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Abstract Plastics are an important new component of the global sedimentary system, and much concern exists about their transport, fate and impact. This study presents the first system-scale assessment of sedimentary storage of microplastic for an estuary, Narragansett Bay, RI (USA), and the measurements of shoreline and seabed sediments add to the growing body of literature demonstrating high coastal concentrations. Microplastic concentrations in sediments ranged from 396 to over 13,000 MP particles kg −1 dry sediment (DW), comparable to other shoreline and seafloor sites located near urban centers. As previously reported for fine sediment and other pollutants, estuarine plastic storage is extensive in Narragansett Bay, especially within the upper urbanized reaches. Over 16 trillion pieces of plastic weighing near 1000 tonnes is calculated to be stored in surface sediments of the Bay based on a power-law fit. This work highlights that estuaries may serve as a significant filter for plastic pollution, and this trapping may have negative consequences for these valuable, productive ecosystems but offer potential for efficient removal. 

Microplastics are ubiquitous contaminants in marine ecosystems worldwide, threatening fisheries production, food safety, and human health. Ingestion of microplastics by fish and large zooplankton has been documented, but there are few studies focusing on single-celled marine predators, including heterotrophic dinoflagellates. In laboratory experiments, the heterotrophic dinoflagellate species Oxyrrhis marina and Gyrodinium sp. readily ingested both algal prey and polystyrene microplastic spheres (2.5–4.5 μm), while Protoperidinium sp. did not ingest microplastics. Compared to algae-only fed dinoflagellates, those that ingested microplastics had growth rates reduced by 25–35% over the course of 5 days. Reduced growth resulted in a 30–50% reduction of secondary production as measured as predator biomass. Ingestion rates of algal prey were also reduced in the microplastic treatments. When given a mixture of microplastics and algal prey, O. marina displayed a higher selectivity for algal prey than Gyrodinium sp. Observations in the coastal ocean showed that phylogenetically diverse taxa ingested microplastic beads, and thus heterotrophic dinoflagellates could contribute to trophic transfer of microplastics to higher trophic levels. The results of this study may suggest that continued increase in microplastic pollution in the ocean could lead to reduced secondary production of heterotrophic protists due to microplastic ingestion, altering the flow of energy and matter in marine microbial food webs. 

Subseafloor microbial activities are central to Earth’s biogeochemical cycles. They control Earth’s surface oxidation and major aspects of ocean chemistry. They affect climate on long timescales and play major roles in forming and destroying economic resources. In this review, we evaluate present understanding of subseafloor microbes and their activities, identify research gaps, and recommend approaches to filling those gaps. Our synthesis suggests that chemical diffusion rates and reaction affinities play a primary role in controlling rates of subseafloor activities. Fundamental aspects of subseafloor communities, including features that enable their persistence at low catabolic rates for millions of years, remain unknown.