April 15, 2018
By Ernest Granson
While plastics are one of the most useful products of the modern era, their use has a significant downside because most mass-produced plastics don’t fully biodegrade.
They disintegrate with exposure to air and sunlight into tiny particles, called microplastics, and then accumulate in huge amounts, finding their way into the environment. According to the publication Science and its recent global analysis of all mass-produced plastics ever made, plastic debris has been found in all major ocean basins, with an estimated 4 to 12 million metric tonnes (Mt) of plastic waste generated on land entering the marine environment in 2010 alone.
As a first step to reducing the growing amount of permanent plastic, a scientist at Arizona State University (ASU) in Tempe is developing a fully biodegradable plastic derived from bacteria.
Plastics, also called polymers, are mainly manufactured synthetically by the separation or extraction of chemicals originating from oil, natural gas or coal. However, polymers are also found in other naturally occurring substances such as the cellulose, lignin, and starch in plants and wood. And, according to Taylor Weiss, Senior Sustainability Scientist at the Julie Ann Wrigley Global Institute of Sustainability at ASU, fully biodegradable plastic can be produced using cyanobacteria and bacteria in partnerships.
Cyanobacteria, sometimes called “blue green algae,” are one of the largest and most important subgroups of all bacteria. Cyanobacteria have existed for more than 3.5 billion years and, through the process of photosynthesis, helped to generate the earth’s oxygen atmosphere. In addition to being able to photosynthesize sunlight and carbon dioxide into sucrose, cyanobacteria have the capability to take part in a symbiotic relationship with other organisms. These are the two attributes that Weiss is harnessing to produce polymers that will become feedstock for biodegradable plastics.
Specifically, he teams up a photosynthesizing cyanobacterium (Synechococcus elongatus 7942) that makes sucrose with Halomonas boliviensis, a bacterium which consumes sucrose to grow and make polyhydroxybutyrate (PHB). The PHB type of polymer has properties similar to various synthetic thermoplastics like the polypropylene produced from fossil fuel feedstock. These characteristics have been long been recognized and development is taking place in a number of industrialized areas to utilize such bacteria.
As matter of fact, as Weiss points out, bacteria aren’t the only source of feedstock for bioplastics.
“Finding renewable sources of bioplastics isn’t difficult,” he says. “For instance, manufacturing bioplastics from crops, like corn, isn’t a problem. From a sustainability perspective, however, using corn requires using valuable cropland needed to support the human food supply. Producing bioplastics using cyanobacteria doesn’t require large sections of land or even agricultural-quality water.”
The critical issues that Weiss must tackle is scalability and production costs. As he puts it, “If you want a lot of plastic, how do you grow that much? We, and other teams, can produce these products in the lab, but how can we manufacture enough of them? The goal is to scale-up the structures, but in an effective manner, using the same process as we do in the lab.”
Weiss says the cyanobacteria are trapped in hydrogel beads, made from a seaweed extract, and look much like green tapioca beads. These beads are then suspended in a saltwater tank filled with Halomonas. Because the cyanobacteria make sucrose constantly, trapping them in a hydrogel bead makes them easy to remove from the saltwater, where the Halomonas are free to grow and make PHB. The PHB polymer exists as a single microparticle inside the bacterial cell, which can be removed like a wrapper.
“Essentially, it’s like harvesting little bioplastic pellets,” he says.
The use of the hydrogel is an important part of the process for two reasons: because the cyanobacteria are encapsulated, they can be retrieved and reused for further production, instead of growing a new crop; and the confinement of the cyanobacteria nearly eliminates its growth, so the energy normally expended on growth is instead channelled towards more sucrose production.
Weiss says another positive aspect is the stamina and resilience of the partnership which easily withstood five months of use without any special action to prevent contamination. Not only did they survive, he says, production actually improved.
Weiss’ objective at the ASU AZCATI research labs is to utilize what is the largest algae testbed facility in the U.S. to push the production process to larger scales.
“There are already companies manufacturing biopolymers, so I suspect you’ll see more products emerging in the next three to five years, and the market will expand over the next 10 years,” he says. “But right now, it’s mainly high-value products that can be expensive to make, such as biodegradable medical sutures. Such sutures are very strong, but dissolve safely. The value isn’t there yet for other mass-produced products such as pop bottles. Economics could change the situation, however. For instance, the United Kingdom is considering an added tax on disposable coffee cups, just as many other governments have placed on plastic bags. Because hot beverage cups are paper, but lined with a polypropylene film, recycling is very difficult and expensive. Cups produced with PHB however could handle the heat, be recyclable, and biodegradeable.
But biodegradable products are just the starting point. Using plug and play ingredients, this type of process can be used for a variety of products including drug manufacturing, nutraceuticals, and fuels. We work with many commercial research partners at the university, so the objective is to mix and match different bacterial partnerships for different product markets.”