Research paper Biomethanation potential of marine macroalgal Ulva biomass in sequencing batch mode: Changes in process performance and microbial community structure over five cycles Heejung Jung a , Jaai Kim a, ** , Joonyeob Lee b , Seokhwan Hwang b , Changsoo Lee a, * a School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 44919, South Korea b School of Environmental Science and Engineering, POSTECH, Pohang, Gyungbuk 37673, South Korea article info Article history: Received 16 March 2015 Received in revised form 4 April 2016 Accepted 13 May 2016 Keywords: Anaerobic digestion Macroalgae Microbial community structure Sequencing batch reactor Sulfate-reducing bacteria Ulva abstract Anaerobic digestion (AD) of Ulva biomass, a promising next-generation feedstock for energy production, was investigated in sequencing batch mode. Over five cycles of operation, the methane yield decreased more than twofold (from 0.15 to 0.07 L/g COD fed ), while the organic treatment efficiency (i.e., chemical oxygen demand (COD) removal) remained fairly constant (53.7e61.1%). Such changes in reactor per- formance were related with structural variations in the microbial community, particularly the bacterial community, with repeated cycles. Methanosaeta- and Methanolinea-related populations were most likely the main aceticlastic and hydrogenotrophic methanogens, respectively, in the reactor. The emergence and prevalence of sulfate-reducing bacteria (SRBs), primarily a Solitalea-related population, most likely resulted in increased consumption of organic substrates for sulfate reduction, rather than methane production, in later cycles. Our observations suggest that the metabolic properties of the reactor changed with the transition of the bacterial community structure over cycles, and the metabolic shift had a negative effect on methanogenesis. The sequencing batch operation strategy applied in this study was not suitable for maximizing methane production from Ulva biomass, although the treatment efficiency was fairly stable. Robust control of SRB activity is necessary for more stable and efficient biomethanation of Ulva biomass in sequencing batch mode. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Human activities have produced significant adverse effects on the environment in recent centuries, leading to the global pollution and energy shortage problems faced today. With growing concerns regarding sustainability worldwide, increasing attention is paid to waste-to-energy biotechnologies as viable solutions to these global issues. A good example of such a technology is anaerobic digestion (AD), whereby waste organic carbons are converted into CH 4 and CO 2 . Among various feedstocks for biofuels, seaweeds (i.e., mac- roalgae) are gaining great interest as a next-generation feedstock due to their higher biodegradation potential relative to terrestrial feedstocks [1]. Many different seaweeds can be effectively converted to methane via AD. However, most previous studies have tested readily biodegradable seaweed species of comparatively high agricultural value. However, such approaches seriously lower the economic feasibility of using seaweeds as feedstock, so increasing attention has recently been paid to seaweeds of little commercial value, for example, Ulva. Unwanted blooming of Ulva species causes marine green tides that lead to serious pollution of coastal areas worldwide. A massive Ulva green tide covering over 3800 km 2 occurred in the Yellow Sea, China in the summer of 2008, and over 1.5 million wet tons of waste Ulva were dumped on the beaches of Qingdao [2]. Large amounts of waste Ulva are difficult to handle as the decomposition of Ulva biomass releases malodorous substances such as sulfide. Therefore, biomethanation of Ulva is environmentally as well as economically appealing, although its methane potential has been reported to be lower than some other, more easily digestible seaweeds [3]. AD of Ulva biomass has been conducted in a number of previous studies, with the emphasis mostly being placed on testing the * Corresponding author. ** Corresponding author. E-mail addresses: jaai@unist.ac.kr (J. Kim), cslee@unist.ac.kr (C. Lee). Contents lists available at ScienceDirect Biomass and Bioenergy journal homepage: http://www.elsevier.com/locate/biombioe http://dx.doi.org/10.1016/j.biombioe.2016.05.018 0961-9534/© 2016 Elsevier Ltd. All rights reserved. Biomass and Bioenergy 91 (2016) 143e149