The plastisphere consists of ecosystems that have evolved to live in human-made plastic environments. All plastic accumulated in marine ecosystems serves as a habitat for various types of microorganisms, with the most notable contaminant being microplastics.^[1][2] There are an estimate of about 51 trillion microplastics floating in the oceans.^[3] Relating to the plastisphere, over 1,000 different species of microbes are able to inhabit just one of these 5mm pieces of plastic.^[4]

Plastic pollution acts as a more durable "ship" than biodegradable material for carrying the organisms over long distances.^[5][6] This long-distance transportation can move microbes to different ecosystems and potentially introduce invasive species^[1] as well as harmful algae.^[7] The microorganisms found on the plastic debris comprise an entire ecosystem of autotrophs, heterotrophs and symbionts.^[8] The microbial species found within plastisphere differ from other floating materials that naturally occur (i.e., feathers and algae) due to plastic's unique chemical nature and slow speed of biodegradation. In addition to microbes, insects have come to flourish in areas of the ocean that were previously uninhabitable. The sea skater, for example, has been able to reproduce on the hard surface provided by the floating plastic.^[9]

History

Discovery

The plastisphere was first described by a team of three scientists, Dr. Linda Amaral-Zettler from the Marine Biological Laboratory, Dr. Tracy Mincer from Woods Hole Oceanographic Institution and Dr. Erik Zettler from Sea Education Association.^[10][11] They collected plastic samples during research trips to study how the microorganisms function and alter the ecosystem. They analyzed plastic fragments collected in nets from multiple locations within the Atlantic Ocean.^[11] The researchers used a combination of scanning electron microscopy and DNA sequencing to identify the distinct microbial community composition of the plastisphere.^[11] Among the most notable findings were "pit formers," crack and pit forming organisms that provide evidence of biodegradation.^[11][12] Moreover, pit formers may also have the potential to break down hydrocarbons.^[11] In their analysis, the researchers also found members of the genus Vibrio, a genus which includes the bacteria that cause cholera and other gastrointestinal ailments.^[13] Some species of Vibrio can glow, and it is hypothesized that this attracts fish that eat the organisms colonizing the plastic, which then feed from the stomachs of the fish.^[14] Studies carried out in the Baltic Sea^[15] and in the Mediterranean Sea^[16], also found microorganisms of the genus Vibrio, in plastic films and fragments, and in plastic fibres, respectively.

Anthropogenic sources

Plastic itself was invented in 1907 by Leo Baekeland using formaldehyde and phenol.^[17] Since then, the material has been used in anything from clothes to artificial heart valves. As a result, the use of plastic has increased twenty-fold since 1964, and it is expected to double by 2035.^[18] Despite efforts to implement recycling programs, recycling rates tend to be quite low. For instance, in the EU, only 29% of the plastic consumed is recycled.^[19] The plastic that does not reach a recycling facility or landfill, will most likely end up in our oceans due to accidental dumping of the waste, losses during transport, or direct disposal from boats.^[19] In 2010, it was estimated that 4 to 12 million metric tons (Mt) of plastic waste entered into marine ecosystems.^[20]

The smaller, more inconspicuous microplastic particles have been aggregating in the oceans since the 1960s.^[21] A more recent worry in the pollution of microplastics comes from the use of plastic films in agriculture. 7.4 million tons of plastic films are used each year to increase food production.^[22] Scientists have found that microbial biofilms are able to form within 7–14 days on plastic film surfaces, and have the ability to alter the chemical properties of the soil and plants that we are ingesting.^[23] Microplastics have been recorded everywhere, even the Arctic due to atmospheric circulation.^[24]

Research

Diversity

Large scale sequencing studies have found alpha diversities to be lower in the plastisphere relative to surrounding soil samples due to a decrease in species richness in the plastisphere.^{[25][26][27][28]} Polymer film fragments affect microbes in different ways, leading to mixed effects on microbial growth rates in the plastisphere.^[25][28][29] Certain polymer degrading bacteria release toxic byproducts as a result of the degradation of the plant fragment, serving as a deterrent to the colonization of the plastisphere by susceptible species.^[25] Phylogenetic diversity is also decreased in the plastisphere relative to nearby soil samples.^[25]

The bacterial and microbial communities in the plastisphere are significantly different from those found in surrounding soil samples, creating a new ecological niche within the ecosystem.^[25][30][31] The specific growth of bacteria caused by film fragments is a primary cause for the creation of a unique bacterial community.^[25][32] Changes in bacterial community composition over time in the plastisphere have also been shown to drive changes in surrounding land.^[25][28][33]

