Microbial hitchhikers on microplastics: The exchange of aquatic microbes across distinct aquatic habitats

Microplastics (MPs) have the potential to modify aquatic microbial communities and distribute microorganisms, including pathogens. This poses a potential risk to aquatic life and human health. Despite this, the fate of ‘ hitchhiking ’ microbes on MPs that traverse different aquatic habitats remains largely unknown. To address this, we conducted a 50-day micro-cosm experiment, manipulating estuarine conditions to study the exchange of bacteria and microeukaryotes between river, sea and plastisphere using a long-read metabarcoding approach. Our findings revealed a significant increase in bacteria on the plastisphere, including Pseudomonas , Sphingo-monas , Hyphomonas , Brevundimonas , Aquabacterium and Thalassolituus , all of which are known for their pollutant degradation capabilities, specifically polycyclic aromatic hydrocarbons. We also observed a strong association of plastic-degrading fungi (i.e., Cladosporium and Plectosphaerella ) and early-diverging fungi (Cryptomycota, also known as Rozellomycota) with the plas-tisphere. Sea MPs were primarily colonised by fungi (70%), with a small proportion of river-transported microbes (1% – 4%). The mere presence of MPs in seawater increased the relative abundance of planktonic fungi from 2% to 25%, suggesting significant exchanges between planktonic and plas-tisphere communities. Using microbial source tracking, we discovered that MPs only dispersed 3.5% and 5.5% of river bacterial and microeukaryotic communities into the sea, respectively. Hence, although MPs select and facilitate the dispersal of ecologically significant microorganisms, drastic compositional changes across distinct aquatic habitats are unlikely.


INTRODUCTION
Microplastic pollution poses a global environmental issue of increasing concern in recent years (Amelia et al., 2021;van Sebille et al., 2015).Microplastics (MPs) such as plastic particles less than 5 mm in size, can enter aquatic ecosystems through industrial or domestic sewage and littering and can become a persistent part of the ecosystem, impacting numerous aquatic organisms (Amaral-Zettler et al., 2020;Worm et al., 2017).These particles serve as a floating substrate for microbial growth and, thus, can host and transport numerous microbial species (Oberbeckmann & Labrenz, 2020).
Yet, we know little about the fate of microbes on microplastics that cross the boundaries of distinct aquatic habitats.However, considering the significant amount of microplastics emitted by rivers into marine systems (Lebreton et al., 2017), it is crucial to understand the effects of freshwater-origin plastisphere when they reach marine habitats and coalesce with marine communities.In estuaries, such river-sea community coalescence is a particularly common phenomenon.Community coalescence is an emerging concept that involves whole-scale mixing of communities and their surrounding environments (Rillig et al., 2015) but is scarcely studied at interfaces like river-sea junctions (Mansour et al., 2018).
Previous studies suggest that the low salinity of the Baltic Sea exerts great influence on microorganisms from freshwater rivers, shifting the final, mixed community towards that of the sea (Rocca et al., 2020;Song et al., 2022;Székely et al., 2013).Nevertheless, there is also evidence for riverine prokaryotes that can successfully grow in the brackish condition of the northern Baltic Sea, as reported by Kisand et al. (2005).Salinity has a relevant role in differentiating bacterial diversity and biomass on plastic particles as well (Li et al., 2019;Qiang et al., 2021).Recently, it has been shown that microplastics can affect aquatic microbial communities by altering their compositions (Kettner et al., 2017), facilitating species dispersal (Mas o et al., 2003;Simkanin et al., 2019;Song et al., 2022) and even promoting gene exchange among bacteria (Arias-Andres et al., 2018).Harmful algal species (dinoflagellates) (Mas o et al., 2003) and (potentially) pathogenic bacteria (e.g., Vibrio spp., Escherichia spp.and Pseudomonas spp.) (Curren & Leong, 2019;Frère et al., 2018;McCormick et al., 2014) can hitchhike MPs, creating hubs for antibiotic resistance genes (Arias-Andres et al., 2018;Pham et al., 2021).MPs might even amplify the dispersal of fungi by providing refuge or opportunity to settle (Kettner et al., 2017;Lacerda et al., 2020).
Studies targeting plastic-associated microeukaryotes, however, have been largely overlooked (Amelia et al., 2021), or had a mere explorative focus (Davidov et al., 2020).Hence, we have a very limited knowledge of microbial communities of a broader spectrum (e.g., considering both bacteria and microeukaryotes) that form biofilms on microplastics.Besides, to our knowledge, no study has yet investigated systematically how MPs influence the exchanges of aquatic microbes from a community coalescence perspective.
