What inspired you to study the microbial communities of ocean
plastic garbage patches?
I did my Ph.D. working on the dispersion of larvae from sea
urchins.
I changed to work in
bacterial diversity on aggregates, and in one of my projects, I
worked on fibers from waste treatment plants. In 2014, I
coordinated an expedition working on plastic.
When Romain [Troublé]
proposed to me this leg of the Tara Pacific expedition, I said,
"Yes, no problem!"
On the Tara, how do you collect samples from the Great Pacific
Garbage Patch, and how are these samples analyzed once they
arrive back on land?
For this experiment, we collect plastics with a Manta net [a
fine-mesh collection device used while the boat is in motion].
We can assess the
size class larger than 300 microns. We track at 3 knots because
we need very calm conditions.
In the Great Pacific Garbage Patch, we deployed several Mantas.
One is for genomic analysis, one is for assessing the spatial
distribution of the fragments, and another is to identify all
kinds of biodiversity associated with plastic.
We deployed up to
five Manta nets a day, so we spent all day doing that and
sorting plastics. For one throw of the Manta net, a 30-minute
throw, we'd find 500 fragments of plastics.
We place the plastics in tubes for genomic sequencing. We
sequence 16S and 18S ribosomal RNA [which is essential for
protein synthesis] for specific bacteria. We also do metagenomic
analysis with
Genoscope [the French National Sequencing Center].
It gives us the whole picture - the free communities of microbes
and the communities associated with plastic aggregates.
Identifying species
associated with plastics will make it possible to determine
zones of bacterial colonization and to better understand the
dispersion of alien, toxic and pathogenic species.
The
majority of plastic that we found in the Mediterranean is
polyethylene and polypropylene, two major components of
packaging.
We are also going to
scan samples from the Great Pacific Garbage Patchto establish a
plastic-to-plankton ratio, like an indicator of the state of the
environment. If there is more plastic than plankton, fish will
be starved because they eat the plastic instead of the plankton.
In this area, the ratio is very, very high.
Previous studies
found there is about one kilogram of plankton for each six
kilograms of plastic.
Contrary to popular descriptions of the Great Pacific Garbage
Patch as a "continent" of discarded plastic, it is more like a
swirling "soup" of trillions of trapped plastic fragments.
Researchers on the
Tara deploy a Manta net to collect samples of this detritus
(below-left). The collected plastic
material (below-right) ranges in size from a few millimeters down to
microns.
©
Samuel Bollendorff
Tara Expeditions Foundation
When people
think about the Great Pacific Garbage Patch, they often imagine
something visible, like an island. What is it really like to be
in the middle of the patch?
The Great Pacific Garbage Patch is not an island you can clean
up with waste boats. It's just a lot of microplastics spread
over a very large area, and a very small concentration of
plankton.
Publications about
the Great Garbage Patch have outlined the presence of
macroplastics [plastic fragments typically larger than five
millimeters], but we found mainly microplastics.
We counted some large
objects, but not many in relation to the extent of the garbage
patch.
When the sample arrives on the boat, we have a lot of
microplastics - I cannot give one term [to describe it]. We use
"soup," but, you know, it's a poor soup, because there's a lot
of plastic and few plankton.
How is the
microbial community in ocean garbage patches different from what
is found elsewhere in the ocean?
Microbes attached to plastics are a
distinct biological
community with different physical [and] chemical characteristics
from free-living microbes.
Microbes really take
advantage of being on the surface of plastics - they are very
opportunistic. When they have no available substrate, they stay
in a dormant state for a long time.
But when they find a
substrate like plastic, they develop a lot. In the Mediterranean
Sea, we found a lot of benthic cyanobacteria on plastics. These
cyanobacteria are filamentous and often live in the deep. They
have the capability to adapt to very different environments.
They can adapt for
different concentrations of nutrients because they are able to
fix nitrogen.
In the Mediterranean, we also found that the keystone species of
bacteria on the plastic specialized in degrading hydrocarbons.
Plastic is a polymer, composed of a chain of monomers.
What these bacteria
do is use the carbon in the monomers [for energy] - this is the
way that they reproduce.
In general, the functions of microbial communities [on plastic]
will be nitrogen fixation, gene transfer and degrading plastic.
In our results, the bacteria attached to plastic are the ones
that are able to degrade hydrocarbons.
