encounter with sinking
particles.
Judging from our preliminary results, the
cell densities of eukaryotic microbes are
extremely low, meaning that they do not
seem to contribute much to the processes
in the water column overall. Most of these
protists are therefore likely to be
concentrated on particles that sink from the
surface water to the deep. If this is the
case, the abundance of protists in the deep
sea may be a sensitive indicator of particle
flux, or in more general terms, of the input
of organic material from the surface (which
may include biodegradable dissolved
organic matter). One of the interesting
ecological questions is whether there are
specialists in the deep sea that remain
dormant for long periods of time and
become alive when sinking particles
become available. Or, perhaps there are
ubiquitous species in the surface water that
may sink with particles and only become
competitive when they have reached
sufficiently low temperatures and high
pressures.
This research requires a highly
interdisciplinary approach combining the
expertise of physical, chemical, biological
oceanographers, and ocean engineers. It
would not be possible without recent
developments in molecular techniques for
the identification of particular organisms
and groups of organisms. In future cruises
to the subtropical and tropical Atlantic and
the Arabian Sea on the RV Pelagia, we will
explore a wide range of hypotheses on the
distribution and abundance of protists of
the deep sea, particularly the idea of using
their abundance as a sensitive indicator of
organic carbon input.