Hence, in this present 50-day-long microcosm experiment, we assessed the effects of MPs on bacterial and microeukaryote communities during the mixing of freshwater river and brackish seawater.By tracking the sources of microbial species of the established biofilms on microplastics (that is, the plastisphere), our goal was to gain a deeper understanding of the fate of microbial hitchhikers of microplastics that are crossing boundaries of distinct aquatic habitats, from freshwater rivers to brackish seawater.We hypothesised a compositional shift of the plastisphere community towards that of the sea community when river communities are being mixed into a brackish environment due to selection processes induced by the environmental conditions of the recipient seawater.Furthermore, we assumed that only a subset of taxa from the planktonic community, comprising ecologically relevant (i.e., pathogenic and pollutants-degrading) species, establishes biofilms on MPs, hence, being dispersed across aquatic used to perform the treatments described below.After sufficient biofilm formation (i.e., day 30): (i) river community (MPÀ) coalesced with the sea (MP+/À) communities, separately, in a 1:1 ratio to mimic estuary conditions; (ii) river (MP+) community coalesced with the sea (MP+/À) communities, separately, in a 1:1 ratio; (iii) River MPs (30 and 15 particles) were transferred into the sea (MP+/À) community, respectively (to have equal final number of MPs in all MP-treated microcosms, 15 MPs of sea (MP+) community were removed); (iv) simultaneous community coalescence and MP transfer of river (MP+) community and sea (MP+/À) community was performed as described in treatment of (ii) and (iii).One of the treatments (i.e., sea (MPÀ) mixed with river (MP+) community and river MPs) did not result in a sufficient amount of extracted DNA and PCR products.Our aim in performing these above-mentioned treatments was to cover all potential scenarios where the effects of microplastics on aquatic microbes are influenced by the mixing of natural communities (e.g., influx from river to sea).
Every fifth day 20% sample volume of each microcosm was exchanged with its respective medium.For this, each replicate 'A' of the control and treatment communities received medium from replicate 'A' (i.e., R m , S m ).Likewise, each replicate 'B' received medium from replicate 'B', and so forth.Both community coalescence, medium refreshment and MP transfer were carried out under sterile conditions using sterile disposable pipettes and tweezers, respectively.All cultures (60 mL each) with four replicates were maintained in sterile culture flasks with filter caps (Sarstedt, Nümbrecht, Germany).The incubation was carried out at 10 C under a photoperiod set to 10:14 h light: dark cycle to mimic ambient conditions (Andersson et al., 1994) with $50 μmol photon m À2 s À1 .Twice a day, the microcosms were mixed by gently shaking and randomly placed to minimise the potential biases of the differential light in the experiment room.
At the end of the experiment (day 50), MPs were gently transferred into sterile Petri dishes, washed twice with MQ water to remove non-attached microorganisms and stored at À80 C. The planktonic community of each treatment was collected onto 0.2 μm 47 mm membrane filters (Pall Supor) by vacuum filtration, and the filters were stored at À80 C.

Metabarcoding and sequence analyses
Planktonic and plastic-associated (plastisphere) DNA was extracted from the filters and microplastics (n = 20), respectively, using the Qiagen DNeasy PowerSoil Pro Kit (Hilden, Germany) following the manufacturer's protocol.DNA extracts were quantified with Qubit 1Â HS Assay Kit (ThermoFisher Scientific).
