Oceans as bioenergy pools for methane production using activated methanogens in waste sewage sludge

Highlights

• CO₂ dissolved in seawater can be a carbon source for methane production.

• Methane energy was generated from seawater (carbonate ion) by enriched methanogens.

• Microbial communities adapted to seawater salinity improved methane production.

• 81% of ¹³CH₄ was generated from microbial conversion of NaH¹³CO₃.

Abstract

The dissolved CO₂ that causes ocean acidification has great potential for bioenergy production. In this study, we demonstrate that activated methanogens in waste sewage sludge (WSS) are useful for converting bicarbonate in seawater into methane. These activated methanogens were adapted in different seawater sources for methane production through repeated batch experiments that resulted in an increase of 300–400 fold in the methane yield. During these repeated batch experiments, the microbial communities in WSS adapted to the high salinity of seawater to generate more methane. Microbial community analysis showed the dominance of Achromobacter xylosoxidans, Serrati sp. and methanogens including Methanobacterium sp., Methanosarcina sp., and Methanosaeta concillii. Using a ¹³C-labeled isotope, we demonstrate that 81% of the methane is derived from microbial conversion of NaH¹³CO₂ in artificial seawater. Therefore, this study shows that oceans, with the largest surface area on Earth, have a potential as a substrate for methane energy production via an acclimated consortium approach.

Introduction

Oceans cover 71% of the Earth’s surface and hold 97% of the terrestrial water [1]. Oceans contain dissolved materials and ions, microorganisms, and dissolved gases from the atmosphere. The oceans absorb one third of the atmospheric carbon dioxide (CO₂) derived from anthropogenic activity which then acts as the main contributor for ocean acidification [2], [3]. The amount of dissolved CO₂ has been increasing each year, and it is easier for CO₂ to dissolve in water at lower temperatures [2]. CO₂ dissolution in water produces carbonic acid (H₂CO₃), hydrogen ions (H⁺), bicarbonate ions (HCO₃⁻), and carbonate ions (CO₃²⁻) by the following reactions: CO₂ + H₂O $\leftrightharpoons$ H₂CO₃ $\leftrightharpoons$ H⁺ + HCO₃⁻ $\leftrightharpoons$ 2H⁺  + CO₃^2- which cause excess protons in the form of H⁺ which then acidifies the ocean [4]. The increment in CO₂ dissolution in seawater is indicated by the reduction in marine pH by 0.3–0.4 pH units since ocean pH is predicted to be reduced from pH 8.1 in 2000 to pH 7.7 in of 2100 with the corresponding increase in dissolved organic carbon (11–20%) and bicarbonate (17–20%) [5]. Of course, ocean acidification affects many marine ecosystems [5].

In general, the ocean is the best carbon sink since the dissolved carbon is used to make coral reefs in marine sediments. Calcium carbonate also precipitates biologically by the reaction of CaCO₃ ↔ CO₃²⁻ + Ca²⁺ to form the shells and skeletons of marine organisms [3], [6]. In seawater, the ratio of dissolved carbon species is 0.5% [CO₂]: 86.5% [HCO₃⁻]: 13% [CO₃⁻] so bicarbonate is the dominant species while dissolved CO₂ is present in small concentrations [7].

World energy demands require renewable energy sources to replace fossil fuel to facilitate sustainable development [8]. Methane gas is colorless, odorless, safe, and has proven to be a good energy source for electricity and power generation [9], [10]. Moreover, methane gas can be used as a substrate for other value-added products such as methanol and other hydrocarbons [11], [12]. During anaerobic degradation of high molecular weight organics, methane evolution occurs in four steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis [13]. Different microbial communities including Bacteria and Archaea are involved by chemolitotrophic activity in order to produce methane [14]. Biological methane production is cost effective by using waste sewage sludge (WSS) that has been enriched with different kinds of microorganisms [15]. In marine environments, many attempts have been made to produce methane using microalgae for oil production [16] as well as in deep ocean basins by taking advantage of the available carbon in marine sediments and their Archaea [17]. Much research has been conducted utilizing organic carbon available in WSS as a source of carbon for methane [18], [19]. However, to the best of our knowledge, no studies have been conducted for methane production from seawater by taking advantage of CO₂ dissolution and carbonic species accumulation. The usual limitation is the salinity constraints that affect methanogens in seawater [20]. We have developed methods for producing enriched methanogens that capture CO₂ gas and convert it into methane [15]. Thus, in this study, we explored the possibility of methane production from bicarbonate in seawater by using our enriched terrestrial methanogens from WSS.

This paper demonstrates that enriched methanogens that were grown under a limited carbon condition (grown for 50 days until all organic carbons were depleted) are capable of capturing carbon from seawater. A ¹³C labelled isotope of NaHCO₃ was used in artificial seawater to show the potential of our enriched consortia in assimilating carbonate species from seawater. Therefore, this work demonstrates that methane production from seawater by enriched methanogens may provide renewable energy as well as provide the benefit of reducing ocean acidification.