The early plastisphere of biodegradable and non-biodegradable plastic films in marine environment

Abstract

Introduction Plastic items are the most common form of marine debris and plastic accumulation in marine ecosystems is a serious environmental issue. One possible solution to reduce the amount of synthetic polymers released in the environment is to substitute them with bio-based polymers. When released in marine environments, plastics are readily colonized by biofilm-forming microorganisms, the so called “plastisphere community”, which may include pathogenic, invasive and plastic-degrading species; the composition and influence of the plastisphere on the fate of plastic is largely unknown. The aim of this study is to investigate the early biofilm formation and the structure and diversity of marine plastisphere on non-biodegradable and bio-based films in marine coastal waters. Materials and Methods A field experiment was designed to investigate the plastisphere communities on non-biodegradable polyethylene (HDPE) and polyethylene terephthalate (PET) and on the biodegradable Mater-Bi (EF 04 P) and polylactic acid (PLA) after a short-term exposure in the coastal area of Palermo (Italy), at 2 m and 9 m seawater depth. After exposure for 30, 90 and 120 days the plastic square samples were collected, gently rinsed and cut in subsamples. Bacterial isolation of the total cultivable fraction of each biofilm was carried out on Marine broth while the HC-degrading bacteria were isolated on a mineral medium with hexadecane as sole C source. Single colonies with different phenotypes were purified and further characterized. Total DNA was extracted from the plastic films for future analysis. The qualitative assessment of biofilm structure and signs of degradation processes on the polymer surfaces were studied by scanning electron microscopy (SEM). Results After 30 days exposure biofilm formation was already visible on all plastic surfaces. The number of cultivable bacteria isolated on Marine Agar ranged from 105 to 106 cells for biodegrable plastics, and from 103 to 105 cells on non-biodegradable films, with higher counts at 2 m depth. The amount of DNA per cm2 extracted from each plastic type confirmed this result. About 5 to 7 colony phenotypes was observed and approximately 30 isolates from each sample were selected for Gram staining and molecular identification (in progress). The SEM images revealed the presence of diatoms, filamentous structures and bacterial cells adherent to the surface of plastic films. Alterations in the surface properties were visible on the biodegradable plastic. Conclusion The preliminary results of this field experiment revealed that the plastisphere is influenced by plastic type and habitat conditions. The biofilms on different polymer types will be further analyzed using molecular fingerprinting methods. Microbial degradation will be further analyzed by Fourier transform infrared spectroscopy (FTIR) for detection of modifications on the surface chemistry of plastic films.