A review of the biodegradability of plastics in the ocean

    In recent years, the global annual production and usage of plastics have significantly increased. Most plastics are difficult to rapidly biodegrade and will transform into large particles or microplastics when they enter the ocean, which can be ingested or entangled by marine organisms, posing a threat to the marine ecosystem. The biodegradability of microplastics depends on their chemical composition, availability of nutrients in the ocean, microbial species, and processing time.
 
    As shown in Figure 1, it takes several days to months for microplastics within a depth of 40 meters to biodegrade, months to years for microplastics within a range of 40-200 meters, and centuries for microplastics deeper. Factors such as oxygen content, temperature, phosphorus, and nitrogen are important limiting factors for microbial growth and biodegradation of marine microplastics. Microplastics may alter the microecological balance in the ocean, where only a small portion of active nitrogen, including dissolved organic nitrogen, ammonia, and nitrate, can be ingested by microorganisms. A broad consensus is that nitrogen content is relatively low in the short term, and microalgae 'adaptability to nutrient poor environments has made them the dominant species in the ocean for three billion years. A study has found that blue-green algae on microplastic particles are more abundant than bacteria in the surrounding marine environment.

    Figure 1 Degradation of Microplastics in the Ocean - From the Original Text
The National Oceanic and Atmospheric Administration (NOAA), American Society for Testing and Materials (ASTM), and International Organization for Standardization (ISO) have released methods for analyzing artificial materials, plastics, synthetic materials, and their biodegradability. The ASTM and ISO standards have also developed breath determination methods for evaluating biodegradability in marine environments. Under aerobic conditions, the minimum requirement for plastic samples to biodegrade into carbon dioxide is 60% to 70%, with durations ranging from 3 months (ASTM D6691-09), 6 months (ASTM D7473-12), and 24 months (ISO 18830, ISO 19679, ASTM D7991-15).
 
    Due to the limited amount of nitrogen required by bacteria in seawater, the estimated biodegradation rate using the ASTM method may be 6000 times higher than the actual rate in the ocean. The biodegradation time range used in testing may also be overestimated. Therefore, in practical situations, the biodegradation rate of microplastics in marine environments may be lower than estimated.
 
    In addition, microplastics have a significant impact on marine ecosystems. Microplastics provide a huge surface area and are quickly covered by inorganic and organic materials, providing carbon and nutrients for biofilm formation in marine environments. Currently, microplastics have been found in all oceans, and within the same water area, the microorganisms on microplastic fibers are significantly different from suspended microorganisms on surrounding particles or in water. Therefore, we need to model the organic load and nitrogen availability to prevent the existing marine microecological balance from being disrupted and developing in an unfavorable direction.
 
key findings
After the decrease in nitrogen content, algae may still be the main group, which may have adverse effects on the marine food chain. It is necessary to improve the testing methods for biodegradability and model the availability of organic load and nitrogen in future research.