In the oceans, floating plastic waste is colonized by bacteria and a biofilm is created. Plastic is mixed with organic additives that make the material elastic and malleable. To find out whether these additives influence the activity of the bacteria and, thus, the nutrient cycle, Cristina Romera-Castillo, University of Vienna, Austria, and colleagues have investigated how much organic material is extracted from plastic waste and what role sunlight plays in this process.

For this purpose, the researchers mixed different types of plastic films and granules for several days in germ-free seawater and irradiated it with artificial sunlight or left it in the dark. They then mixed the water with typical bacteria from the Mediterranean Sea and analyzed the growth and activity of the microbes.

Immediately after contact with water, the plastic parts released organic compounds, both in the dark and under the influence of light. The exact amount fluctuated depending on the type of plastic. Polypropylene packaging released significantly more organic substances than products made of polyethylene. The added bacteria took up about 60 % of the dissolved organic compounds from the plastic within about five days. The activity of the microbes was stimulated and they grew better. Sunlight, in contrast, reduced their growth. The UV radiation leads to structural changes in the organic compounds dissolving from the plastic, so that bacteria can absorb them less efficiently.

The researchers estimate that the approximately ten million tons of plastic that land in the sea each year emit about 23,600 tons of organic compounds. According to current forecasts, plastic pollution in the oceans will increase tenfold over the next ten years. Thus, the organic material dissolved from the plastic will drastically increase and stimulate bacterial growth. As a result, the entire natural carbon cycle in the ocean could change.

• Dissolved organic carbon leaching from plastics stimulates microbial activity in the ocean,
  Cristina Romera-Castillo, Maria Pinto, Teresa M. Langer, Xosé Antón Álvarez-Salgado & Gerhard J. Herndl,
  Nat. Commun. 2018.
  https://doi.org/10.1038/s41467-018-03798-5