Biofilm growth and its implications on the sinking behavior of microplastics

Biofilm growth and its implications on the sinking behavior of microplastics

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Executive Summary

The LabPlas project studied the impact of biofilm growth on the buoyancy and sinking behavior of plastics in marine environments. The results showed that biofilm colonization increases the wet mass of the plastic, reducing buoyancy and accelerating immersion. During the in situ experiment (180 days), plastic films were particularly affected, with 76% of them sinking, compared to just 6% of fragments. These results underline the importance of biofilm interactions in plastic transport and call for updates to sedimentation models and targeted mitigation strategies.

Introduction and Context

Plastic debris in marine environments threatens ecosystems, aquatic life and human health. As plastics degrade, the growth of biofilms can alter their buoyancy, hampering their distribution. Global plastic production has increased exponentially, leading to a significant accumulation of plastic waste. Research has shown that plastic leakage into the ocean is significant, with estimates ranging from 4.8 to 12.7 million tons per year. However, the discrepancy between plastic inputs and observed floating debris suggests that plastics sink due to several processes including biofouling. The LabPlas project filled this gap by studying biofilm formation on naturally aged plastics and assessing its effect on plastic buoyancy.Additionally, as plastic waste enters aquatic environments, it undergoes multiple degradation processes such as photodegradation, mechanical fragmentation, and biodegradation. These processes, combined with biofilm growth, affect plastic transport and sedimentation patterns. Understanding these interactions is crucial for improving ocean plastic pollution mitigation efforts and predicting long-term ecological impacts.

Key Findings

Potential Implications for Marine Life

Biofilm Growth and Sinking

Plastic Type and Sinkage

Experiment Overview – at Sea

• Collect buoyant plastics from manta net
• Individual monitoring of each plastic
• Incubation of plastics at sea
• Development of biofilm

During 10 months From October 26, 2022 to July 12, 2023

Every 3 weeks/1 month

24-well plate (perforated)

Cage below the surface

Experiment Overview – In Lab

Wet Mass Measure By Louis

Microscopic photography By Louis

Passage in column

Policy Implications and Recommendations

• Resilient Research Infrastructure: Develop robust experimental models that account for environmental variability, ensuring more accurate and reproducible data collection.
• Public Awareness and Policy Interventions: Increase public education on plastic pollution’s impact on marine ecosystems and implement stricter regulations on plastic waste management to reduce environmental contamination.

Policy

• Enhance Sedimentation Models: Incorporate biofilm growth and plastic morphology into plastic dispersion models to improve predictions of plastic transport and accumulation.
• Support Ongoing Research: Fund long-term in situ studies to better understand biofilm-plastic interactions and refine mitigation strategies.

Conclusion

The LabPlas project findings underscore the significant role of biofilm growth in altering the behavior of plastic debris in marine ecosystems. This study integrates in situ microplastic aging and biofilm formation with controlled laboratory experiments to assess plastic sinking dynamics. By considering natural weathering, microbial colonization, and density changes, these methods provide a more accurate representation of plastic transport mechanisms in marine environments. The results emphasize the importance of incorporating biological processes into plastic pollution management strategies. Addressing these interactions through informed policy measures will improve marine plastic pollution management and promote healthier marine environments. The findings contribute to improving predictive models for plastic fate in aquatic systems and understanding the role of biofouling in the vertical transport of microplastics.

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The experimental setup was adapted from Waldschläger & Schüttrumpf (2019) to measure both the ascent and descent speeds of plastics. It consists of a transparent column made of plexiglass. The column is 1m high and 20cm in diameter and is large enough to neglect any wall effects (Baba & Komar, 1981). To support the column, a metallic structure was constructed, and a valve was installed at the base of the column to allow the injection of plastics from below. Lighting is provided by an LED bar attached to one of the column support posts. The temperature in the room is set so that the water in the column is around 20°C, and the column is filled with seawater collected from the Bay of Villefranche-sur-Mer just before the experiment. The plastic is then subjected to the same conditions as the ones in which it will be incubated. Overall, the experimental setup seems well-designed to measure the ascent and descent speeds of plastics under controlled conditions. Velocity calculation: The velocity of the plastics is then determined by noting the time it takes a plastic to travel the distance between the 20 cm markers attached to the column. The 3 velocity replicates are measured for each plastic. Three other polystyrene polymer sphere particles were used to validate the experiments, with a certified average diameter of 4.9 mm (±0.1 mm) and 1.94 mm (±0.05 mm) with a certified density of 1.03 g/cm³ and a certified average diameter PS sphere of 4.95 ± 0.05 mm with a certified density of 0.9–0.94 g/cm³. The plastics have distinct rise rates depending on their type, with a higher average rise rate for fragments (about 32 mm/s) than for films (about 10 mm/s). When data were plotted according to the surface/volume ratio there was a continuity between fragments and film behaviour. Most of the films reached their sinking point around December 2022 (2 months after initial incubation), while the first fragments reached this point after January 2023 (3 months of incubation) and most have not yet sunk. Sinking speed distributes in a relatively similar way as rising speed according to surface/volume ratio and plastic type. The sinking velocity of the plastic particles varied from 0.7 to 53 mm/s, with an average of 7.43 mm/s.

Biofilm accumulation increased the wet mass of the plastic, thereby decreasing its buoyancy. After 36 days of incubation, plastics showed a 14 mg wet mass increase and 34% biofilm coverage. Over the 269-day duration of the experiment, surface coverage by biofilm ranged from an average of 30% to a maximum of 60%.

Among the two selected plastic types studied, films were significantly more affected by biofouling-induced sinking, with 76% sinking over the experiment. In contrast, only 6% of fragments sank during the same period. These findings highlight the critical role of plastic morphology in influencing marine sedimentation processes. The results suggest that different plastic forms exhibit distinct buoyancy behaviours, which may lead to varying ecological impacts in marine environments.

The sinking of plastics due to biofilm growth could have significant consequences for benthic organisms and deep-sea ecosystems. As plastics settle into marine sediments, they may introduce pollutants or disrupt habitat structures, impacting marine biodiversity.