Trillions of persistent plastic particles of varying sizes are scattered throughout the world's oceans, where they often accumulate in ocean gyres known as 'garbage patches'. Two of these regions were the focus of research expeditions by Helmholtz researchers in 2019. As part of the MICRO-FATE project led by the UFZ researchers aboard the research vessel SONNE analysed plastic particles in the North Pacific Garbage Patch between Singapore and Canada while colleagues from the GEOMAR Helmholtz Centre for Ocean Research Kiel aboard the research vessel POSEIDON as part of the PLASTISEA project investigated the North Atlantic Garbage Patch southwest of the Azores. "From a taxonomic perspective, the plastisphere has been well studied. Less is known, however, about the functional strategies that enable microorganisms in the biofilm to survive the extreme conditions of a nutrient-poor environment and high UV exposure at the ocean's surface", says UFZ hydrobiologist and co-author Dr Mechthild Schmitt-Jansen.
During their ocean expeditions, the researchers collected macroplastics from the ocean surface and extracted DNA from the plastisphere. They then sequenced their metagenomes – that is the total DNA of a biological community – and compared the structure and function of microbial metagenomes of the plastisphere in the Pacific and Atlantic oceans with that of plankton naturally occurring in the sea. The analyses focused on functional genes. These sections of DNA encode important functions for organisms and thus form the basis of biological processes. "Functional genes contain genetic information that enables microbes to produce proteins, control metabolic processes, build cell structures, and regulate signalling processes within the cell", says GEOMAR microbiologist and co-author Dr Erik Borchert.
In their analysis of around 340 key functional genes, Helmholtz researchers found that the bacterial metagenome of the plastisphere differs considerably from natural plankton communities in the Pacific and Atlantic in terms of both, structure and function. The metagenome thus contains more of those functional genes that enable microbes in the plastisphere to survive under the extreme conditions of the open oceans. "The microorganisms in the biofilm have more gene copies, thereby enabling them to absorb nutrients effectively, utilize and break down carbon sources, and either ward off UV radiation through effective mechanisms or repair damage to the genome quickly", says UFZ biologist and lead author Dr Stefan Lips. They can also use alternative energy sources such as anoxygenic photosynthesis, which does not produce oxygen.
Differences were also observed in the taxonomic structure of the biofilm: although the species composition within the bacterial groups differs between the Atlantic and the Pacific, the functionally relevant bacterial groups are comparable in both oceans.
The research team also found that the genomes of microbes in the plastisphere are considerably larger than those of naturally occurring marine plankton. Over the course of evolution, plankton have adapted their genomes to nutrient-poor environments and greatly reduced their genome size. The microbes in the plastisphere do not need to do this because they benefit from the shared metabolic processes of the microorganisms on the plastic particles – and thus from a better availability of nutrients. Furthermore, relatively high concentrations of chlorophyll a were found in the biofilm compared with plankton. "This shows that, in relative terms, microbes in the plastisphere have the potential to produce more biomass than the surrounding plankton", says Schmitt-Jansen. "This creates eutrophic niches in the nutrient-poor environment of the open oceans".
The research results show how microorganisms in the plastisphere adapt to the harsh living conditions in nutrient-poor subtropical ocean gyres. "This is not a good sign for the oceans, because only their original, natural state is considered healthy – and any deviation from that is seen as a deterioration", says Lips. Whether biofilm growth on plastic disrupts the geochemical cycles of these sensitive ecosystems remains a subject of ongoing research. "Because microbes use plastic as a habitat rather than a source of nutrients, it is unlikely that they will help to remove plastic from the oceans", says Borchert. That is why it is imperative that we put a stop to plastic pollution as soon as possible.
This collaborative study by the UFZ and GEOMAR was conducted as part of the InnoPool projects P-LEACH and AI MareExplore and was funded by the Helmholtz Association and the BMFTR projects MICRO-FATE and PLASTISEA .
Further information:
An ARTE film about the plastisphere (featuring Mechthild Schmitt-Jansen): https://www.arte.tv/en/videos/114189-000-A/plastic-and-marine-life/
