by Chloe Mayne
Anthropogenic plastic pollution in the ocean has become extremely harmful to marine organisms and their environment. These problems include ingestion, entanglement, leaching of chemicals and adsorption of organic pollutants. The most common marine debris are small plastic fragments, which often are larger items that have been degraded. Microorganisms likely interfere with the degradation process as a result of biofilm formation on plastic surfaces. They may block plastic from UV radiation and photo-catalysis, which would increase plastic longevity. Inversely, microorganisms may accelerate degradation. Fouling microorganisms are extremely important to understanding the problems with plastic pollution, yet they have not been adequately studied. In this experiment, Carson et al. (2013) examine the abundance and diversity of microorganisms on plastics in the North Pacific Gyre.
Three important questions are addressed: what is the density of microorganisms on the surface of the ocean, how does the density vary across the Eastern North Pacific Gyre and how does the density vary in relation to plastic fragment properties of size, type, and roughness. The results found bacillus bacteria and pennate diatoms to be most abundant. The bacteria had an unevenly distributed abundance while diatoms had high densities on rough surfaces and where plastic concentrations were highest.
In July 2011, Carson et al. collected neustonic plastic in the Eastern North Pacific Gyre. The voyage on the R/VSea Dragon began in Honolulu, Hawaii and collected 17 trawl samples across the gyre in a northeastern direction. All of the trawl samples contained plastics. A manta trawl was used to collect the samples by towing the trawl for one hour alongside the ship. The exact trawl length was calculated using a flowmeter. To maximize the debris present on the surface and to avoid waves, the trawls were only utilized during sea states of 4 or less. The contents from the trawl were rinsed into a sieve and preserved with 5% formalin in a jar. Before and after each trawl, the sea surface temperature and salinity were documented.
Before examination, the samples were washed in water. From each trawl sample, eight fragments (1–10mm in size) were selected for analysis. Two of the trawls did not contain the correct size fragments, so only 100 pieces were used in the experiment. Each individual fragment was rinsed with ethanol, covered with a thin layer of gold, and examined under the Scanning Electron Microscope (SEM) for presence of microorganisms. A portion of each fragment was examined from top-to-bottom at 6000x magnification and the organisms were counted. Three random transects of the piece of plastic were analyzed and the data were recorded.
The fragments could not be aged and so each piece was identified as having either a “smooth” or “rough” surface. Species of microorganisms could not be identified so they were classified based on appearance. Diatoms were divided into three groups (pennate diatoms with a raphe, pennate diatoms without a raphe, and centric diatoms) while bacteria were divided into coccoid and bacillus. Dinoflagellates, coccolothopores and radiolarians were also recorded. The rest of the trawl samples were sorted and analyzed under dissecting microscopes. The plastic was dried and sorted by type before it was counted and weighed. Analyses of variance and linear regressions were performed to explain the abundance and diversity of microorganisms.
This experiment found a large number of plastic debris in the Eastern North Pacific Gyre. The trawls contained whole objects, fragments, pellets, line, foam, and films. The average number of pieces was 219 per trawl. Of the 100 pieces sampled, 59 were polyethylene, 33 were polypropylene and 8 were polystyrene. Each piece had microorganisms attached, the most abundant being bacillus bacteria and pennate diatoms. Coccoid bacteria, centric diatoms, dinoflagellates, coocolithophores, and radiolarians were found at much lower densities.
Bacillus bacteria density significantly varied with each trawl site and was also related to the type of plastic. Polystyrene fragments had the greatest abundance of the bacillus bacteria. A negative relationship was found between the bacillus bacteria and sea surface temperature, salinity, and longitude. High densities of bacteria were irregularly distributed within the gyre, while diatoms had the highest densities in the center of the transect. Trawl site and diatom density were significantly related. Density was also high on rough fragments and sites where there was more plastic. Diatoms had a significant negative correlation with temperature and salinity. As the transect moved northeast, density of pennate diatoms with a raphe decreased while those without a raphe increased. Centric diatoms were extremely low in density in this experiment.
The average number of fragments found in this experiment was larger than in the South Pacific and North Atlantic Gyres. While the concentration is highly variable, there was an increase of plastic in the center of the transect. The distribution of plastic-associated organisms was quite irregular. Some variation was explained by the number of microorganisms found on the specific type of plastic. Diatom abundance peaked in the center of the gyre along with the plastic concentration. Higher amounts of plastic may make it easier for these microorganisms to colonize the new surfaces. Plankton abundance did not have a similar distribution. Since there was no standardization during the trawl collections, it is possible that vertical migration changed the plankton abundance based on morning or afternoon trawls.
Surface roughness was related to a high diatom density but not high bacteria density. Rough surfaces are a sign of plastic degradation, which may speed up as a result of colonization. This could result in the sinking or ingestion of an item, which becomes part of the food chain. This process may be accelerated in the gyre center, as there is a greater abundance of plastics and microorganisms.
With global plastic production increasing rapidly, it is expected that there will be a large amount of plastic debris on the ocean surface. Future research is important to understand what can be done once the input of plastic debris slows or stops. Degradation of plastics in the ocean, though not understood, is most likely a result of microbial biofilms. The future of plastic pollution is based on understanding and identifying the microorganisms on the ocean surface and the effects they have on the debris.
Carson, H., Nerheim, M., Carroll, K., Eriksen, M., 2013. The plastic-associated microorganisms of the North Pacific Gyre. Marine Pollution Bulletin 75, 126–132. http://bit.ly/1vpD8L5