In another study which looked at the factors influencing the diversity of the plastisphere, the researchers found that the highest degree of unique microorganisms tended to favor plastic pieces that were blue.^[34]

A recent experiment carried across the Atlantic Ocean and the Mediterranean Sea aimed at studying the colonisation and genetic variety of plastics in the marine environment, identified tardigrades in in situincubated plastics for the first time.^[35]

Taxonomy

The growth of specific bacteria in their plastisphere occurs because of the ability of certain bacteria to degrade polymers. Phyla of bacteria that have increased presences in the plastisphere relative to soil samples without plastic micro-fragments include Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi, Firmicutes, Planctomycetes, and Proteobacteria.^{[25][36][37][38][39]} Furthermore, bacteria of the order Rhizobiales, Rhodobacterales, and Sphingomonadales are enriched in the plastisphere.^[25] Interactions within the unique bacterial community composition in the plastisphere influence local biogeochemical cycles and ecosystems' food web interactions.

Community metabolism

The metabolism of bacterial communities in the plastisphere are enhanced.^[25] KEGG Pathway enrichment analyses of plastisphere samples have also demonstrated increases in genetic and environmental information processing, cellular process, and organismal systems.^[25] Enhanced metabolic functions for communities in the plastisphere include nitrogen metabolism, insulin signaling pathways, bacterial secretion, organophosphorus compound metabolism, antioxidant metabolism, Vitamin B synthesis, chemotaxis, terpenoid quinone synthesis, sulfur metabolism, carbohydrate metabolism, herbicide degradation, fatty acid metabolism, amino acid metabolism, ketone body pathways, lipopolysaccharide synthesis, alcohol degradation, polycyclic aromatic hydrocarbon degradation, lipid metabolism, cofactor metabolism, cellular growth, cell motility, membrane transport, energy metabolism, and xenobiotics metabolism.^{[25][39][40][41]}

Relationship to carbon, nitrogen, and phosphorus cycling

The presence of hydrocarbon degrading species in the plastisphere proposes a direct link between the plastisphere and the carbon cycle.^[25][42][43] Metagenome analyses suggest that genes involved in carbon degradation, nitrogen fixation, organic nitrogen conversion, ammonia oxidation, denitrification, inorganic phosphorus solubilization, organic phosphorus mineralization, and phosphorus transporter production are enriched in the plastisphere, demonstrating the potential impact on biogeochemical cycles by the plastisphere.^{[25][44][45][46][47][48][49][50]} Specific bacterial phyla present in the plastisphere due to their biodegradation abilities and their role in the carbon, nitrogen, and phosphorus cycles include Proteobacteria and Bacteroidetes.^{[25][42][43][51][52]} Some carbon-degrading bacteria are able to use plastics as a food source.^[53][54]

Research in the South Pacific Ocean has investigated the plastisphere's potential in CO₂ and N₂O contribution where fairly low greenhouse gas contributions by the plastisphere were noted. However, it was concluded that greenhouse gas contribution was dependent on the degree of nutrient concentration and the type of plastic.^[55]

Significance to human health

KEGG Pathway enrichment analyses of plastisphere samples suggest that sequences related to human disease are enriched in the plastisphere.^[25] Cholera causing Vibrio cholerae, cancer pathways, and toxoplasmosis sequences are enriched in the plastisphere.^[13][25] Pathogenic bacteria are sustained in the plastisphere in part due to the adsorption of organic pollutants onto biofilms and their usage as nutrition.^[25][39][40] Current research also aims to identify the relationship between the plastisphere and respiratory viruses and whether the plastisphere affects viral persistence and survival in the environment.^[56]

Degradation by microorganisms

Some microorganisms present in the plastisphere have the potential to degrade plastic materials.^[19] This could be potentially advantageous, as scientists may be able to utilize the microbes to break down plastic that would otherwise remain in our environment for centuries.^[57] On the other hand, as plastic is broken down into smaller pieces and eventually microplastics, there is a higher likelihood that it will be consumed by plankton and enter into the food chain.^[58] As plankton are eaten by larger organisms, the plastic may eventually cause there to be bioaccumulation in fish eaten by humans.^[58] The following table lists some microorganisms with biodegradation capacity^[19]

Oftentimes the degradation process of plastic by microorganisms is quite slow.^[19] However, scientists have been working towards genetically modifying these organisms in order to increase plastic biodegradation potential. For instance, Ideonella sakaiensis has been genetically modified to break down PET at faster rates.^[68] Multiple chemical and physical pretreatments have also demonstrated potential in enhancing the degree of biodegradation of different polymers. For instance UV or c-ray irradiation treatments, have been used to heighten the degree of biodegradation of certain plastics.^[19]