Contents lists available at ScienceDirect Algal Research journal homepage: www.elsevier.com/locate/algal Efficient hydrolysis of glycogen from engineered Synechocystis sp. PCC 6803 catalyzed by recyclable surface functionalized nanoparticles for ethanol production Rajendran Velmurugan a , Aran Incharoensakdi a,b, ⁎ a Cyanobacterial Biotechnology Laboratory, Department of Biochemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand b Academy of Science, Royal Society of Thailand, Bangkok 10300, Thailand ARTICLEINFO Keywords: Cyanobacterium Nanocatalyst Ultrasound Metal oxide Hydrolysis ethanol ABSTRACT The availability of sugars is a key factor for industrial ethanol production. In this study, recyclable surface functionalized metal oxides were prepared and used as catalysts to obtain sugars directly from engineered Synechocystis sp. PCC 6803 (hereafter Synechocystis) biomass. The individual, sulphonated and tungstenated forms of Al 2 O 3 , CaO, Fe 3 O 4 , TiO 2 and ZrO 2 were screened for the hydrolysis of Synechocystis glycogen under ultrasound irradiation. Among them, the sulphonated ZrO 2 produced a maximum fermentable sugar yield of 40.2 g/L, whereas the tungstenated ZrO 2 produced 37.8 g/L. This improvement was due to the combined action of ultrasound irradiation and acidic groups conferred by sulphur or tungsten on metal oxides. In recycling, tungstenated metal oxides were superior to sulphonated metal oxides in terms of recovery and hydrolysis effi- ciency. The compatibility of the tungstenated metal oxides mediated hydrolysis with fermentation using Saccharomyces cerevisiae MTCC-170 produced the highest ethanol concentration (16.5 g/L), which favors the overall process. 1. Introduction Biofuel production from renewable feedstock is necessary to reduce the dependency of fossil fuel and to reduce the greenhouse gases emission. As an alternative, the production of ethanol fuel from various feedstocks has been developed; however, the recalcitrance of lig- nocellulosic biomass and edible nature of starch-based feedstock limits its commercialized production [1,2]. Similar to starch, glycogen is a storage type of polymer that is considered a better feedstock for glucose than lignocellulosic biomass, as it does not require pretreatment pro- cesses [3]. Certain photosynthetic microorganisms, especially cyano- bacteria are natural producers of glycogen in abundant quantity from atmospheric carbon fixation [4]. In addition, cyanobacteria can be metabolically engineered to accumulate more glycogen which provides another advantage of using cyanobacterial biomass to produce ethanol. To obtain the glycogen for fermentable sugar production, it has to be extracted prior to hydrolysis and fermentation processes [5]. Various studies were developed for the extraction of intracellular poly- saccharides based on alkali and enzymatic treatments, where the ex- tracted polysaccharides have been hydrolyzed as a separate step using acids or enzymes [5,6]. When compared to the lignocellulosic biomass, the extraction of carbohydrates from cyanobacteria is not difficult be- cause of their brittle cell membrane architecture [5]. However, the catalyst systems used for the extraction/hydrolysis are neither eco- nomically feasible nor environmentally acceptable [7]. Hence, the current techniques are associated with sequential steps, requirement of more catalysts and wastewater generation [7]. The development of a technique with the consideration of these drawbacks could make a major impact in biochemical conversion of glycogen. Rather than pro- cessing biomass in multiple consecutive steps to get the fermentable sugar molecule, it would be desirable to perform simultaneous extrac- tion and hydrolysis with a single recyclable catalyst [6]. Recently, Mollers et al. [8] developed a method utilizing lysozyme and two α- glucanases in an attempt to obtain fermentable sugar from cyano- bacterial biomass. However, the enzyme mediated conversion is con- sidered as an expensive process [9]. The metal oxides are explored as solid catalysts for the hydrolysis of polysaccharides due to the posses- sion of Brønsted and Lewis acid sites on the surface [10,11]. As metal oxides are nano-sized, they can act more efficiently in the reaction medium at the solid/liquid interface. Furthermore, they are recyclable, thus, reducing the concern of environmental pollution [11]. In metal oxide based solid acids, protons help to balance the net negative charge https://doi.org/10.1016/j.algal.2019.101621 Received 21 March 2019; Received in revised form 31 July 2019; Accepted 31 July 2019 ⁎ Corresponding author. E-mail address: aran.i@chula.ac.th (A. Incharoensakdi). Algal Research 43 (2019) 101621 2211-9264/ © 2019 Elsevier B.V. All rights reserved. T