Roberts, C., Flintrop, C.M., Khachikyan, A., Milucka, J., Munn, C.B. and Iversen, M.H. (2024), Microplastics may reduce the efficiency of the biological carbon pump by decreasing the settling velocity and carbon content of marine snow. Limnol Oceanogr, 69: 1918-1928. https://doi.org/10.1002/lno.12615
Let it snow
Many people romanticize the first snowfall of the season, but did you know snow is just as important in the ocean? Marine snow, or the sinking of organic material to the ocean’s deeper layers, plays a crucial role in the global carbon cycle by transporting nutrients to deep-sea species. This process, called the biological carbon pump, moves nutrients from the ocean’s surface to its depths.
A significant threat to marine snow and the biological carbon pump is the incorporation of microplastics, particularly microfibers from washing machine effluent, into sinking aggregates. Although most studies on microplastics focus on bead-shaped particles from face washes or the breakdown of larger items, little research has explored the impact of plastic microfibers on essential sinking processes like marine snow and the implications for nutrient transport.
The impact of plastic
A 2011 study by Browne et al. revealed that microfibers from washing machine effluent are the largest source of secondary microplastics in the oceans today. Therefore, experiments are needed to look at their impact on biological processes like marine snowfall.
To investigate, researchers from the UK and Germany conducted experiments using the diatom Skeletonema marioni, known for its ability to form blooms and aggregates. They tested how plastic microfibers become integrated into biological aggregates that contribute to marine snow. Roller tanks simulated ocean movement with four microfiber concentrations, using cut black paracord rope as the source: control (no fibers), low (~250 fibers/L), medium (~700 fibers/L), and high (~850 fibers/L). Fibers were cut to 2.5–3 mm and rinsed in ultra-pure water.
Over five time points between 24 and 163 hours, the team took videos to estimate aggregate number, size, and sinking velocity. Afterward, aggregates were filtered onto GF/F filters and analyzed for organic nitrogen and carbon content to estimate nutrient transport. Basic statistics then assessed the significance of changes across the conditions.
Slowing the fall
The results showed that aggregates containing microfibers formed smaller clumps, sank more slowly, and potentially reduced carbon export from the surface to the deep ocean. The fibers not only made the aggregates smaller, slowing their descent due to less gravitational pull, but they also seemed to add buoyancy. Since marine snow is essential for drawing down natural and human-made CO2, reduced production and slower sinking could disrupt nutrient cycling.
Further, the incorporation of microplastics made the aggregates more fragile, producing smaller particles. If aggregates remain suspended in the upper ocean for longer, they may be consumed by zooplankton aggregate-feeders like copepods, salps, polychaetes, and protozoans. This consumption can then alter the sinking rates of zooplankton fecal pellets—another crucial source of marine snow—thereby further reducing the efficiency of the biological carbon pump.
Overall, the findings indicate that microfibers can significantly impair the biological carbon pump, a key component of the global carbon cycle. While this study used only one type of polymer and diatom, the results suggest that increasing microfibers could decrease carbon flux by 8–45%, despite variability. The reduction in export flux with more microfibers was statistically linked, highlighting the need to address these small yet impactful filaments in our increasingly plastic-filled world.
References
Browne, M. A., P. Crump, S. J. Niven, E. Teuten, A. Tonkin, T. Galloway, and R. Thompson. (2011). Accumulation of microplastic on shorelines worldwide: Sources and sinks. Environ. Sci. Technol. 45: 9175–9179. doi:10.1021/es201811s.
Cover photo by NOAA National Ocean Service, via Wikimedia Commons
I’m a former oceanographer with an MSc in Biological Oceanography from UConn where I studied mixotrophy in marine ciliates. After a year in Poland (studying freshwater critters) I moved to California. I currently work as a lab technician at Stanford. Outside of science, I enjoy a good book, a long run, and frozen fruit.
