Evidence of Coupled Autotrophy and Heterotrophy on Plastic Biofilms and Its Influence on Surrounding Seawaters

Abstract

We measured phytoplankton primary production and heterotrophic bacterial activities on microplastics and seawater in the Northwestern Mediterranean sea during two 3-month spring periods over consecutive years. Microorganisms growing on 5-mm diameter low-density polyethylene (LD-PE) film faced two contrasted conditions from year to year. A classic phytoplanktonic spring bloom linked to a high nutrient supply characterized the 2018 conditions, in contrast to less favorable situation in 2019 marked by the absence of bloom during the sampled period. For the first time, we observed a clear coupling between primary production and heterotrophic prokaryote production on microplastics during both years, but with different intensity between years that reflected the crucial role of the environmental conditions in shaping microbial activities on plastics. Indeed, we proved that high primary production on plastics could support the whole (net autotrophy) or the majority of the bacterial carbon demand needed for heterotrophic activities, supplemented by other carbon sources if surrounding waters are highly productive. We also propose that microbial activity on plastics influence the microbial community in the surrounding seawaters, especially when the environmental conditions are less favorable. An illustrative image of the role of plastics in the environment could be that of an electrical inverter in a circuit that allows for the attenuation of both positive and negative variations. Our results highlight the potential role played by the plastisphere shaping biogeochemical cycles in a context of an increasing amount of plastic particles in the marine environment.

Keywords: Microplastic, Plastisphere, Biogeochemical cycles, Primary production, Bacterial production