For the bacterial community profiling, we amplified the V1-V9 region of the 16S rRNA gene using the 27F (Lane, 1991) and 1492R (Stackebrandt & Liesack, 1993) primers with an expected amplicon of $1.4 kbp (Table S1).To do so, PrimeStar GXL polymerase (TaKaRa Bio) was used following the suggested rapid protocol by the manufacturer, using F I G U R E 1 Overview of the experimental design.Pre-filtered (<200 μm) water samples from two sites (river and sea) were collected to inoculate our microcosms (river microbes = brown dots, sea microbes = blue dots).Control and treatment communities consisted of MP-free (MPÀ) and MP-polluted (MP+) river (1) and sea (2) communities.On day 30, community coalescence was performed by mixing river and sea community at a 1:1 mixing ratio to create coalesced cultures (treatments of i, ii and iv), and microplastics (MPs; empty circles) from river communities were transferred into sea communities (treatments of iii and iv).Sterile-filtered (<0.2 μm) media were used during the experiment (i.e., every fifth day) to refresh all communities (as indicated by orange arrows).All experimental treatments (each of 60 mL) consisted of four replicates.At the end of the experiment (i.e., day 50), planktonic and plastisphere communities were identified separately, using a long-read metabarcoding approach.Parts of the figure were drawn by using pictures from Servier Medical Art.Servier Medical Art by Servier is licensed under a Creative Commons Attribution 3.0 Unported License (https://creativecommons.org/licenses/by/3.0/).5-10 ng template DNA.To characterise the microeukaryotic community, amplification was done by using the V4_Balzano_F/D11_3143R primer pair to amplify a $4.5 kbp region of the eukaryotic rRNA operon (Latz et al., 2022) (Table S1).The PCR was performed using the same PrimeStar polymerase according to Latz et al. (2022) and using 5-10 ng template DNA.
The barcoded PCR products were purified with 0.8Â AMPure magnetic beads (Beckmann) following the manufacturer's protocol.Thereafter, the purified PCR products were quantified using the Qubit 1Â HS Assay Kit (ThermoFisher Scientific) and pooled in equimolar amounts.The final pools were concentrated with 1.8Â AMPure magnetic beads in 51 μL nuclease-free water (ThermoFisher), and again quantified (Qubit 1Â HS Assay Kit).
1 μg of bacterial and microeukaryotic library was used for the ONT library preparation using the 1D sequencing (SQK-LSK109; Oxford Nanopore Technologies), following some modifications described in Vass et al. (2022).Sequencing was performed separately using a MinION Mk1C instrument (ONT) operated with a Spot-ON Flow Cell (R9.4.1 chemistry).Real-time high-accuracy base-calling (HAC) was executed using the MinKNOW software (v22.05.6).In the end, 3.87 M and 2.46 M reads were yielded with Q > 9 from our bacterial and microeukaryotic amplicon libraries, respectively.
Bacterial consensus sequences (16S rRNA gene) were assigned against SILVA SSU v138.1 reference database (Quast et al., 2012), while the 18S, 28S rRNA genes and the full length internal transcribed spacer (ITS) were first extracted from the microeukaryotic consensus sequences with ITSx (Bengtsson-Palme et al., 2013) and were assigned against the PR2 v4.14 database (Guillou et al., 2013), SILVA LSU v138.1 reference database (Quast et al., 2012) and the UNITE +INSD v9.0 database (Abarenkov et al., 2023), respectively.All taxonomic assignments were done by using BLAST+ (v2.13.0+) and keeping hits with at least 80% identity.Results of the BLASTn searches were processed with phyloR (https://github.com/cparsania/phyloR) to keep top hits (highest bit score) and to assign taxonomy levels.In the case of microeukaryotes, each OTU was manually inspected and consensus classification was determined down only to the level that could be robustly supported by at least two of the three reference databases, using the two out of three rule (e.g., if an OTU classified as taxon A by two reference databases but as taxon B by the third one, then taxon A is selected and used in the final taxonomy table).Unassigned, mitochondrial or chloroplast OTUs were discarded.In the end, 1625 bacterial (Tables S4  and S5) and 399 eukaryotic OTUs (Tables S6 and S7) were identified, thus, kept for downstream analyses.

Data analyses
All statistical analyses and visualisations were conducted in R version 4.0.4(R Core Team 2021).Assessments of alpha diversity (i.e., observed OTU richness, Shannon, Simpson and Chao1 indices) and beta diversity (based on Bray-Curtis distance) were performed using the 'microeco' R package (v.0.6.5)(Liu et al., 2021) and the results (i.e., nonmetric multidimensional scaling-NMDS) were plotted using 'ggplot' package (Wickham, 2009).Differences in alpha diversity across inoculum sources and treatments were tested with ANOVA followed by Duncan's test (p < 0.05) as a post-hoc test.To test compositional differences between samples, permutational multivariate analysis of variance (PERMANOVA, permutations: 999) was performed using the 'vegan' R package (Oksanen et al., 2016).
To identify taxa favoured by microplastics, we applied ANOVA-like differential expression analysis (ALDEx2; v1.30) (Fernandes et al., 2013) which applies clr transformation on the data before it identifies planktonic and microplastic-associated microbes (grouped by genus with relative abundance >0.5%) using Benjamini-Hochberg corrected p-value of Welch's t-test (significance level of 0.05).
