We use plastics daily thanks to their durability. Unfortunately, this makes them also incredibly hard to degrade, allowing them to accumulate in unexpected places like the oceans. While pictures of sea animals entangled in plastic waste make their way around the world, what escapes our eyes may be an even more complicated problem: the effects of microbes colonizing marine plastic debris. This #FEMSmicroBlog explores for this year’s World Ocean Day how the microbial plastisphere impacts the health of our oceans. #MicrobiologyEvents
A new microbial niche: “the plastisphere”
The term “plastisphere” was first coined in 2013. It describes the ecological niche created when microbes colonize the surface of plastic litter in aquatic environments. Like many other microbial environments, plastisphere formation starts with biofilm formation.
As a densely populated micro-ecosystem, the plastisphere is made up of primary producers, predators, symbionts, and decomposers. Diatoms are considered to be one of the earliest colonizers of plastic surfaces. Over time, as the community matures, they are replaced by other eukaryotes, such as heterotrophic dinoflagellates.
Apart from phototrophs, the plastisphere contains sunlight-converting photoheterotrophs, like Erythrobacter and Roseobacter, and many heterotrophs, that require organic carbon sources, including members of the Gammaproteobacteria, Firmicutes, and Actinobacteria families. Fungi have also been reported in communities growing on plastic debris, even though studies are still rare on the fungal marine plastisphere.
With the plastisphere microbiome differing greatly from the surrounding aquatic communities, it is not clear yet whether plastic material selects for microbial communities. One stark difference is the abundance of heterotrophs such as Candidatus and Pelagibacter. While rich in surrounding waters, they are scarce in ocean plastics.
Another difference between the microbial communities on plastic debris and the surrounding waters concerns fungi. For example, members of the genus Malassezia are abundant in marine environments but have so far not been identified in plastic biofilms.
Like many other biofilm surroundings, environmental conditions, substrate morphology, particle size, and surface properties determine the composition of biofilm communities on plastic litter. Researchers even found considerable seasonal and locational differences between plastisphere communities, while the communities growing on a certain type of plastic appear to be similar.
One small step for microbes, a giant problem for our oceans
As both pathogenic and non-native microbes can hitchhike on plastic debris, long-term impacts for ecologically sensitive areas are inherent. For example, researchers have identified pathogens from the Vibrio, Pseudomonas, and Escherichia genera in the plastisphere, increasingly during summer months.
The long-range transport of pathogen-loaded plastic debris gets even more complicated when the pathogens do not simply ride the debris but also become incorporated in the biofilm matrix. As biofilms provide a beneficial and protective environment for microbes, incorporated pathogens can become even more infectious than their planktonic counterparts. Between sea water, natural aggregates, and plastisphere communities, metabolic pathways related to infection mechanisms were shown to be increasingly expressed in plastisphere samples.
Additionally, drug-resistant bacteria were reported to be present on plastic pieces at 100-5000 times the concentrations than in the surrounding water. Hence, it seems that horizontal gene transfer within plastic biofilms can even impact the formation of pathogenicity islands, where drug resistance is common place.
Microbial colonization on plastic also contributes to the vertical transport of pollutants within marine sediments. The microbial load and biofilm substances on plastic debris results in increased density and decreased buoyancy of the plastic particle leading to sinking aggregates. Thus, plastic waste introduces not only plastic material but also microbes and their metabolites into marine sediments and oceanic food webs.
Microbes as the clean-up crew?
While it seems that microbes make the plastic problems in our oceans even more challenging, they could also be part of the solution. Under controlled conditions, a few marine bacteria and fungi have shown potential to degrade different types of plastic materials.
For example, researchers have isolated plastic-degrading bacteria, such as Alcanivorax xenomutans, Marinobacter sediminum, Marinobacter gudaonensis, Thalassospira xiamenensis and Nocardioides marinus, from Pacific Ocean sediments. Only recently, the fungus Parengyodontium album was isolated from plastic debris in the North Pacific and shown to mineralise polyethylene to CO2.
Yet, Ideonella sakaiensis is likely the best-characterised plastic degrader. Reported first in 2016, this bacterium degrades polyethylene terephthalate, the most widely used type of plastic, into environmentally benign monomers. Currently, more research is underway to improve this bacterium’s efficiency and consistency.
Furthermore, several marine bacterial communities and biofilms were shown to have potential plastic degradation abilities. However, full characterisation of their degradation rates and specificities is still required for us to consider them the plastic-degradation heroes of our oceans.
Since we currently do not have a solution to all the plastic waste in our oceans, all we can do is not increase the burden! So, let’s be part of the solution, not the problem. Happy World Ocean Day 2024!
About this blog section
The section #MicrobiologyEvents for the #FEMSmicroBlog reports about events and meetings relevant to our network. These include world awareness days, FEMS-sponsored meetings or meetings of Member Societies and many more.
| DO YOU WANT TO BE A GUEST CONTRIBUTOR? |
| The #FEMSmicroBlog welcomes external bloggers, writers and SciComm enthusiasts. Get in touch if you want to share your idea for a blog entry with us! |