Elucidating the pivotal functions of fulvic acid in enhancing Monoraphidium sp. QLZ-3 for cadmium remediation and bioresource recovery

Abstract

Heavy metal pollution poses substantial challenges to human health and aquatic ecosystems. This study investigates a coupled technology for lipid production and cadmium adsorption utilizing microalgae regulated by fulvic acid (FA). Under the combination of 40 mg L⁻¹ FA and cadmium (Cd) treatment, Monoraphidium sp. QLZ-3 exhibited the highest biomass (3.27 g L⁻¹), lipid content (52.73 %), and lipid productivity (193.26 mg L⁻¹ d⁻¹), which were enhanced by 20.10 %, 15.81 % and 40.27 % respectively compared with the control. Notably, FA application significantly increased cadmium removal efficiency to 100 %. Moreover, the synergistic effect of FA and Cd enhanced the biomass, lipid production, and energy yield (92.38 kJ L⁻¹) by accelerating nitrogen consumption, inhibiting carbohydrate synthesis, and elevating levels of reactive oxygen species and mitogen-activated protein kinase. FA had a minimal impact on fatty acid composition and biodiesel properties. The majority of the biodiesel quality parameters met the specifications for commercial biodiesel. Proteomic analysis revealed that exogenous FA promoted cell growth and lipid accumulation by upregulating the tricarboxylic acid cycle, the nitrogen assimilation pathway, and activating Ca²⁺ signaling in QLZ-3 under cadmium treatment. Additionally, calcium ion (Ca²⁺) and reactive oxidative species (ROS) were identified as key factors in promoting cell growth and lipid synthesis under the influence of Cd and FA. These findings collectively indicate that FA can boost both biomass and lipid production, as well as the efficient removal of Cd²⁺, providing a theoretical foundation for the optimization of microalgal biomass and lipid production and the bioremediation of heavy metal contamination in aquatic environments.