Changes in bacterial assemblage on LDPE microplastics in coastal sediment with time

Microplastics have accumulated in the oceans and sediments worldwide in recent years, with coastal area found acting as sinks which allow the accumulating of these contaminants. Low-density polyethylene (LDPE) is a widely produced polymer, accounting for 21% of global production, and is widely documented as a component of marine plastic debris. Therefore, LDPE was chosen as the model polymer for this research.

The paper looks into the potential for microplastics to function as sites for attachment of naturally occurring bacterial assemblage within coastal marine sediments. It aims to quantify attachment of naturally occurring bacteria onto plastic fragments within sediments, investigate variation in the structure and diversity of plastic-colonizing bacterial assemblages over time and across sediment types and identify the predominant bacterial genera attaching onto the plastic surfaces.

A 14-day microcosm experiment was used to investigate bacterial colonization of LDPE microplastics within three types of coastal marine sediment from Spurn Point, Humber Estuary, UK. The nature of the sediments from the three sties differs from one another – fine grained sand, medium grained sand and silt.  Scanning electron microscopy and Catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH) was used to observe the attachment of bacteria cells onto LDPE microplastics with time. Quantitative polymerase chain reaction (Q-PCR) and terminal restriction fragment length polymerization (T-RFLP) were carried out for 16s rRNA genes to quantify and identify the bacterial assemblage respectively.

Structure and diversity of LDPR-associated bacterial communities

T-RFLP analysis shows that the bacterial communities present on LDPE microplastics differed significantly from those within the sediments. Initially, sediment type-specific communities were found on LDPE microplastics. Subsequently, significant variations were observed in the structure of the LDPE-associated bacterial communities over time. Specifically, there were significant shifts in the structure of LDPE-associated bacterial communities by day 7 and 14 of the experiment. There was also observed convergence in the structure of the bacterial assemblage across the three sediment types. The bacterial communities on LDPE microplastics became less diverse over time. In contrast, sediment bacterial communities from each site remained significantly different from each other throughout the 14-day experiment and no significant temporal shift was observed in the diversity of sediment bacterial communities.

The taxonomic identities of LDPE-associated bacterial genera

16s rRNA gene sequences from the genera Arcoabacter and Colwellia were found to dominate the LDPE-associated bacterial assemblages, together comprising between 84 and 93% of sequences from the three sites. Neighbor-joining phylogenetic trees revealed a high degree of sediment-specific clustering within each genus on LDPE-affiliated communities.

In conclusion, the evidence for successional formation of plastisphere-specific bacterial assemblages can be attributed to the rapid colonization of bacteria within coastal sediments. The experiment has also shown evidence of convergence of bacterial assemblage on LDPE microplastics over time.

The mechanism of how the change in bacterial assemblage on microplastics occurs is still not fully understood. Would it be chemical cues that were produced from the primary colonies that attracted the subsequent bacterial assemblage which resulted in the displacement of the primary colonizers? Or is there a synergistic effect with the formation of different layers of biofilms on the LDPE microplastics? Would the Arcobacter and Colwellia spp. be again the dominant genera on other plastic materials such as PVA OR PVC?

Moerover, Arcobacter and Colwellia spp. have previously been affiliated with the degradation of hydrocarbon contaminants within low-temperature marine environments.  Hydrocarbon-degrading bacteria have also been discovered on plastic fragments in seawater. Would these bacterial aids in “cleaning up” the polluted ocean? Like those oil engulfing bacteria does. 

There are still a lot more to study about the interaction between the microorganisms and its environment. We have still yet to fully understand the effect of the bacteria communities have on plasticsphere and vice versa. Also the interactions among the types of bacteria biofilm formed and the effects it has on its surrounding.

Harrison, J.P., Schratzberger, M., Sapp. M. and Osborn, A. M. (2014) Rapid bacterial colonization of low-density polyethylene microplastics in coastal sediment microcosms. BMC Microbiol, 14, 232.