Little is known about the direct effects of microplastics (MPs) and their organic additives on marine bacteria, considering their role in the nutrient cycles, e.g., N-cycles through the N2-fixation, or in the microbial food web. To fill this gap of knowledge, we exposed marine bacteria, specifically diazotrophs, to pure MPs which differ in physical properties (e.g., density, hydrophobicity, and/or size), namely, polyethylene, polypropylene, polyvinyl chloride and polystyrene, and to their most abundant associated organic additives (e.g., fluoranthene, 1,2,5,6,9,10-hexabromocyclododecane and dioctyl-phthalate). Growth, protein overproduction, direct physical interactions between MPs and bacteria, phosphorus acquisition mechanisms and/or N2-fixation rates were evaluated. Cyanobacteria were positively affected by environmental and high concentrations of MPs, as opposed to heterotrophic strains, that were only positively affected with high concentrations of ~120 μm-size MPs (detecting the overproduction of proteins related to plastic degradation and C-transport), and negatively affected by 1 μm-size PS beads. Generally, the organic additives had a deleterious effect in both autotrophic and heterotrophic bacteria and the magnitude of the effect is suggested to be dependent on bacterial size. Our results show species-specific responses of the autotrophic and heterotrophic bacteria tested and the responses (beneficial: the “good,” deleterious: the “bad” and/or both: the “double-sword”) were dependent on the type and concentration of MPs and additives. This suggests the need to determine the threshold levels of MPs and additives concentrations starting from which significant effects can be observed for key microbial populations in marine systems, and these data are necessary for effective environmental quality control management.

We investigated the effects of increasing seawater temperature and CO2 concentration based on a high business‐as‐usual climate change scenario by year 2100 on the photosynthetic performance and productivity of Mediterranean seagrass Posidonia oceanica and alkaline phosphatase and N2‐fixing activities of microbes associated with different plant parts during winter when the plants may be thermally more vulnerable. Our results suggest that elevated CO2 and temperature benefit the overall photosynthetic performance of P. oceanica. Despite the benefits, the magnitude of respiration increased with elevated CO2 resulting in a negative carbon balance for P. oceanica in winter. This trend is contradictory to the general notion of decreased respiration in plants with increasing CO2, and warrants future investigation on the mechanisms behind the opposite trend. Changes of alkaline phosphatase activities found here may not be a direct consequence of the different treatments, but indirectly, through changes in the demand for dissolved inorganic phosphorus for N2 fixers. Of the several groups of N2 fixers tested for nifH expression (a proxy for activity of nitrogenase, the enzyme required for N2 fixation), only the unicellular N2‐fixing cyanobacterial phylotypes, UCYNB and UCYNC, actively transcribed with a positive nifH transcription response of UCYNC to elevated CO2 and temperature. Our results suggest that in future climate scenarios, the structure and diversity of N2 microbial communities associated with seagrasses may change and high‐light the importance of investigating the responses of different groups individually in their natural habitat substrates.

The accumulation of microplastics (MPs) pollution at depths suggests the susceptibility of benthic organisms (e.g. seagrasses and their associated macro- and micro-organisms) to the effects of these pollutants. Little is known about the direct effects of MPs and their organic additives on marine bacteria, e.g. in one of the most ecologically significant groups, the diazotrophs or N2-fixing bacteria. To fill this gap of knowledge, we exposed marine diazotrophs found in association with the endemic Mediterranean seagrass Posidonia oceanica to pure MPs which differ in physical properties (e.g. density, hydrophobicity and/or size), namely, polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC) and polystyrene (PS) and to their most abundant associated organic additives (e.g. fluoranthene, 1,2,5,6,9,10-hexabromocyclododecane [HBCD] and dioctyl-phthalate [DEHP]). Growth, protein overexpression, direct physical interactions between MPs and bacteria, phosphorus (P) acquisition mechanisms and N2-fixation rates were evaluated. Our results show species-specific responses of the autotrophic and heterotrophic N2-fixing bacteria tested and the responses were dependent on the type and concentration of MPs and additives. N2-fixing cyanobacteria were positively affected by environmental and high concentrations of MPs (e.g. PVC), as opposed to heterotrophic strains, that were only positively affected with high concentrations of ∼120 µm-size MPs (detecting the overexpression of proteins related to plastic degradation and C-transport), and negatively affected by 1 µm-size PS beads. Generally, the organic additives (e.g. fluoranthene) had a deleterious effect in both autotrophic and heterotrophic N2-fixing bacteria and the magnitude of the effect is suggested to be dependent on bacterial size. We did not find evidences that specific N2-fixation rates were significantly affected by exposure to MPs, albeit changes in bacterial abundance can affect the bulk N2-fixation rates. In summary, we reported for the first time, the beneficial (the “good”), deleterious (the “bad”) and/or both (the “double-sword”) effects of exposure to MPs and their organic additives on diazotrophs found in association with seagrasses.