Fast expectation-maximisation for microbial source tracking (FEAST) probabilistic model (Shenhav et al., 2019) was used to estimate if taxa in the plastisphere (i.e., biofilm-forming microbes on the microplastics) originate from the river and/or sea community, or, in case of sea plastisphere, if taxa are originated from river MPs that were transferred to sea communities at a later stage (i.e., day 30).Here, we considered the sea control community and river control community and its MPs as 'sources', while sea or river plastisphere as 'sink' communities.

Compositional differences between microbial communities
The two inocula, from river and seawater, used in this microcosm experiment were compositionally distinct.While both freshwater river and sea community harboured a great fraction of Proteobacteria (Figure 2A), the river consisted of mainly bacteria from the orders Burkholderiales, Cytophagales and Sphingomonadales, and the sea was dominated by bacteria belonging to Burkholderiales, Rhodobacterales and, to a lesser extent, by Pseudomonadales (Figure S1).Ochrophyta taxa (i.e., chrysophytes; see Figure S2) built up almost the entire microeukaryotic river (MPÀ) control communities, while the sea (MPÀ) control communities showed more membership diversity, consisting of mostly Ochrophyta, Perkinsea, Cercozoa and Dinoflagellata taxa (Figure 2B).
Community compositions have been shifted as river and sea inoculum communities have been exposed to the applied treatments (PERMANOVA: bacteria-R 2 = 0.387, F = 3.39, p = 0.001; microeukaryotes-R 2 = 0.327, F = 2.48, p = 0.001), including the community coalescence of river and sea communities and/or exposing them to MPs (Figure S3).River communities experienced a great loss in relative abundances of Cytophagales and Sphingomonadales bacteria as being mixed into the recipient sea community (Figure S1), resulting in similar community compositions towards that of the sea (MPÀ) inoculum communities (Figure S3).Several microbial groups reached higher abundances in community coalesced and/or MP-exposed treatments than in their controls (i.e., river/sea [MPÀ] control communities).Namely, increased relative abundances of Cryptomycota (=Rozellomycota) and Ascomycota coincided with most treatments exposed to MPs.Bacterial phylum Campylobacterota increased in relative abundance in treatments where the sea (MP+/À) community was mixed with the river community or received MPs from the river community, but not when simultaneous community mixing and MP transfer were applied.Certain classes of Ochrophyta (i.e., Dictyochophyceae), Discoba and Ciliophora increased in abundance in most treatments that involved community coalescence (Figure 2A; Figures S1 and S2).Plastispheres of both river and sea communities were built up mainly by Proteobacteria and to a lesser extent by Bacteroidota (Figure 2A).Sea MPs, however, were predominantly colonised by fungi ($ 70% of relative abundance), while their river counterparts were populated by ochrophytes (>90%) (Figure 2B).Interestingly, the mere presence of MPs in both river and sea control communities elevated the relative abundance of fungi (e.g., with a substantial change from 2% up to 25% in case of sea) (Figure 2B).Furthermore, the relative abundances of Cytophagales, Legionellales and Rhodobacterales increased in the planktonic fraction of river and sea, respectively (Figure S1).
In addition to the compositional differences, the alpha diversity of bacteria was higher ( p < 0.05) in the plastisphere compared to the planktonic fraction (Figure S4), while the plastisphere harboured equally or less diverse microeukaryotic communities than its planktonic counterpart (Figure S5).

Effects of community coalescence on the plastisphere
Using the microbial source tracking approach, we were able to estimate the proportion of communities that originated from an inoculum community or its plastisphere.Our results indicate that, as river MPs have been transferred into the sea, their plastisphere composition changed substantially, replaced by taxa with unknown and sea origin (Figure 4A,B).On average, 49.8% and 59.3% of this final plastisphere of riverorigin MPs are sourced from planktonic bacterial and microeukaryotic fractions of the sea, respectively.Only small proportions, 3.5% and 5.5% of their initial, riverorigin bacterial and microeukaryotic communities, respectively, remained detectable on the plastisphere.
Microbes from the planktonic fraction of river (MP +/À) communities contributed little (3.76%-4.20%) to the final bacterial plastisphere of sea MPs (i.e., particles that have been incubated in the sea community throughout the entire experiment and exposed to river community on day 30), which were mainly populated by sea microbes and taxa with unknown source.In the case of microeukaryotes, their contributions were even less, with 0.51%-1.25%.
