Biodegradable material vs non-degradable materials: s consumption of compostable plastics less harmful than oil-derived plastics?

Walter Alain Martínez-Muñoz¹, Jesús Guillermo Jiménez-Cortés¹, Cecilia Soldatini², Yuri V. Albores-Barajas^3,4 & Alex Córdoba-Aguilar^1*

¹ Departamento de Ecología Evolutiva, Instituto de Ecología, Universidad Nacional Autónoma de México, 04510 Ciudad de México, México.

² Centro de Investigación Científica y de Educación Superior de Ensenada - Unidad Académica La Paz, Calle Miraflores 334, Bellavista 23050, La Paz, Baja California Sur, Mexico.

³ Secretaría de Ciencia Humanidades Tecnología e Innovación. Av. Insurgentes Sur 1582, Col. Crédito Constructor. Alcaldía Benito Juárez, C.P. 03240, Mexico City.

⁴ Departamento de Ciencias Marinas y Costeras, Universidad Autónoma de Baja California Sur, 23080, La Paz, Baja California Sur, Mexico.

*Corresponding author: acordoba@iecologia.unam.mx, Phone/FAX +52 555622-9003

Abstract

Given the harmful impact of plastics on organisms’ fitness, one question arises: is compostable material (derived from non-oil sources) any better? Here we assessed the fitness effects of consuming two oil-derived plastics (polyethylene and polystyrene) and one compostable product in insects, utilizing Tenebrio molitor beetles as the study system. Animals were fed during the larval stage either of four different treatments: a) polyethylene + apple/wheat; b) polystyrene + apple/wheat; c) compostable product + apple/wheat; and d) apple/wheat alone. Upon reaching the adult stage, insects were provided with wheat and apple for 7 days, allowed to mate, and lay eggs. We recorded developmental rate and mortality from larvae to pupa, weight and fecundity, and survival probability from one stage to the next. Mortality was higher when animals consumed any type of plastic. The probability of survival was also affected, particularly in the pupal and adult stages. Feeding with any type of plastic oil-derived or compostable plastic led to a reduction in body size and reproductive success (measured as surviving larvae). Notably, in some cases, the group fed with compostable plastic was the most affected. Delays in development at different stages could increase mortality, while the decrease in egg production in females and the reduction in adult size could imply carry-over effects on demography. Perhaps, the additional materials in compostable products imply toxic effects like those caused by plastics. Thus, the effects of compostable products are not any better than those of plastics.

Key words: plastic, compostable material, fitness, insect,Tenebrio , life history, survival, weight, fecundity

Introduction

In a world full of threatening factors, plastics are part of the top list of stressors (MacLeod, et al. 2021; Santos et al. 2021; Martinho et al. 2022). Plastics are produced through the polymerization of monomers derived from oil and gas. Two well-known examples are polyethylene (PE) and polystyrene (PS) which are extensively used for plastic bags and containers (such as Styrofoam®) respectively (Andrady & Neal, 2009). Given the negative effects of these products, a proposed alternative is the use of bioplastics such as compostable plastics (CP), which are polymeric, non-oil derived materials from natural substances (Arikan & Ozsoy, 2015; Atiwesh et al. 2021). Unlike plastics, CP production presumably implies reduced greenhouse effects and energy-saving balance, faster degradation (Atiwesh, et al. 2021).

Although there were doubts about the effects of plastic ingestion on animal physiology, we now have a clearer understanding. Studies using insects have shown negative effects at molecular levels, such as a high production of reactive oxygen species, antioxidant enzymes, lipid peroxidation indicators, and oxidative damage in Tenebrio molitor larvae fed with PE (Peng et al. 2023). Similarly, also in T. molitor larvae, when PS was part of the diet, negative effects were observed on the lipidome, with high levels of ceramides and cardiolipins being produced, which are implicated in apoptosis and cellular stress processes (Tsochatzis et al. 2022). At the DNA level, PS consumption affected the expression of genes that encode heat shock and oxidative stress proteins during development, specifically at the pupal formation stage Chironomus riparius dipteran larvae (Carrasco-Navarro et al. 2021). These negative effects also take place systemically. InC. riparius larvae, an increase in basal phenoloxidase activity was found after PE ingestion (Silva et al. 2021). Also, Bombyx mori larvae fed with PS, showed an increased expression of antimicrobial peptide (AMP) genes, such as lysozymes and cecropins (Muhammad et al. 2021).

While the environmental advantages of using non-petroleum-derived plastics are known, there is still unconclusive evidence on their effects on insects. For example, consumption of polybutylene adipate terephthalate, a biodegradable and compostable copolymer, by T. molitor larvae over 4 weeks did not affect survival, mass, or molting rate (Kokalj et al. 2024). When black soldier fly was fed with the biodegradable polyester polylactic acid (PLA), their larval development, survival, as well as pupal development and rate, were not affected (Heussler et al. 2024). Conversely, T. molitor larvae fed with PBAT during the first and second generations, increased and decreased their molting rate in the first and second generation respectively (Kokalj et al. 2024). Finally, one other aspect that has lagged behind is that of the effects of both non-biodegradable plastics and bioplastics on insects when their ingestion is constant and long-term (i.e., during all or almost all the larval phase until adulthood). These elements are crucial since several future scenarios indicate that plastic waste in the environment will be a constant for all organisms across ecosystems.

Here we show the effects of ingesting plastic versus compostable material on insect fitness (measured as development rate, mortality, weight, hatching rate, fecundity, and survival probability among different age stages) using the mealworm beetle Tenebrio molitor (Linnaeus, 1758) as our study subject. We chose this animal based on previous research indicating that larvae can consume polymers for short periods of time (Yang et al., 2015), and that microplastic, oil-derived consumption impairs survival, growth, and development (Matyja et al., 2020). The novelty of our work lies in its comparison of the fitness effects of ingesting plastics vs CP, as well as in examining how fitness surrogates covariate with each other. Our working hypothesis is that the ingestion of biodegradable plastics will have little or no effect on the variables analyzed, compared to PS or PE.