Abstract
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.
Highlights
Marine coastal ecosystems are the most impacted zones by the pollution of plastics
Culture media were composed of synthetic seawater medium (ASN-III) + Turks island salts 4X for Halothece sp., BG110 for F. muscicola and marine broth (MB) for the rest of the heterotrophic bacteria (Rippka et al, 1979)
Speciesspecific growth responses of the bacteria tested with the addition of MPs were exemplified here when polyvinyl chloride (PVC) and PP addition significantly enhanced the growth of the autotrophic cyanobacterial diazotrophs (Halothece sp. and Fischerella muscicola) by 1.5- to 4- fold (p < 0.05, n = 3, Figures 1B,C)
Summary
Marine coastal ecosystems are the most impacted zones by the pollution of plastics. Up to 10 million tons of plastic enter annually in the oceans (Almroth and Eggert, 2019). Tetu et al (2019), Sarker et al (2020), and Piccardo et al (2020) revealed the importance of concentration levels of leached plastic in cyanobacteria and heterotrophic bacteria Considering these previous results, experimental studies investigating the effect of MPs and their additives should take into account the response of different bacterial test species and different concentration levels of the pollutants. We report the responses of different bacterial species, N2-fixing phototrophic and heterotrophic bacteria to different concentrations of pure MPs (i.e., PE, PP, PVC, and PS) and their most predominant organic chemical additives [fluoranthene, 1,2,5,6,9,10-hexabromocyclododecane (HBCD) and dioctyl-phthalate (DEHP)]. Detrimental or both effects, depending on the species tested and the type and concentrations of MPs and additives added
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