One of our treatments included the equal mixing of MPs from both river and sea and their further incubation in the sea community (e.g., mimicking a scenario when both waterbodies are polluted by microplastics from the start).Here, we assumed that this mixed plastisphere could equally be traced back to the plastisphere of the river control community and to the planktonic sea community.Interestingly, we found that, on average, only 11.3%-11.4% and 0.5%-14.2% of the final bacterial and microeukaryotic plastisphere, respectively, could be attributed to river-origin plastisphere.Thus, when MPs from different sources got mixed (e.g., in estuaries), their plastispheres resembled a greater extent that of the recipient sea community.

DISCUSSION
Studying microbes on plastic particles has been of paramount interest in microbial ecology, recently.Nevertheless, we lacked knowledge of how microplasticassociated microbial communities (that is, the plastisphere) change when crossing distinct boundaries of aquatic ecosystems, and how community coalescence influences microbes of the plastisphere and its surrounding environment.Although our results provide experimental evidence of the selection and dispersal of ecologically relevant microbes by microplastics, we found that members of the river-origin plastisphere are being greatly outcompeted by the taxa of the recipient community.Thus, it is less likely that hitchhiked microbes establish dominance and cause drastic compositional changes when reaching new and distinct aquatic habitats.

Community coalescence disrupts the establishment of dispersed microbes
Environmental changes, selecting species along riversea continuum, can be induced by numerous factors.Li et al. (2019) suggested that salinity and nutrients are mainly the factors affecting biomass on microplastics along a river-estuarine transect.Specifically, the authors articulated that the growth rates of biofilm in freshwater sites are greater than those in saline conditions.Although we did not measure biofilm growth rates and biomass, our SEM images (Figure S6)-taken from randomly selected microplastic particles of the river and sea communities at the end of the experimentsupported visually this difference in biofilm formation on microplastics between freshwater river and brackish seawater.Nevertheless, microbes carried by MPs from freshwater river communities cannot take advantage of greater biofilm development.River communities tend to suffer from species loss as being introduced into saline environments and by doing so, the composition of the final, mixed community shifts towards that of the sea (Rocca et al., 2020;Song et al., 2022).This is in line with our detected compositional patterns, proving that indeed the compositions of both planktonic and plastisphere communities tended to converge towards that of the sea communities.Potential reasons behind this phenomenon might be attributed to differences in competitiveness of the coalesced communities (Lech on-Alonso et al., 2021), or the lack of sufficient propagule pressure that would otherwise contribute to the establishment success of riverine microbes (Vila et al., 2019).
Microplastics may potentially balance this species loss by acting as floating 'species pool islands' in the new, recipient environment, however, our results indicate that even members of the plastisphere are replaced by resident microbes, leaving only a small fraction of the river-origin plastisphere behind.Using a microbial source tracking approach, we were able to assess (i) to what extent river microbes hitchhike on MPs and thereafter maintain populations in the sea and mixed river-sea environments, and (ii) how sea plastisphere is impacted by community coalescence with river communities.When river-origin microplastics end up in the sea, the bacterial and microeukaryotic communities on microplastics change drastically and are mostly replaced by planktonic organisms in the seawater.Thus, although MPs can disperse microbes, these floating tiny islands do not necessarily facilitate the successful establishment of microbes along their journey.We also found that community coalescence had very little influence on the bacterial and microeukaryotic plastisphere communities.It is, nonetheless, important to note that community dynamics can influence the efficiency of microbial source tracking methods, such as the one we applied in this present study (Wang et al., 2023).For example, the great variation found among replicates of the same treatment during source tracking suggests alternative community assemblies during biofilm formation, as a result of stochastic community assembly (Zhu et al., 2023).Hence, the likelihood that certain microbes can be transported by microplastics most likely depends on the early stage of community development of the plastisphere.

