Panel 7: Marine Microplastics

Agglomeration of Nano and Microplastic Debris in Seawater by Bacterial Exopolymers
STEPHEN SUMMERS, TONY GUTIERREZ, AND THEODORE HENRY
HERIOT WATT UNIVERSITY

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Can Citizen Scientists Identify Microplastics Ingested by Atlantic Cod? A Single Blind Comparison Across Four Methods
NATALIE RICHÁRD
UNIVERSITY OF NEWFOUNDLAND

Comments 6

  1. Hi Stephen
    Thank you for a really informative presentation, i am really interested in your work! Not only in the aggregation of the micro/nano plastics with the organic matter thus potentially increasing the number of organisms which may accidentally/intentional ingest the plastics while feeding on the marine snow, but also in how these aggregates may act as a mechanism transporting plastics through the water column.

    i am really interested to know how the exoploymer changes the buoyancy of the aggregate/plastics? have you done any work into this, or looked at the sinking rates?
    also, if there are any differences in the ingestion of the plastics with and without the exopolymer present (thus acting to aggregate or not to aggregate)? is this something you plan to look in to?

    With best wishes
    Winnie

    1. Hi Winnie,

      The sinking rates of the aggregates are quite variable. The type of exopolymer alone can vary in density. Though, the fate is generally thought to be an aggregate of polymer and other detritus has a negative buoyancy, resulting in the marine snow we see sinking to the depths. How the addition of plastic affects this method of mass carbon transportation really depends on the plastic type and the concentration of this plastic in the aggregate. A heavy PVC particle (~1.45g/cm3) caught up in the aggregate will accelerate the downward velocity of the marine snow, resulting in accumulation of carbon and plastic at a greater rate than expected. Contrary though, is a lighter plastic such as expanded polystyrene. The marine snow will rise until the trapped air escapes. I have done some initial work looking at the sinking velocity of aggregates containing various sizes of non-expanded polystyrene and as the density of this plastic is similar to the density of seawater we found, as expected, that the addition of plastic had no effect on sinking velocity.
      The work I have done, and that of others I have read normally rely on a single plastic type for the examination of sinking rates. In the natural environment, I would expect to see several plastic types all aggregated together with exopolymers and detritus, making the influence on the sinking rates highly variable.

      Your question about ingestion of the plastics is something we are currently investigating. We expect to see that ingestion rates will be altered once the plastic is contained within an aggregate, however, the data is still pending. Another hypothesis, specific to the smaller nanoplastics, is that retention times in the gut may be altered as a result of aggregation. The expected larger concentrations of plastic ingested as well as the increase in size of the ingested particle (plastic+exopolymer+detritus) may cause the plastic the be retained in the gut for longer periods. Again, this is something we are looking at but have yet to analyse the data.

      Thanks for your comments

      Steve

      1. Thank you for your reply Steve,
        you make some good points here, i too have done much reading of the sinking rates of plastics and only see a few studies, which as you say, focus on a single polymer type. It would be really interesting to look at mixed polymer aggregates, however I feel at this point any experiments to elucidate sinking rates of micro/nano plastics both free i.e. not incorporated into faeces, detritus, exoploymers, biofouling, and with the influence of all of those factors, would really provide some advances to our understanding to the behaviour of plastics in the oceans.
        I am also very interested to hear about the retention, it seems like a logical hypothesis to me. Such fascinating work, with real implications to understand the long term fate of marine plastics. I will keep my eye out for any papers in the near future!

  2. Thanks Stephen for this wonderful presentation,
    it is interesting to see that even nanoplastics are involved into the forming of these aggregates. Before I thought that for the interaction with plastic microbes need some surface space to form biofilms. I would be interested in the relations and differences between approaches that deal with colonisation of surface (plastisphere) and aggregations with exopolymers and other particles in the marine snow if you like to comment on that more. Best, Sven

    1. Hi Sven,

      Sorry for the delay in replying. I also assumed that the plastisphere microbial community would form as a biofilm on the surface. However, I have run several experiment both with and without bacteria present and in each case the exopolymers contribute to the formation of aggregates. This describes the initial stages of aggregate formation, in which nanoplastics and eukaryotes can be captured by the gel like exopolymer. I would assume though that once the aggregate has formed the microbes (and alga) would then contribute to the aggregate mass through biofilm formation, in it’s classic sense.

      I am currently exploring the microbial colonisation of these aggregates and have yet to analyse the sequencing data obtained. However, I would expect the microbial diversity to be strongly influenced by the conditions which the plastic aggregates have been exposed to, probably more so that the type/size of plastic itself. For example, plastics originating from a polluted region may have sorped pollutants on their surface and these pollutants will influence the microbes in these plastisphere communities.

      Steve

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