Pathogenic organisms are also associated with plastic, so we
will evaluate the potential of harmful microbial taxa. I worked
on a project where my colleagues found the Vibrio gene in the
North Atlantic where was associated with microplastics in the
North Atlantic.
They found Vibrio all
the time, a group of bacteria including those that cause
cholera. Most marine bacteria are harmless, but several taxa can
cause disease in humans and animals.
Two researchers aboard the Tara, Melanie Billaud and
Nils Haentjens, inspect a sample collected with the Manta
net.
Plastic
concentrations in the Great Pacific Garbage Patch are 10 times
those in the Mediterranean, one on the world's dirtiest seas.
© Samuel Bollendorff
Tara Expeditions Foundation
How do you think results from your Great Pacific Garbage Patch
project will compare with those from your Mediterranean project?
We will compare the Mediterranean system with the Pacific system
to see if we have the same kind of microbial attachment [to
plastic].
Is it a common
mechanism, or is it a specific mechanism for the Mediterranean?
In the Mediterranean, the diversity on plastics is higher than
the diversity of free-living bacteria. This is very interesting.
How can bacteria use
these plastics? Why is the diversity higher? Maybe because
bacteria are specialized for some kind of substrates and can use
plastics as a substrate.
Another colleague, a
student, did an experiment that shows us that biofilms start on
degraded plastic. It's very difficult to start with new plastic.
Microbes need this
kind of degraded surface to colonize.
How do
microbes in ocean garbage patches interact with the rest of the
ocean food chain? Could these changes eventually affect humans?
The plastisphere system harbors toxic microbial species.
There is a lot of
this plastic in oysters or mussels - if you eat oysters
directly, you also eat the stomach. If there are pathogenic
[microbes], the transfer is direct when you eat filter feeders.
Another threat is that filter feeders [fish that eat by
straining small organisms like krill from seawater] confuse the
thin plastic with plankton - if you are a fish, you cannot
separate them.
With the ingestion of
plastic, fish will be starved, and the food chain will be
impacted. Everything is connected.
Earlier
this year, German researchers reported that bacteria living on
microplastics have
unusually high gene exchange rates, giving
rise to antibiotic resistance. How might this affect the rest of
the ecosystem?
They inoculated the plastic - they realized a kind of mutation
to adapt microbes to a new environment.
It's usually a very
long adaptation process, but they show that this can happen very
rapidly on plastic, because there are a lot of different groups
of bacteria.
This could be harmful for human health. If you are in
Bangladesh, for example, and these plastics arrive on the river,
you could have an epidemic situation, because the bacteria adapt
themselves very fast.
"There are no magic devices that can clean the oceans,"
according to Pedrotti, even if studies of the microbes that grow
on the seaborne plastics can help with finding ways to degrade
them.
Cleaning coastal
areas, however, may be an effective first step in limiting the
pollution.
Maria-Luiza Pedrotti
Villefranche Oceanographic Laboratory
©
Samuel Bollendorff
Is it possible to clean up ocean garbage patches completely?
It's not feasible, in my opinion, when you see the extent of the
phenomenon.
We cannot collect
small plastic, like
microplastic, and we cannot be efficient in
large areas. Even biodegradable plastic is not good for the
ocean.
Yes, we could ban
plastic bags and replace them with biodegradables, but it
wouldn't work. If you throw biodegradable plastic into the sea,
it may not biodegrade under those conditions, and it can still
affect the ocean.
There are no magic devices that can clean the oceans.
But cleaning coastal
areas, locally, is a very good solution because it helps to
remove the plastics before they enter the greater ocean
ecosystem.
Even so, some scientists - including your team - think garbage
patch microbes can teach us something valuable about how to
break down waste.
The idea is that maybe it will be a solution to stop pollution.
Bacteria and fungi
have the ability to efficiently degrade polyethylene,
polypropylene and other polymers. What bacteria do is release
enzymes to cut polymers into monomers.
How do you
plan to explore the workings of the plastisphere over the next
few years?
Plastics can facilitate microbial colonization and be vectors of
potential invaders, harmful algae and pathogens in the sea.
Understanding that is
the first step, but the second is to try to cultivate the
organisms that are attached to the plastic.
We need more research. Studies on plastic-associated microbial
communities are lacking; we don't understand exactly how these
microbes are affecting ocean ecology globally.
To be credible, we
have to be humble.