Selective microbes on microplastics
In line with previous studies, the compositions of the plastispheres differed from their planktonic counterparts (Kettner et al., 2017;Oberbeckmann et al., 2018;Qiang et al., 2021).This generally suggests that selective planktonic taxa participate in biofilm formation.We found a substantial increase in the abundance of Pseudomonas taxa together with other ecologically relevant bacteria on the plastisphere (i.e., Sphingomonas, Hyphomonas, Brevundimonas, Aquabacterium and Thalassolituus).Numerous studies have reported the importance of the genus Pseudomonas to degrade and metabolise plastic polymers (Roberts et al., 2020;Wilkes & Aristilde, 2017) and aromatic compounds (Ramasamy et al., 2023).The significant enrichment of the genus Flavobacterium and Hyphomonas was also catalogued in another study, conducted in the southern part of the Baltic Sea (Oberbeckmann et al., 2018).Most of these bacteria are known for their capability to degrade polycyclic aromatic hydrocarbons (PAHs), including carbazole (Maeda et al., 2009), and alkanes during dieselpolluted marine environments (Murphy et al., 2021).Hence, the enriched presence of the plastisphere in our study may suggest their active role in plastic degradation and their potential wide-range usefulness in bioremediation.Nevertheless, the catalytic mechanism of the enzymes involved in pollutant degradation of these bacteria is yet to be explored.The increased abundance of taxa within the genus Brevundimonas raises some concerns, given its emerging opportunistic pathogenicity (Ryan & Pembroke, 2018).The reason why numerous bacterial genera such as Duganella, Polynucleobacteria and Candidatus decreased on plastisphere remains unclear but might point to the toxicity of MPs (Pencik et al., 2023).
Targeting microeukaryotes, with a particular focus on the ones with vast metabolic potential to degrade plastics such as fungi, has remained peripheral in plastic-focused studies (Kettner et al., 2017).Although several functional groups of fungi are well-known degraders of litter, recent studies suggest their relevance in plastic-degrading processes as well (Rogers et al., 2020;Singh & Gupta, 2014).Their potential role in microplastic degradation is therefore reasonable but overlooked.A previous study highlighted that microfungi belonging to Cryptomycota (a.k.a Rozellomycota), Ascomycota and even Chytridiomycota thrive on the plastisphere (Kettner et al., 2017).Our study generally supports their findings, except for chytrids that had neglected dominance in the mycoplankton and on the plastisphere.The population of chytrids (as a proxy of their summed relative abundance) showed elevated levels only in the planktonic fraction of MP-polluted seawater (see, Figure S2).This might suggest that microplastic pollution potentially increases the abundance of parasitic fungi in coastal communities, but the mechanism behind it is unclear (note that the relative abundance of Bacillariophyta-including diatoms as common host of chytrids-was similar in all sea (MP +/À) communities).The enrichment of fungal Cladosporium and Plectosphaerella taxa has been evidenced in aquatic habitats in previous studies (Forero-L opez et al., 2022;Lacerda et al., 2020), suggesting their plastic-degrading potentials (Kim et al., 2022;Srikanth et al., 2022).These findings do not merely support that a subset of aquatic microbes is enriched on the plastisphere but also highlight relevant ecological roles in biodegradation processes.
Although past works suggest no obvious impact of microplastics on the composition of planktonic bacterial communities (see, e.g., Qiang et al., 2021), the enriched fungal taxa in sea plastisphere had a clear influence on sea planktonic communities by elevating the relative abundance of fungi by about 20%.Discrepancies among studies may occur as different concentrations and types of microplastics are applied, or when the durations of experiments, affecting biomass development on plastics, differ.

CONCLUSION
Given its semi-enclosed nature, the Baltic Sea is vulnerable to environmental changes, including climate change, eutrophication and anthropogenic pollution.Microplastics are one of the emerging pollutants, occurring in the form of fragments, flakes and fibres (Narloch et al., 2022).Rivers and wastewater treatment plants are known entrance pathways for microplastics to coastal marine areas (i.e., estuaries) (Siegfried et al., 2017;Talvitie et al., 2015).Studying microplastics in estuaries can help us understand the effects of such pollutants across aquatic ecosystems and may aid in the development of effective mitigation strategies.This study contributes to knowledge on how microbial colonisation on microplastics in the source river habitat affects the recipient community in the northern Baltic Sea, following their coalescences.Our study suggests that microbial-colonised microplastics in northern rivers will not have major effects on the recipient sea communities, likely due to the competitive advantage and resilience of the sea microbial communities, which can be attributed to more favourable, coalescence-induced environmental conditions.This shows that microbial communities in northern coastal areas exhibit some degree of resilience to dispersed microorganisms from freshwater habitats, even when their abundances are enhanced by floating microplastics.