Phytoplankton and bacteria interactions have a significant role in aquatic ecosystem functioning. Associations can range from mutualistic to parasitic, shaping biogeochemical cycles and having a direct influence on phytoplankton growth. How variations in phenotype and sampling location, affect the phytoplankton microbiome is largely unknown. A high-resolution characterization of the bacterial community in cultures of the dinoflagellate Alexandrium was performed on strains isolated from different geographical locations and at varying anthropogenic impact levels. Microbiomes of Baltic Sea Alexandrium ostenfeldii isolates were dominated by Betaproteobacteria and were consistent over phenotypic and genotypic Alexandrium strain variation, resulting in identification of an A. ostenfeldii core microbiome. Comparisons with in situ bacterial communities showed that taxa found in this A. ostenfeldii core were specifically associated to dinoflagellate dynamics in the Baltic Sea. Microbiomes of Alexandrium tamarense and minutum, isolated from the Mediterranean Sea, differed from those of A. ostenfeldii in bacterial diversity and composition but displayed high consistency, and a core set of bacterial taxa was identified. This indicates that Alexandrium isolates with diverse phenotypes host predictable, species-specific, core microbiomes reflecting the abiotic conditions from which they were isolated. These findings enable in-depth studies of potential interactions occurring between Alexandrium and specific bacterial taxa.

Marine litter, especially microplastics (plastic fragments < 5 mm), has been a subject of increasing interest in recent decade, due to its ubiquitous distribution in the marine environment. Most marine litter will eventually sink to the seafloor, and many field studies to date have confirmed the accumulation of microplastics in fine-grained soft sediments. The numbers of microplastics in the environment are expected yet to increase; thus, the seafloor sediments represent both current and future hotspots for microplastic pollution, making it important to investigate the fate and potential impacts of plastic litter in these habitats. In this thesis, the interactions between microplastics, the benthic invertebrate community and harmful contaminants were examined in four different mesocosm studies that together shed light on how the size, properties (polymer type and associated contaminants) and vertical distribution of plastics on the seafloor may affect the benthic fauna. The most common benthic invertebrates in the northern Baltic Sea, the Baltic clam Limecola balthica, polychaete Marenzelleria spp. and amphipod Monoporeia affinis, were selected for the experiments that investigated how the activities of the benthic community shape the vertical distribution of microplastics in the sediment. A follow-up study further examined the bacterial communities developing on the surface of different biodegradable (cellulose acetate, poly-L-lactic acid) and conventional (polyamide, polystyrene) mesoplastics together with the capacity of plastics to sorb polycyclic aromatic hydrocarbons (PAHs) from the sediment. Lastly, the effects of acute (5 days) and chronic (29 days) exposure to tyre rubber fragments on L. balthica were examined, using a suite of biomarkers and cell ultrastructural examination of clam tissues. The results demonstrate that bioturbation by common benthic fauna buried microplastics in the sediment up to a depth of 5 cm and at the same time reduced their bioavailability to the invertebrates feeding from the sediment surface. In the experiments, 25% of the exposed clams ingested microplastics from the sediment surface, but the availability of microplastics decreased with depth; only 1% of the clams were found to ingest microplastics that were placed at depths of 2–5 cm in the sediment. In addition to the location of the microplastics, their bioavailability was also governed by the species-specific particle-size range for ingestion. Furthermore, the redistribution of buried microplastics at the sediment surface by bioturbation was negligible, supporting the hypothesis of seafloors acting as a final sink for microplastics. When incubated in the sediments, the bacterial communities developed on biodegradable cellulose acetate diverged from the other polymer types examined and harboured potentially biodegrading bacteria. The results also showed that all the polymer types examined sorbed PAHs from the sediments, but had varying PAH sorption capacities, indicating that if ingested, the microplastics’ role as PAH vectors is dependent on the polymer type. However, comparison of the PAH concentrations in plastics and in the sediment also suggested that the ingestion of plastics is not likely to increase the PAH burden of deposit-feeders. In contrast, the contaminants already present in microplastics may pose elevated risk for benthic fauna, as was found in the study carried out with tyre rubber. Both PAHs and trace metals were quantified from the tyre rubber, and the clams exposed to an environmentally relevant concentration of tyre rubber fragments exhibited multiple sublethal responses, indicating oxidative stress and damage to vital cellular structures. In essence, this thesis provides novel information that contributes to fulfilling the current knowledge gaps regarding the fate and impacts of microplastics on the seafloor, and will further aid in assessing the potential risks microplastics pose to the benthic fauna, especially in the study area of the northern Baltic Sea. It remains unclear whether the impacts of microplastics could span from the individual to the population dynamics and ecosystem functioning, but the results obtained call for further research on the complex interactions taking place in the seafloor to better understand the impacts of microplastics on the marine environment.