How Microplastics Impact Life at the Bottom
Microplastics are prevalent in the news and in the oceans. These small pieces of plastic have been found in wildlife, on beaches, and even in human fecal matter! Because there is such a wide variety of plastic sources, microplastics come in a multitude of plastic types and shapes: ranging from Styrofoam circles to nylon fibers. These different types and shapes of plastic behave differently in the environment, with their density in particular determining where they end up in the ocean. As such, it’s important to understand how microplastics can potentially impact different areas.
As much as the potential impact of microplastics on bigger ocean critters has been documented and studied, less attention has made its way down to the bottom of the water. In the soft sediments of the photic zone (where light still reaches the bottom surface), lies a collection of photosynthetic organisms referred to as the microphytobenthos (MPB). This diverse collection of organisms plays a very important role in the marine environment, both as a food source and in nutrient cycling. The MPB can make up to 90% of the primary production in estuaries. In other words, it’s both the bottom of the water body and the bottom of the food chain (really “web“). Similar to microplastics, organisms in this layer come in all different types and serve a variety of roles. Two major components are diatoms and cyanobacteria. Diatoms are generally more nutritious and can help prevent eutrophication, while cyanobacteria are less nutritious and sometimes toxic, and they can also accelerate eutrophication.
The health and productivity of this bottom layer was the focus of a recent study by a group of researchers from the University of Auckland in New Zealand. The research team, led by Julie Hope, investigated the impact of polyethylene terephthalate (PET) microfibers on the health and function of the MPB and a common bivalve. PET microfibers make up a large fraction of the microplastics in the oceans, and largely come from polyester fabrics and clothing. The bivalve both feeds on the MPB and stirs up sediments when it burrows to provide nutrients the MPB needs to grow. These researchers thought that microplastics would impact this relationship on multiple levels, upturning the fragile balance between nutrients and organisms that leads to a successful ecosystem.
How did the research team assess the health of the MPB and bivalves? It’s not like the researchers could ask them questions. Rather, they looked for certain fatty acids in the organisms, referred to as biomarkers, that are produced in response to certain functions or states of the organism. In addition to indicating the general organism health, the ratios between these different fatty acids can tell the researchers about the composition of the MPB (the relative amount of diatoms and cyanobacteria) and the energy reserves of the bivalves. They also looked at the movement and burrowing activity of the bivalves. The study system is summarized in Figure 2.
They found that the presence of microplastics increased the presence of cyanobacteria over diatoms, greatly reducing the nutrition of this layer for bivalves and other higher-level consumers. This in turn reduced the energy stores of the bivalves, decreasing their burrowing activity. This reduced burrowing activity in turn limits nutrient cycling, which impacts the growth of the MPB. Since the organisms in the MPB serve as the foundation of many aquatic food webs, disruptions to the health and function of these organisms could have large implications for the rest of their ecosystem.
Hope is not lost!
There is still time to reduce plastic consumption and better manage waste. Here are some ways to reduce your microplastic footprint in addition to being thoughtful when recycling and doing laundry. Furthermore, scientists are hard at work developing new technologies for biodegradable plastics and plastic degradation. Specifically with PET, several bugs and bacteria have been found to munch on PET, with the most recent development making significant strides in efficiency that is bringing the technology one step closer to large-scale adoption.
Source article: J. Hope, G. Coco, & S. Thrush. Environ. Sci. Technol. 2020. https://doi.org/10.1021/acs.est.0c00514
Cover Image Source: https://www.flickr.com/photos/dimsis/4907380853
