The particular characteristics of the plastic matrix, such as its floating ability and hydrophobicity, have created a new unique substratum for microbial colonization [25,50,51]. The new micro-niche thus created becomes occupied by a specific biofilm called the plastisphere [25,42,52,53,54,55].

The total mass of the plastisphere in the oceans cannot be neglected, representing about 0.01–0.2% of the total microbial biomass in their surface waters [42]. However, because of the unknown total amount of plastic discarded in the oceans, the total mass of the plastisphere may be much higher than this [3,42]. Indeed, some authors have described MPs and their associated plastisphere as the eighth continent [52,56,57]. More research on the plastisphere and its importance in biogeochemical cycling and the resulting environmental balance [58] is fundamental.

As a result of the different physicochemical conditions in fresh and saline water, the microbiota in these two ecosystems is distinct, which can impact the structure and evolution of the microbial populations in these environments [53] The microbial ecology of the plastisphere, however, is mainly controlled by the composition of the colonized plastic [59]; MPs work as a filter for microorganisms in the environment.

As hydrophobic organic surfaces with large surface area:volume ratios, MPs readily attract organic matter from the water column, including organic carbon sources and pollutants such as pesticides [60] and hydrocarbons [61]. In addition, many of the chemical compounds added to plastics during their industrial production are toxic to the colonizing microorganisms. These characteristics turn the MP surface into a very complex substratum that is highly selective for colonization by specific microbial species.

Nowadays, thanks to new technologies based on metagenomics, it has been possible to observe the complexity and partially understand the operation of the plastisphere. Reisser et al. (2014) [62] and Dussud et al. (2018) [33] confirmed the influence of certain properties of plastic fragments such as composition, size, degree of degradation, and surface roughness. Amaral-Zettler et al. (2015) [42] noted important differences between the microorganisms colonizing MPs in two different oceans, and between planktonic and sessile cells on MPs in the same environment. Oberbeckmann et al. (2018) [2] and Debroas et al. (2017) [63] showed that the microbial communities present on the surfaces of marine MPs are very different from those in surrounding middle and upper waters or on other particle types (Figure 1). The authors reported greater abundance and richness of colonizing bacterial assemblages on a natural substrate compared with MP communities. This suggests that the modern universal availability of MPs in our oceans not only affects the structure, composition, and functional properties of attached bacteria but also represents a potential ecological risk as a function of the high stability, pathogenicity, and stress tolerance of the bacterial communities present on the MP surface.

Some bacterial groups, such as the phyla Bacteroidetes, Proteobacteria, Cyanobacteria and Firmicutes, are more often found colonizing MPs than other types of particles [25,33]. Certain bacterial taxa, then, seem to be more resistant to the toxic compounds of the plastic matrix, either naturally, or because of ready metabolic adaptation. The latter may be linked to processes such as attachment, degradation or chemotaxis [25,33].

Under the protective impact of the plastisphere, MPs can translocate the local microbiota to other areas, “rafting’’ microorganisms from their origins to other ecosystems [52,59]. Plastic items produced by humans and discharged into the marine environment as wastes can therefore be responsible for the migration and transportation of allochthonous species in aquatic environments (Figure 1). In this way, it has been suggested, pollution-resistant [64] or antibiotic-resistant [64,65] microbial groups may spread worldwide [52,66].

Human and non-human pathogenic bacteria have been detected in the plastisphere, again indicating the importance of this protective milieu for disease transmission. One of those most commonly reported is the genus Vibrio, which contains species pathogenic to humans [67] and to crustaceans [68]. E. coli pathotypes have also been detected in marine plastispheres [69]. In addition, micro-algae and cyanobacteria responsible for algal blooms have been implicated in plastisphere-associated transfer [33]. The adherent organisms may be released from the plastisphere when it breaks down because of a change in environmental conditions or through the action of biodegradative organisms within it.

According to Ward et al. (2022) [70], there are significant changes in colony formation during the first weeks of plastisphere production, revealing a complex ecological succession during the period of colonization of the micro-niche. Erni-Cassola et al. (2020) [71] reported that bacteria capable of using hydrocarbons as a carbon source play an important role in the initial stages of the process of colonization of the plastic surface. Similarly, Teughels et al. (2006) [72] and Rummel et al. (2017) [45] believe that the first stages of ecological succession and resulting colonization are dominated by species more adapted to more hostile environments, pioneer substrate-specific taxa capable of degrading plastics, later replaced by more generalist biofilm component species [41]. Initially, bacteria and diatoms are the major biofilm components, but other organisms, such as microalgae, fungi and heterotrophic protists (flagellates and ciliates), also populate these micro-niches. They may bring other degradative activities to the plastisphere. Degradation of plastics in the marine environment has, however, been less studied than in freshwater or soil, and degradation rates are practically unknown [73]. Goudriaan et al. (2023) [74] discuss the problems and deficiencies of studies on biodegradation of plastics in the marine environment. Unambiguous proof of microbial degradation and quantification of the normally low degradation rates are two problematic areas. There are, however, numerous studies of biodegradation in other environments [75,76,77,78,79,80,81,82].

During biofilm maturation and microbial succession, biological transformations occur in parallel with physical and chemical changes that include degradation and oxidation of the polymer itself by microbiota living on the plastic particle surface in an ecologically complex multilayer micro ecosystem [46]. Microorganisms may be both stimulated and inhibited within the highly variable physicochemical microclimate of the MP surface, depending on the additives and contaminants present. The plastic biodegradation process depends on many variables, such as polymer composition and resulting molecular weight, particle surface physical characteristics and environmental parameters [83,84,85]. The process has been evaluated by monitoring a varied group of parameters. These are substrate weight loss, changes in mechanical properties and/or chemical structure of the polymer, and the percentage of carbon dioxide released. The initial tests of microbiological biodegradation sought to prove that microbial activity would result in physical changes in the polymer matrix, such as mechanical strength, degree of crystallinity and water absorption [86,87]. The various plastic biodegradation processes are directly related to the compositional particularities of each polymer, just as the active sites of enzymes are particular to their specific substrate configurations. The main polymeric compounds can be divided into three groups: polymers whose basic molecule is formed by linear carbon chains (polyethylene—PE, polypropylene—PP, polystyrene—PS, and polyvinyl chloride—PVC); polymers with ester-linked backbones and side chains (polyethylene terephthalate—PET, and polyurethane—PU); and polymers with hetero/carbamate(urethane) linkages (polyurethanes—PUs) (Figure 2).