This plastic disappears in the deep sea—and microbes make it happen

Despite the growing popularity of bio-based plastics, plastic pollution remains one of the world's most pressing environmental issues. According to the OECD's Global Plastics Outlook (2022), about 353 million metric tons of plastic waste were produced globally in 2019, with nearly 1.7 million metric tons flowing directly into aquatic ecosystems. Much of this waste becomes trapped in large rotating ocean currents, known as gyres, forming the infamous "garbage patches" found in the Pacific, Atlantic, and Indian Oceans.

To tackle this, researchers have been searching for plastics that can be degraded more reliably in deep-sea environments. One promising candidate is poly(d-lactate-co-3-hydroxybutyrate) or LAHB, a lactate-based polyester biosynthesized using engineered Escherichia coli. So far, LAHB has shown strong potential as a biodegradable polymer that breaks down in river water and shallow seawater.

Now, in a study made available online on July 1, 2025, and published in Volume 240 of the journal Polymer Degradation and Stability on October 1, 2025, researchers from Japan have shown for the first time that LAHB can also get biodegraded under deep-sea conditions, where low temperatures, high pressure, and too limited nutrients make breakdown of plastic extremely difficult. The study was led by Professor Seiichi Taguchi at the Institute for Aqua Regeneration, Shinshu University, Japan, together with Dr. Shun'ichi Ishii from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Japan and Professor Ken-ichi Kasuya from Gunma University Center for Food Science and Wellness, Japan.

"Our study demonstrates for the first time that LAHB, a microbial lactate-based polyester, undergoes active biodegradation and complete mineralization even on the deep-sea floor, where conventional PLA remains completely non-degradable," explains Prof. Taguchi.

The research team submerged two types of LAHB films -- one containing about 6% lactic acid (P6LAHB) and another with 13% lactic acid (P13LAHB) -- alongside a conventional PLA film for comparison. The samples were submerged at a depth of 855 meters near Hatsushima Island, where deep-sea conditions, cold temperatures (3.6 °C), high salinity, and low dissolved oxygen levels make it hard for microbes to degrade plastic.

After 7 and 13 months of immersion, the LAHB films revealed clear signs of biodegradation under deep-sea conditions. The P13LAHB film lost 30.9% of its weight after 7 months and over 82% after 13 months. The P6LAHB film showed similar trends. By contrast, the PLA film showed no measurable weight loss or visible degradation during the same period, underscoring its resistance to microbial degradation. The surfaces of the LAHB films had developed cracks and were covered by biofilms made up of oval- and rod-shaped microbes, indicating that deep-sea microorganisms were colonizing and decomposing the LAHB plastic. The PLA film, however, remained completely free of biofilm.

To understand how the plastic decomposes, the researchers analyzed the plastisphere, the microbial community that formed on the plastic's surface. They found that different microbial groups played distinct roles. Dominant Gammaproteobacterial genera, including Colwellia, Pseudoteredinibacter, Agarilytica, and UBA7957, produced specialized enzymes known as extracellular poly[3-hydroxybutyrate (3HB)] depolymerases. These enzymes break down long polymer chains into smaller fragments like dimers and trimers. Certain species, such as UBA7959, also produce oligomer hydrolases (like PhaZ2) that further cleave these fragments, splitting 3HB-3HB or 3HB-LA dimers into their monomers.

Once the polymers are broken down into these simpler building blocks, other microbes, including various Alpha-proteobacteria and Desulfobacterota, continue the process by consuming the monomers like 3HB and lactate. Working together, these microbial communities ultimately convert the plastic into carbon dioxide, water, and other harmless compounds that ideally return to the marine ecosystem.

The findings of this study fill a critical gap in our understanding of how bio-based plastics degrade in remote marine environments. Its proven biodegradability makes it a promising option for creating safer, more biodegradable materials.

"This research addresses one of the most critical limitations of current bioplastics -- their lack of biodegradability in marine environments. By showing that LAHB can decompose and mineralize even in deep-sea conditions, the study provides a pathway for safer alternatives to conventional plastics and supports the transition to a circular bioeconomy," says Prof. Taguchi.