Rugulopteryx okamurae: Effect of microwave pretreatment on PHB fermentation

Polyhydroxyalkanoates (PHA) are biodegradable and sustainable bioplastics that are a promising alternative to conventional petrochemical-based plastics and contribute significantly to the reduction of environmental pollution. This study investigates the potential of using the invasive macroalga Rugulopteryx okamurae as a feedstock for PHA production, specifically polyhydroxybutyrate (PHB), through fermentation with Cupriavidus necator. The macroalga was pretreated using microwave-assisted extraction at different temperature conditions, followed by enzymatic hydrolysis to enhance the release of fermentable sugars and subsequent PHB production. Results indicate that microwave pretreatment at 160 °C yielded the highest growth of C. necator (CDW = 1.75 g·L⁻¹). However, pretreatment at higher temperatures (180 °C and 220 °C) led to reduced yields, probably due to the release of biological inhibitors. Additionally, the hydrolysate medium´s carbon-to-nitrogen (C/N) ratio was analysed, revealing that a lower C/N ratio, which was obtained in the hydrolysates from pretreated macroalgae, favoured bacterial growth. Higher C/N ratios obtained in the hydrolysate from non-pre-treated macroalgae were more conducive to PHB production, achieving the highest PHB accumulation (94 %). The study demonstrated that non-pretreated R. okamurae biomass led to the highest PHB accumulation, surpassing yields from other pretreatment methods like solid-state fermentation or hydrothermal acid pretreatment. This research highlights the potential of using R. okamurae, an underutilized and invasive macroalga, as a renewable resource for PHA production, offering a sustainable alternative to conventional plastics and addressing environmental challenges such as beach-cast biomass accumulation. Future studies could focus on optimizing pretreatment processes and exploring more efficient ways to convert macroalgae biomass into bioplastics.