Calcite carbonate sinks low-density plastic debris in open oceans

Marine plastic pollution is a major global concern, yet the fate of plastic in deep seas remains largely unknown. Low-density plastics, such as polyethylene (PE) and polypropylene (PP), constitute over 50% of total plastic waste. These plastics float on the surface, drift via ocean currents, and fragment into smaller pieces, known as low-density microplastics (LDMPs) (size <5mm). Field measurements show a discrepancy between the estimated amount of LDMPs entering the ocean and the amount observed on the surface. This 'missing plastic' puzzle suggests significant LDMPs sinks in the ocean. Despite the challenges of deep-sea sampling, studies since 2013 have detected substantial LDMPs from the twilight zone to the abyssal seafloor. A tentative 'whole ocean' mass balance suggests that a large proportion of LDMPs settle on the seafloor, but a clear understanding of the settling mechanisms remains elusive. Previous models, primarily focusing on biofouling, struggle to explain the accumulation of LDMPs in sediments. Biofouling alone, while explaining low surface concentrations, cannot account for the observed deep-sea enrichment. Other processes, such as ingestion by organisms or aggregation with marine snow, account for only a negligible fraction of LDMP sinking. Therefore, understanding the mechanisms behind LDMPs’ vertical transport and accumulation in the deep ocean is a crucial step toward addressing this environmental problem.

Existing research highlights the significant presence of LDMPs in deep-sea environments, challenging previous assumptions about their fate in the ocean. Studies have reported considerable amounts of LDMPs in deep ocean water and sediment samples, raising questions about the mechanisms responsible for their vertical transport. While biofouling has been proposed as a key factor, models based solely on this process are insufficient to explain the observed enrichment of LDMPs in deep-sea sediments. Other processes such as ingestion by marine organisms, aggregation with biogenic particles, and incorporation into fecal pellets have been investigated, but their contributions to the overall sinking flux are relatively small. The lack of a comprehensive understanding of LDMP settling dynamics underscores the need for further research in this area.

This study develops a new one-dimensional hydrodynamic model to evaluate the impact of microbially induced calcium carbonate precipitation (MICP) on LDMP settling. The model considers MICP alongside biofouling. The model incorporates new hydrodynamic equations to improve the accuracy of simulations for irregular LDMP shapes. LDMPs are modeled with densities ranging from 0.85–1.00 g cm⁻³, equivalent sizes between 1 µm and 5 mm, and shapes including spheres, fibers, and films. The MICP process is controlled by algal photosynthetic intensity, which is dependent on light penetration. Calcite, with a density of 2.63 g cm⁻³, acts as an effective ballast. The model compares the settling dynamics of LDMPs with and without MICP, examining the effects of size and shape on settling behavior. Monte Carlo simulations are used to generate the trajectories of 500 random LDMPs over 200 days under tropical Pacific Ocean conditions. The model also considers the potential for calcite dissolution with depth, especially below the calcite saturation depth (CSD). The shape factor (Ψ) is introduced to account for the irregular shapes of LDMPs, calculated using sphericity and circularity. The model incorporates a detailed biofouling sub-model, which includes algae attachment, growth, mortality, and respiration, with parameters determined from the literature and oceanographic data. The model considers the effect of light intensity on algae growth. The RTOFS data set is employed to provide realistic seawater temperature, salinity, and density profiles for specific geographic locations used in the simulations, enabling more precise calculations of LDMP trajectories.

The model demonstrates that microbially induced calcium carbonate precipitation (MICP) significantly affects LDMP settling in open oceans. Calcite, a byproduct of MICP, acts as an effective ballast, substantially increasing the density of LDMPs. Even a thin layer of calcite precipitate is sufficient to increase LDMP density above seawater density, initiating sinking. The model reveals two main settling patterns: (1) damped oscillation near the surface for larger, near-spherical LDMPs, and (2) direct settling to the seafloor for smaller, irregular-shaped LDMPs. The damped oscillation occurs because biofouling and calcite precipitate build up, causing the LDMP to sink until biofouling decreases with depth, causing buoyancy to increase. The process repeats, with slight calcite accumulation occurring during each cycle. The direct settling pattern is observed for smaller LDMPs due to higher vertical drag coefficients and the sufficient ballast provided by biofouling and calcite precipitates. The model successfully reproduces the observed enrichment of LDMPs in the ocean subsurface and sediment, especially for the size range of 10-200 μm. LDMPs within 100–500 µm sphere shapes are able to settle independently to the ocean floor, while those outside this size range might require aggregation to reach the sediment. Aggregation plays a significant role in the transport of smaller (<10 μm) and larger (1-5 mm) LDMPs to the seafloor. The model simulates the unique vertical distribution of LDMPs in the Arctic Ocean, where seasonal variations in light intensity cause surface accumulation in winter and seafloor accumulation in summer. The model shows that the most favorable size range for independent vertical settling is 10-500 μm, which is consistent with sediment core analyses.

This study's findings provide a more complete understanding of the factors governing LDMP settling. The model successfully addresses the 'missing plastic' problem by demonstrating the significant role of calcite precipitation in driving LDMP sinking. The two distinct settling patterns identified highlight the complex interplay between LDMP size, shape, and biogeochemical processes. The inclusion of MICP and improved hydrodynamic equations significantly enhances the accuracy and explanatory power of the model compared to previous biofouling-only models. The findings are consistent with field observations of LDMP concentrations in both the water column and sediment. This study demonstrates that the seafloor is a major sink for LDMPs, particularly those in the 10-500 μm size range. The study’s implications extend to the potential impact of deep-sea LDMP accumulation on benthic ecosystems, global carbon cycles, and ocean calcite cycling.

This study demonstrates the critical role of microbially induced calcium carbonate precipitation (MICP) in the vertical settling of low-density microplastics (LDMPs) in the open ocean. The developed model successfully reproduces observed LDMP distribution patterns, explaining the 'missing plastic' phenomenon and the abundance of LDMPs in deep-sea sediments. The study emphasizes the importance of considering both biofouling and MICP in future projections of LDMP fate and their impact on marine ecosystems. Further research should focus on field measurements of calcite-coated LDMPs in deep-sea environments to validate the model and explore the broader ecological implications.

While the model provides significant insights into LDMP settling, several limitations should be noted. The model is one-dimensional and does not consider horizontal currents, which may affect LDMP transport. The model simplifies the complex biofouling process, focusing primarily on algae growth. The influence of other organisms and environmental factors on biofouling is not fully explored. Furthermore, the model assumes a uniform distribution of LDMP shapes and sizes, which might not perfectly reflect reality. Further investigation into regional variations in MICP rates and CSD is needed to improve model accuracy for diverse oceanographic conditions.