Biodegradation of low-density polyethylene by marine bacteria from pelagic waters, Arabian Sea, India

Highlights

• Biodegradation of polyethylene by marine bacteria M16, M27 and H1584 is ecofriendly.

• Growth, hydrophobicity, metabolic activity and FT-IR data support biodegradation.

• These bacteria can be explored for bioremediation.

Abstract

Sixty marine bacteria isolated from pelagic waters were screened for their ability to degrade low-density polyethylene; among them, three were positive and able to grow in a medium containing polythene as the sole carbon source. The positive isolates were identified as Kocuria palustris M16, Bacillus pumilus M27 and Bacillus subtilis H1584 based on the 16S rRNA gene sequence homology. The weight loss of polyethylene was 1%, 1.5% and 1.75% after 30 days of incubation with the M16, M27 and H1584 isolates, respectively. The maximum (32%) cell surface hydrophobicity was observed in M16, followed by the H1584 and M27 isolates. The viability of the isolates growing on the polyethylene surface was confirmed using a triphenyltetrazolium chloride reduction test. The viability was also correlated with a concomitant increase in the protein density of the biomass. Polyethylene biodegradation was further confirmed by an increase in the Keto Carbonyl Bond Index, the Ester Carbonyl Bond Index and the Vinyl Bond Index, which were calculated from FT-IR spectra.

Introduction

The development and use of synthetic plastic has changed the nature of waste in last 3–4 decades (Sheavly, 2005). Over this period, it has replaced natural material in various aspects of human life and become an indispensable part of our society. Although the durability of plastic is ones of its most beneficial qualities, this same property is a major problem for our environment (Sivan, 2011). Plastics are chemically synthesized long-chain polymers (Scott, 1999) and are globally produced on a substantial scale. As per a recent estimate of the Central Pollution Control Board, New Delhi, India, 8 million tons of plastic products are consumed every year in India alone. A study on plastic waste generation in 60 major Indian cities revealed that approximately 15,340 tons per day of plastic waste is generated in the country (Central Pollution Control Board (CPCB) New Delhi, India, 2013). Low-density polyethylene (mainly used as carry bags) constitutes the major portion of this waste problem.

In the last two decades, the rate of plastic deposition has increased tremendously, and plastic has intruded into the marine environment. Plastic is found floating in oceans everywhere from the polar regions to the equator and has become one of the most common and persistent pollutants of seas and beaches worldwide (Frias et al., 2010, Moore, 2008, Teuten et al., 2009). Plastic debris is one of the largest contaminants of the marine environment. Polyethylene is the most commonly found non-degradable solid waste and has recently been recognized as a major threat to marine life. There are reports that suggest that polyethylene causes blockages in the intestines of fish, birds and marine mammals. In addition, entanglement in or ingestion of this waste has endangered hundreds of different species (Teuten et al., 2009, Secchi and Zarzur, 1999, Spear et al., 1995).

Polyethylene represents up to 64% of the synthetic plastics that are discarded within a short period after use (Byuntae et al., 1991). It is highly resistant to acids, alcohols, bases and esters. It is also biologically inactive and considered a recalcitrant material. Its inertness is due to the high molecular weight, hydrophobicity and lack of functional groups recognized by microbial enzymatic systems (Hamid, 2000). Polyethylene is a concern for waste management due to its accumulation in landfills and natural habitats (Thompson et al., 2009). Hence, a suitable method for disposal that is eco-friendly must be found. Recycling of polyethylene was considered a solution but has failed to provide safe disposal of these materials (Sivan, 2011); in this regard, microbial degradation is one of the best options. Some reports on the biodegradation of plastics indicate that it could be a viable proposition when suitable microorganisms are utilized (Singh and Sharma, 2008, Shah et al., 2008). Studies on polyethylene biodegradation (Albertson, 1980, Albertsson et al., 1987), including the biotic environment (Shah et al., 2008), have been reported. However, few studies have been conducted on non-amended polyethylene (Balasubramanian et al., 2010, Hadad et al., 2005).

In the present study, we screened sixty marine bacterial isolates cultured from the pelagic waters of the Arabian Sea, India, for their polyethylene degradation ability and found three potential strains. In addition, we characterized the cell-surface hydrophobicity and growth kinetics of these bacteria and the biodegradation of polyethylene.