An investigation into the stability and degradation of plastics in aquatic environments using a large-scale field-deployment study.

The fragmentation of plastic debris is a key pathway to the formation of microplastic pollution. These disintegration processes depend on the materials' physical and chemical characteristics, but insight into these interrelationships is still limited, especially under natural conditions. Five plastics of known polymer/additive compositions and processing histories were deployed in aquatic environments and recovered after six and twelve months. The polymer types used were linear low density polyethylene (LLDPE), oxo-degradable LLDPE (oxoLLDPE), poly(ethylene terephthalate) (PET), polyamide-6 (PA6), and poly(lactic acid) (PLA). Four geographically distinct locations across Aotearoa/New Zealand were chosen: three marine sites and a wastewater treatment plant (WWTP). Accelerated UV-weathering under controlled laboratory conditions was also carried out to evaluate artificial ageing as a model for plastic degradation in the natural environment. The samples' physical characteristics and surface microstructures were studied for each deployment location and exposure time. The strongest effects were found for oxoLLDPE upon artificial ageing, with increased crystallinity, intense surface cracking, and substantial deterioration of its mechanical properties. However, no changes to the same extent were found after recovery of the deployed material. In the deployment environments, the chemical nature of the plastics was the most relevant factor determining their behaviours. Few significant differences between the four aquatic locations were identified, except for PA6, where indications for biological surface degradation were found only in seawater, not the WWTP. In some cases, artificial ageing reasonably mimicked the changes which some plastic properties underwent in aquatic environments, but generally, it was no reliable model for natural degradation processes. The findings from this study have implications for the understanding of the initial phases of plastic degradation in aquatic environments, eventually leading to microplastics formation. They can also guide the interpretation of accelerated laboratory ageing for the fate of aquatic plastic pollution, and for the testing of aged plastic samples.

A spatio-temporal analysis of marine diatom communities associated with pristine and aged plastics.

Complex microbial communities colonize plastic substrates over time, strongly influencing their fate and potential impacts on marine ecosystems. Among the first colonizers, diatoms play an important role in the development of this 'plastiphere'. We investigated 936 biofouling samples and the factors influencing diatom communities associated with plastic colonization. These factors included geographic location (up to 800 km apart), duration of substrate submersion (1 to 52 weeks), plastics (5 polymer types) and impact of artificial ageing with UV light. Diatom communities colonizing plastic debris were primarily determined by their geographic location and submersion time, with the strongest changes occurring within two weeks of submersion. Several taxa were identified as early colonizers (e.g. Cylindrotheca, Navicula and Nitzschia spp.) with known strong adhesion capabilities. To a lesser extent, plastic-type and UV-ageing significantly affected community composition, with 14 taxa showing substrate-specificity. This study highlights the role of plastics types-state for colonization in the ocean.

Does plastic type matter? Insights into non-indigenous marine larvae recruitment under controlled conditions.

Marine plastic debris (MPD) are a global threat to marine ecosystems. Among countless ecosystem impacts, MPD can serve as a vector for marine 'hitchhikers' by facilitating transport and subsequent spread of unwanted pests and pathogens. The transport and spread of these non-indigenous species (NIS) can have substantial impacts on native biodiversity, ecosystem services/functions and hence, important economic consequences. Over the past decade, increasing research interest has been directed towards the characterization of biological communities colonizing plastic debris, the so called Plastisphere. Despite remarkable advances in this field, little is known regarding the recruitment patterns of NIS larvae and propagules on MPD, and the factors influencing these patterns. To address this knowledge gap, we used custom-made bioassay chambers and ran four consecutive bioassays to compare the settlement patterns of four distinct model biofouling organisms' larvae, including the three notorious invaders Crassostrea gigas, Ciona savignyi and Mytilus galloprovincialis, along with one sessile macro-invertebrate Spirobranchus cariniferus, on three different types of polymers, namely Low-Linear Density Polyethylene (LLDPE), Polylactic Acid (PLA), Nylon-6, and a glass control. Control bioassay chambers were included to investigate the microbial community composition colonizing the different substrates using 16S rRNA metabarcoding. We observed species-specific settlement patterns, with larvae aggregating on different locations on the substrates. Furthermore, our results revealed that C. savignyi and S. cariniferus generally favoured Nylon and PLA, whereas no specific preferences were observed for C. gigas and M. galloprovincialis. We did not detect significant differences in bacterial community composition between the tested substrates. Taken together, our results highlight the complexity of interactions between NIS larvae and plastic polymers. We conclude that several factors and their potential interactions influenced the results of this investigation, including: (i) species-specific larval biological traits and ecology; (ii) physical and chemical composition of the substrates; and (iii) biological cues emitted by bacterial biofilm and the level of chemosensitivity of the different NIS larvae. To mitigate the biosecurity risks associated with drifting plastic debris, additional research effort is critical to effectively decipher the mechanisms involved in the recruitment of NIS on MPD.

Biosecurity implications of drifting marine plastic debris: Current knowledge and future research.

The introduction and spread of marine non-indigenous species (NIS) and pathogens into new habitats are a major threat to biodiversity, ecosystem services, human health, and can have substantial economic consequences. Shipping is considered the main vector for marine biological invasions; less well understood is the increased spread of marine NIS and pathogens rafting on marine plastic debris (MPD). Despite an increasing research interest and recent progress in characterizing the plastisphere, this manuscript highlights critical knowledge gaps and research priorities towards a better understanding of the biosecurity implications of MPD. We advocate for future research to (i) investigate plastisphere community succession and the factors influencing NIS propagules and pathogens recruitment through robust experimental investigations; (ii) combine microscopy and molecular approaches to effectively assess the presence of specific taxa; (iii) include additional genetic markers to thoroughly characterize the biodiversity associated with MPD and explore the presence of specific marine pests.

Release and degradation of environmental DNA and RNA in a marine system.

Over the last decade, there has been growing interest in the analysis of environmental DNA (eDNA) to infer the presence of organisms in aquatic environments. The efficacy of eDNA/eRNA based tools are highly depend on the turnover rate of the molecule (their release and degradation). Environmental DNA has been shown to persist for days, weeks or years in environmental samples. Environmental RNA (eRNA) is thought to degrade faster than eDNA, however to our knowledge, no experimental studies have explored this. Here we present an aquarium study to investigate eDNA and eRNA shedding rates and degradation for two sessile marine invertebrates. The copy numbers for eDNA and eRNA were assessed using droplet digital PCR targeting the mitochondrial Cytochrome c Oxidase subunit 1 (COI) gene. Environmental RNA persisted after organism removal for much longer than expected with detections for up to 13 h. In contrast, eDNA was detected is samples collected up to 94 h after organism removal. There was no evidence that the decay rates constants for eDNA and eRNA were different (p = 0.6, Kruskal-Wallis tests). Both eDNA and eRNA was detected in biofilms collected at the end of the experiment (day 21). This suggests binding with organic or inorganic compounds or stabilization of these molecules in the biofilm matrix. The finding of the prolonged persistence of eRNA may provide new opportunities for improved biodiversity surveys through reducing false positives caused by legacy DNA and could also facilitate new research on environmental transcriptomics.