Biofuel Production Based on Carbohydrates from Both Brown and Red Macroalgae: Recent Developments in Key Biotechnologies.

Shigeyuki Kawai,Kousaku Murata

doi:10.3390/ijms17020145

Shigeyuki Kawai, Kousaku Murata

Open Access

https://doi.org/10.3390/ijms17020145

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Abstract

Marine macroalgae (green, red and brown macroalgae) have attracted attention as an alternative source of renewable biomass for producing both fuels and chemicals due to their high content of suitable carbohydrates and to their advantages over terrestrial biomass. However, except for green macroalgae, which contain relatively easily-fermentable glucans as their major carbohydrates, practical utilization of red and brown macroalgae has been regarded as difficult due to the major carbohydrates (alginate and mannitol of brown macroalgae and 3,6-anhydro-l-galactose of red macroalgae) not being easily fermentable. Recently, several key biotechnologies using microbes have been developed enabling utilization of these brown and red macroalgal carbohydrates as carbon sources for the production of fuels (ethanol). In this review, we focus on these recent developments with emphasis on microbiological biotechnologies.

Highlights

Based on the medium variant projection, the world population of 7.2 billion in mid-2013 is projected to reach 8.1 billion in 2025 and 9.6 billion in 2050 [1]
It was concluded that a conversion of macroalgal glucan into ethanol is apparently not enough, and a conversion of the other carbohydrates derived from both brown and red macroalgae into ethanol is needed to achieve a high concentration of ethanol [3]
A1, alginate is depolymerized by endo-type alginate lyases (A1-I, A1-II and A1-III) to produce oligo-alginates, which are degraded by exo-type alginate lyase (A1-IV) into an unsaturated uronate that is further non-enzymatically converted to deoxy-L-erythro-5-hexoseulose uronate (DEH)

Summary

Introduction

Based on the medium variant projection, the world population of 7.2 billion in mid-2013 is projected to reach 8.1 billion in 2025 and 9.6 billion in 2050 [1]. Macroalgae have attracted attention as an alternative source of biomass for the production of both fuels and chemicals. The contents of glucan based on a dry weight basis in green, red and brown macroalgae are 22% (Ulva pertusa), 21.8% (Gelidium elegans) and 24.5% (Alaria crassifolia), respectively [5]. The major portion of farmed macroalgae is brown and red macroalgae. Macroalgae, in particular red and brown macroalgae, are undoubtedly a promising renewable energy source capable of producing both liquid fuels and chemicals. It was concluded that a conversion of macroalgal glucan into ethanol is apparently not enough, and a conversion of the other carbohydrates (mannitol, alginate, agarose, agaropectin, carrageenan, etc.) derived from both brown and red macroalgae into ethanol is needed to achieve a high concentration of ethanol [3]. Several developments have been achieved after those reviews were published, and in this review, we further overview this field including the latest developments with emphasis on microbiological biotechnologies

Carbohydrates in Brown Macroalgae

Key Biotechnologies for Production of Ethanol from Mannitol

Alginate Metabolism

Key Biotechnology to Utilize Agar and Carrageenan for Ethanol Production

Findings

Conclusions and Perspectives