Microplastics and the Marine Food Web
Microplastics, small plastic particles less than 5 millimeters in size, have been getting a lot of attention lately (check out these recent Envirobites posts!). Because of their small size, these manmade products may find their way into the marine food web. Importantly, copepods, a zooplankton and important food source for many marine critters, may eat them.
Recent laboratory studies have shown that when copepods ingest microplastics they can suffer serious effects, including a reduction in their ability to reproduce. But what exactly causes a copepod to decide that microplastics looks tasty?
In the vast ocean, the ingestion of microplastics may not just be driven by a chance encounter with the occasional plastic fragment. There may be more to the story: a smell, in fact. Many species of microscopic algae, known as phytoplankton, in the open ocean create a chemical called dimethyl sulfide. When these algal species ‘bloom’, concentrations of this chemical increase dramatically, attracting nearby marine critters – including copepods – in search of a tasty snack.
Jade Procter and colleagues hypothesized that copepods may be more tempted to eat microplastics if they are infused with this ‘dinner scent’ of dimethyl sulfide. If true, this may provide insights on why ocean organisms mistakenly eat microplastics.
The Methods: Do copepods eat infused microplastics?
To conduct this study, the researchers caught female Calanoid copepods from fan observing station in the Western English Channel and kept them in a controlled laboratory that mimicked the natural marine environment. The copepods were starved for 18 hours before being exposed to any microplastics.
Nylon microfibers, commonly found in textiles like washcloths, were used as the test microplastic, since previous studies have found microfibers to be the most representative microplastic type that is currently available to use in a laboratory setting.
Two treatments were used in the experimental design: in the first, microfibers were infused with dimethyl sulfide concentrations typical for a phytoplankton bloom. The second was the control: no dimethyl sulfide was added. A specialized camera was used to measure the change in microfiber concentrations in the water throughout the experiment. Less microfibers in the water indicated the copepods had launched their snack attack.
What did the scientists find?
Dimethyl sulfide infused microfibers were more appetizing to the copepods! The laboratory trials found that copepods ate the snack-smell-infused microplastics at rates up to nearly 300% more than when the microfibers did not have any dimethyl sulfide scent. This study also suggested that the dimethyl sulfide scent absorbed directly onto the surface of the microfibers. As a side study, it took about 9 days of soaking in a dimethyl sulfide solution before those microfibers had detectable amounts of the sulfur-based compound. This suggests that the microplastic particles have to be floating around in that phytoplankton bloom for more than a week before they become tasty.
Thus, ‘hotspots’ of dimethyl sulfide in the ocean, after a big algae bloom, may be a prompt for copepods to seek out and incidentally ingest microplastics. Additionally, the scent of dimethyl sulfide may act as a stimulus for feeding – in other words, when copepods smell that sulfur-based scent, they go on a feeding frenzy.
However, it should be noted that this is a laboratory study: what happens in the lab doesn’t always translate to the natural environment. For example, copepods are picky eaters: they pick the most nutritious food during grazing. But when starved, they are more like us and can become less selective, like in this study.
What does this mean for the marine food web?
This study provided direct evidence to show how microplastics can be introduced into the marine food web far beyond just a random encounter with a drifting plastic fragment. Microplastics have the ability to absorb dimethyl sulfide, and thus can ‘smell’ like a tasty algal cell. If copepods are hungry, this scent cue may prompt them to ingest microplastics, with consequences far up the food web.
Reference: Procter, Jade, Frances E. Hopkins, Elaine S. Fileman, and Penelope K. Lindeque. “Smells good enough to eat: Dimethyl sulfide (DMS) enhances copepod ingestion of microplastics.” Marine pollution bulletin 138 (2019): 1-6. DOI: https://doi.org/10.1016/j.marpolbul.2018.11.014