The “good, the bad and the double-sword” effects of exposure to MPs and their organic additives on N 2 -fixing bacteria

Abstract 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 N 2 -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 N 2 -fixation rates were evaluated. Our results show species-specific responses of the autotrophic and heterotrophic N 2 -fixing bacteria tested and the responses were dependent on the type and concentration of MPs and additives. N 2 -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 N 2 -fixing bacteria and the magnitude of the effect is suggested to be dependent on bacterial size. We did not find evidences that specific N 2 -fixation rates were significantly affected by exposure to MPs, albeit changes in bacterial abundance can affect the bulk N 2 -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.