A domesticated photoautotrophic microbial community as a biofilm model system for analyzing the influence of plastic surfaces on invertebrate grazers in limnic environments.

The environmental fate of plastic particles in water bodies is influenced by microbial biofilm formation. Invertebrate grazers may be affected when foraging biofilms on plastics compared to biofilms on natural substrata but the mechanistic basis for these effects is unknown. For analyzing these effects in ecotoxicological assays stable and reproducible biofilm communities are required that are related to the environmental site of interest. Here, a defined biofilm community was established and used to perform grazing experiments with a freshwater snail. For this, snippets of different plastic materials were incubated in the photic zone of three different freshwater sites. Amplicon sequencing of biofilms formed on these snippets showed that the site of incubation and not the plastic material dominated the microbial community composition. From these biofilms, individual microbial strains as well as photoautotrophic consortia were isolated; these consortia consisted of heterotrophic bacteria that were apparently nourished by microalga. While biofilms formed by defined dual cultures of a microalga and an Alphaproteobacterium were not accepted by the snail P. fontinalis, a photoautotrophic consortium (Co_3) sustained growth and metabolism of this grazer. Amplicon sequencing revealed that consortium Co_3, which could be stably maintained on solid medium under photoautotrophic conditions, reproducibly formed biofilms of a defined composition on three different plastic materials and on glass surfaces. In conclusion, our study shows that the generation of domesticated photoautotrophic microbial communities is a valid novel approach for establishing laboratory ecotoxicological assays with higher environmental relevance than those based on defined microbiota.

Environmental conditions affect the food quality of plastic associated biofilms for the benthic grazer Physa fontinalis.

With an ever-increasing amount of plastic pollution in the various aquatic ecosystems around the world, the effects on organisms are still not fully understood. Most studies focus on direct effects posed by plastic intake or entanglement, but plastic debris can also affect primary production of biofilms and have an indirect impact on its consumers. This study investigates the primary production on three common plastic types in freshwater and its food quality for a benthic grazer. We hypothesized that different polymer types affect biofilm composition as well as the life parameters of its consumers. We incubated polyethylene (PE), polyethylene terephthalate (PET) and polystyrene (PS) as well as glass (control) in a productive freshwater creek for natural biofilm establishment. To account for changes in the environmental conditions, the experiment was conducted twice during winter and late spring, respectively. These biofilms were offered to the freshwater gastropod Physa fontinalis as sole food source. Growth and reproduction of the snails were measured to monitor sublethal effects. Additionally, biofilm composition was observed using confocal laser scanning microscopy (CLSM). In winter, snails feeding off PET and PE showed a significantly lower egg production and lower growth rates were observed on PET. No such effects occurred in spring. CLSM data revealed, that algal growth was significantly lower on PE and PET during the winter treatment compared to PS and glass. Since we could only find these effects during the colder and darker months (January-March), the microbial colonization on PE and PET was inhibited by the substrate under less favorable conditions of temperature and light. Hence, benign conditions may mask the adverse effects of microplastic on food webs. Our findings show that future studies on the plastisphere will need to consider such variations to further understand the influence of plastic pollution on primary production and higher trophic levels.

Transfer and effects of PET microfibers in Chironomus riparius.

Multiple studies in freshwater environments have verified that microplastic particles are present in water columns, sediment, and aquatic organisms. These studies indicated that certain freshwater ecosystems may act as temporary sinks of microplastic particles, leading to accumulation in the sediment and the ingestion by benthic organisms. Polyethylene terephthalate (PET) is one of the non-buoyant polymers that has been frequently found in aquatic sediments. This study aims to investigate a possible transfer of PET microfibers from aquatic to the terrestrial habitats and addressed selected effects (i.e. survival, general stress response, and growth) of PET microfibers using Chironomus riparius, a frequently applied model organism in ecotoxicological research. To assess the growth and development of C. riparius, a modified 28-day sediment chronic toxicity test was conducted, in which the main endpoint is time until emergence of the larvae. In this assay, C. riparius were exposed to artificial sediments spiked with PET microfibers. In addition, weight and head capsule lengths of the larvae were also measured. As a general stress response marker on the molecular level, Heat Shock Protein 70 (HSP70) levels were measured in two involved life stages, i.e. larvae and adults. Using staining method, ingestion of PET microfibers was verified in the adult sample. Our results clearly demonstrated that ingested microfibers by C. riparius larvae can be carried through subsequent life stages and end up in the adults. Accordingly, this is the first proof of aquatic-terrestrial transfer of PET microfibers for C. riparius. However, toxicity test results showed that there was no significant effect on the time until emergence, weight or head capsule lengths in the organisms exposed to PET microfibers compared to control organisms. HSP70 measurements showed no significant effects between control and exposure groups in the same life stage. The result suggests that PET microfibers in the applied concentration do not exert adverse effects both on organism and subcellular level in one generation.