Plastic became an omnipresent material in the
everyday life these days and one has to deal with the waste. A lot of plastic
is not recycled, therefore, a lot of plastic occurs in the environment, also in
the ocean waters (like in the Great Pacific Garbage Patch). Plastic and its
particles can be ingested by animals or can be incorporated in the sediments. Consequently,
plastic debris is a significant problem in environmental pollution and can also
be ingested by humans who consume e.g. fish that on its part has ingested
plastic particles.
Normally, plastic is positively buoyant and
floats at the seawater-air interface. As a result of fouling processes and
degradation it can lose its buoyancy and start sinking and therefore e.g. can
be incorporated in the sediments. Attaching algae to the plastic and other
organisms can increase the degradation process and make the plastic neutrally
buoyant. The attaching organisms can form biofilms, whereby firstly bacteria
will attach to the plastic, followed by unicellular eukaryotes, larvae and
spores.
The study by Lobelle & Cunliffe (2011)
aimed to examine the early biofilm formation on plastic debris to get a better understanding
of the mechanisms of the behaviour of plastic in the oceans.
For that reason, plastic food bags (polyethylene)
were secured to boards and brought into the coastal waters in Plymouth, UK at
2 m depth for a duration of 3 weeks in summer. The plastic bags were
sampled weekly and brought to the laboratory. Lobelle & Cunliffe used crystal
violet to dye the samples and to make the attached biofilm visible. The stained
plastic samples were incubated in ethanol, which was then transferred into a
cuvette and its optical density at 595 nm was measured. Buoyancy was
measured by placing the washed plastic samples in sterile seawater and noting
its position after 10 min. Furthermore, the hydrophobicity was measured
and the attached biofilm was diluted and plated out on Marine Agar 2216 (Difco)
and polyethylene marine agar plates and incubated for 3 weeks to determine the
number of culturable bacteria on the plastic samples.
After the first week of the experiment a visible
biofilm has started to form and significantly increased during the 3 weeks.
Coinciding with this, a decrease in hydrophobicity as well as an increase in
hydrophilicity after 2 weeks were detected. Additionally, the plastic lost its
positive buoyancy after about 2 weeks and floated below the seawater-air
interface when tested in the laboratory and showed neutral buoyancy in week 3.
Apparently, the plastic samples were positively
buoyant again and floated at the seawater-air interface as the control plastic in
week 3, which is contradictory to the finding presented above. The authors
mention that physicochemical effects of biofilm can be reversible and plastic
can be positively buoyant when rapidly defouled, but do not really explain the
finding and it remains unclear.
Over the duration of the experiment an
increasing number of culturable bacteria has been found but no polyethylene-degrading
bacteria were detected. Previous studies have shown that plastic-degrading
bacteria like Pseudomonas or Arthrobacter have only been isolated
from plastic after 12 weeks. Consequently, it takes longer to “infect” plastic
with plastic-degrading bacteria.
The paper by Lobelle & Cunliffe is a bit hard to
follow because it has no subheadings to structure it more. Moreover, it
lacks some information to clarify the results. It is not clear, why the plastic
was placed at 2 m depth. I think that, firstly, the plastic would float in
the SML and would start to sink after a while. I think that the composition of
the bacterial communities differs between the SML and at 2 m depth, so it
would appear more logic to me to place the plastic bags directly at the
surface, because plastic-degrading bacteria may be more abundant in the SML. A
possible reason for the placement could be that the plastic cannot be ingested
by e.g. birds during the duration of the experiment. Additionally, I have not found
the results of the optical density measurements. In some points the study seems
to be incomplete or some parts were not chosen to be published in this 4 page
long paper.
All in all, this study provides a good base for
understanding the influencing factors on plastic debris behaviour. It has been
cited nearly 100 times since its publication and shows that plastic-degradation
may start after more than 3 weeks and is a long process that includes different
factors. It would be interesting to see how the results change when the study
is conducted with the plastic floating directly at the seawater-air interface
and with a different kind of plastic.
Reviewed paper:
Lobelle, D., & Cunliffe, M. (2011). Early microbial biofilm formation on marine plastic debris. Marine Pollution Bulletin, 62(1), 197-200.
http://ac.els-cdn.com/S0025326X1000473X/1-s2.0-S0025326X1000473X-main.pdf?_tid=e830865e-b55f-11e6-b984-00000aab0f27&acdnat=1480333600_8074b98c59785186b7da541593992c3d
Hi Eleni, very interesting read!
ReplyDeleteAfter doing some reading on plastics and seeing how they can provide protective niches for a diversity of different microorganisms I stumbled across the term "Plastisphere".
I was just curious if you had heard anything about possible harmful Vibrios colonizing this "Plastisphere", potentially utilizing it as a vector of infection? There was this great paper by Kirstein et al.,(2016) who found Vibrio parahaemolyticus on numerous types of micro plastics.
So my question is do you think it may be possible that this huge influx of plastics and potential vectors may increase human exposure to harmful pathogens?
Looking forward to your reply,
Stefan
Paper Referenced:
Kirstein, I., Kirmizi, S., Wichels, A., Garin-Fernandez, A., Erler, R., Löder, M. and Gerdts, G. (2016). Dangerous hitchhikers? Evidence for potentially pathogenic Vibrio spp. on microplastic particles. Marine Environmental Research, 120, pp.1-8.
Hi Stefan,
Deletethanks for your comment an the paper!
I am not sure if there are human pathogens colonizing the plastic. However, I think there is a possibility that this could happen. It may all depend on the conditions on the plastic and if the pathogens like Vibrio can deal with these (like increasing hydrophilicity etc).
I think that Johanna reviewed a paper dealing with this subject some weeks ago.
Eleni
Hi,
Deleteyes, I did review that exact paper about a month ago (http://2015-mbio322.blogspot.co.uk/2016/11/the-hitchhikers-guide-to-oceans.html)
I also recently stumbled upon another paper on the subject. Here, the authors used electronic and fluorescence microscopy to show how V. crassostreae (an oyster pathogen) colonizes Polystyrene Microparticles. Apparently, they are secondary colonizers, requiring an established biofilm before adhesion.
Johanna
Reference:
Foulon, V., Le Roux, F., Lambert, C., Huvet, A., Soudant, P., & Paul-Pont, I. (2016). Colonization of polystyrene microparticles by Vibrio crassostreae: light and electron microscopic investigation. Environmental Science & Technology, 50(20), 10988-10996. Link: http://pubs.acs.org/doi/abs/10.1021/acs.est.6b02720
Hi Eleni,
ReplyDeleteThank you for your review. I was just wondering if the results found here may be specific to the area it was tested in (e.g. Plymouth) and if/how the results would change at different places with different plastic-degrading bacteria? Do you think this buoyancy time is reflective of the global situation, or do you think there may be big fluctuations in this time depending on the area?
Thanks,
Amy
Hi Amy,
Deletethanks for your comment. I think that the buoyancy is also effected by the hydrophilicity that increases over a time period. I am not completely sure but I think that the increase in hydrophilicity is influenced by abiotical factors as salinity and temperature. Additionally, it can be influenced by the composition of bacteria on the plastic. You already mentioned, that it is likely that the bacteria found in this experiment may be specific to this area. So I think that other bacteria occuring in the biofilm would change the hydrophilicity/hydrophobicity and therefore the buoyancy.
But all in all I don't think that there would be enormous changes/fluctuations in buoyancy time but slight changes are very likely.
Eleni