Enhancement of lipid production and nutrient removal of Monoraphidium sp. FXY-10 by combined melatonin and molasses wastewater treatment

Highlights

• Algae were grown in molasses wastewater (MWW) with melatonin treatment.

• MT significantly boosted lipid and carbohydrate contents of FXY-10 under MWW.

• Nutrient removal rates increased in the integrated mode compared to control.

• A combined strategy was developed for MWW treatment and biofuel production.

Abstract

Molasses wastewater can be utilized for microalgae cultivation, and the harvested algal biomass is suitable for biofuel production. In this study, the effects of melatonin on the metabolism and activities of three key lipid biosynthetic enzymes of Monoraphidium sp. FXY-10 cultured in molasses wastewater were evaluated. The maximal biomass (1.21 g L⁻¹) and lipid content (68.69%) were obtained during melatonin treatment and were approximately 1.42- and 1.15-fold, respectively, of the group not treated with melatonin. The carbohydrate and protein contents of melatonin-treated cells were also increased. MT treatment upregulated the activities of malic enzyme and acetyl-CoA carboxylase but downregulated phosphoenolpyruvate carboxylase activity, which was correlated with lipid accumulation in FXY-10. MT-treated samples showed greater potentials for removing organic carbon (92.33% chemical oxygen demand removal) and nutrients (90.07% nitrogen removal, 86.04% phosphorus removal) than the non-treated samples. Additionally, the economic feasibility of melatonin treatment demonstrated that a combined strategy is ideal for molasses wastewater treatment and biofuel production.

Introduction

Microalgae have been considered a potential source of biofuel to replace traditional energy crops because they can produce lipids, proteins and carbohydrates [1], [2] and have the capacity for rapid growth and efficient photosynthesis and have a high lipid content [3]. Their use can also avoid competition for arable land because they can be cultivated on non-arable land and in wastewater [4], [5].

Currently, to decrease the cost of producing microalgal biomass, there have been wide applications of wastewater to microalgae cultivation, including industrial wastewater [6], municipal wastewater [7], swine production wastewater [8] and urban wastewater [9]. Molasses wastewater (MWW) is a by-product of sugar refinement, collected from the sugar plant alcohol distillation crude tower. MWW has a high chemical oxygen demand (COD) [10], and contains nitrogen, phosphorus and micronutrients [11]. The output of molasses in China is around 400 million tons per year and its price was maintained at relatively stable levels in recent years [10]. However, MWW is rarely used for the cultivation of microalgae [10], [12], [13]. Additionally, previous studies showed that Chlorella zofingiensis and Scenedesmus sp. Z-4 grew well in MWW, but their lipid contents were only 38% and 28.9%, respectively [12], [13]. Although MWW has been confirmed to be a feasible alternative for microalgae feeding, the lipid content of microalgae grown in MWW is insufficient. However, lipid production can be further enhanced by customizing the conditions so that they are conducive to lipid accumulation in microalgae cultured in MWW.

Microalgae have always been cultivated under abiotic stress conditions to increase lipid production, including high light [14], high salinity [15], and nutrition deprivation [16]. Stress conditions stimulate microalgal lipid accumulation but drastically reduce cell growth [14], [17], [18]. Phytohormones are metabolic messengers that regulate the growth and lipid production of microalgae and moderate abiotic stress conditions, including auxins, abscisic acid, gibberellin [19], fulvic acid [20], and melatonin [21], [22]. Melatonin (MT), a plant hormone that has multiple functions in many aspects of plant physiology [23], [24], alleviates abiotic stress [25], [26]. Tal et al. [27] indicated that exogenous MT can regulate oxidative stress in Chlamydomonas reinhardtii. Furthermore, Ding et al. [28] and Zhao et al. [22] showed that MT could further enhance metabolites synthesis by microalgae under conditions of high light levels and nitrogen deficiency, respectively. Therefore, exogenous MT may stimulate biomass and lipid accumulation in microalgae cultured in wastewater. Additionally, coupling of MT with MWW may be an efficient strategy for microalgae cultivation and biodiesel production

In this study, the effects of MT on the biomass and lipid production of Monoraphidium sp. FXY-10 cultured in MWW were evaluated. The three specific objectives of the study were to analyze (1) the biochemical compositions in FXY-10 cultivated in MWW and treated with MT, (2) the activities of three key lipid biosynthesis enzymes and nutrient removal by algal cells treated with MT and MWW and (3) the material cost analysis and mass balance of a combination strategy for microalgae cultivation and biofuel production.