The complete enzymatic saccharification of agarose and its application to simultaneous saccharification and fermentation of agarose for ethanol production

Agarase cocktail from agarolytic Alteromonas sp. Aga1552 converts homogenized Gelidium amansii into monosaccharide

Marine algae biomass utilization has attracted considerable attention, however, the preparation of monosaccharides from raw algae is still hindered by many technical barriers. In this study, three genes, aga1365, aga1364, and aga1360, encoding key enzymes constituting a complete agar decomposition pathway were expressed and characterized. Recombinant Aga1365, Aga1364, and Aga1360 exhibited high optimal reaction temperatures and excellent thermal stability. Moreover, enzyme cocktail was proved to have higher synergistic effect to prepare monosaccharide from raw seaweed. The enzyme cocktail of Aga1360 (GH117) with Aga1365 (GH16) and enzyme cocktail of Aga1360 with both Aga1365 and 1364 (GH50) were used to synergistically degrade homogenized Gelidium amansii, maximum monosaccharide production of 21.47 mg/g and 39.28 mg/g could be achieved, respectively. This study presents an environment-friendly, time saving and efficient way to prepare monosaccharides from raw seaweed, which also provide a potential strategy to effectively convert algae biomass for biofuel and biochemical production by utilizing the synergistic effects of enzyme cocktail.

Eco-friendly production and probiotic purification of agarose degradation products: Oligosaccharides and 3,6-anhydro-L-galactose

Algal biomass offers a solution to global resource scarcity, with agarose, a key component of Gelidium amansii, containing valuable products like oligosaccharides and 3,6-anhydro-L-galactose. However, current purification methods limit their commercial viability. In this study, we utilized gel filtration chromatography to purify agaro-oligosaccharides and neoagaro-oligosaccharides with varying degrees of polymerization, achieving a novel purification of odd-numbered neoagaro-oligosaccharides. Additionally, by fermenting a mixture of 3,6-anhydro L-galactose and D-galactose with six probiotics, our results demonstrate that five probiotics-Lactobacillus plantarum, Bifidobacterium adolescentis, Streptococcus thermophilus, Lactobacillus acidophilus, and Lactobacillus rhamnosus effectively utilize D-galactose in mixed carbon sources while retaining 3,6-anhydro L-galactose. This approach enables efficient, low-cost, and eco-friendly purification of 3,6-anhydro L-galactose, opening avenues for its widespread utilization.

Constructing Marine Bacterial Metabolic Chassis for Potential Biorefinery of Red Algal Biomass and Agaropectin Wastes

Marine red algal biomass is a promising feedstock for sustainable production of value-added chemicals. However, the major constituents of red algal biomass, such as agar and carrageenan, are not easily assimilated by most industrial metabolic chassis developed to date. Synthetic biology offers a solution by utilizing nonmodel organisms as metabolic chassis for consolidated biological processes. In this study, the marine heterotrophic bacterium Pseudoalteromonas atlantica T6c was harnessed as a metabolic chassis to produce value-added chemicals from the affordable red algal galactans or agaropectin, a byproduct of industrial agarose production. To construct a heterologous gene expression device in P. atlantica T6c, promoters related to agar metabolism were screened from the differentially expressed genes using RNA-Seq analysis. The expression device was built and tested with selected promoters fused to a reporter gene and tuned by incorporation of a cognate repressor predicted from the agar-specific polysaccharide utilization locus. The feasibility of the marine bacterial metabolic chassis was examined by introducing the biosynthetic gene clusters of β-carotene and violacein. Our results demonstrate that the metabolic chassis platform enables direct conversion of low-cost red algal galactans or industrial waste agaropectin into valuable bioactive pigments without any pretreatment of biomass. The developed marine bacterial chassis could potentially be used in a biorefinery framework to produce value-added chemicals from marine algal galactans.

Bifunctional and monofunctional α-neoagarooligosaccharide hydrolases from Streptomyces coelicolor A3(2)

Agar is a galactan and a major component of the red algal cell wall. Agar is metabolized only by specific microorganisms. The final step of the β-agarolytic pathway is mediated by α-neoagarooligosaccharide hydrolase (α-NAOSH), which cleaves neoagarobiose to D-galactose and 3,6-anhydro-α-L-galactose. In the present study, two α-NAOSHs, SCO3481 and SCO3479, were identified in Streptomyces coelicolor A3(2). SCO3481 (370 amino acids, 41.12 kDa) and SCO3479 (995 amino acids, 108.8 kDa) catalyzed the hydrolysis of the α-(1,3) glycosidic bonds of neoagarobiose, neoagarotetraose, and neoagarohexaose at the nonreducing ends, releasing 3,6-anhydro-α-L-galactose. Both were intracellular proteins without any signal peptides for secretion. Similar to all α-NAOSHs reported to date, SCO3481 belonged to the glycosyl hydrolase (GH) 117 family and formed dimers. On the other hand, SCO3479 was a large monomeric α-NAOSH belonging to the GH2 family with a β-galactosidase domain. SCO3479 also clearly showed β-galactosidase activity toward lactose and artificial substrates, but SCO3481 did not. The optimum conditions for α-NAOSH were pH 6.0 and 25 °C for SCO3481, and pH 6.0 and 30 °C for SCO3479. Enzymatic activity was enhanced by Co²⁺ for SCO3481 and Mg²⁺ for SCO3479. The β-galactosidase activity of SCO3479 was maximum at pH 7.0 and 50 °C and was increased by Mg²⁺. Many differences were evident in the kinetic parameters of each enzyme. Although SCO3481 is typical of the GH117 family, SCO3479 is a novel α-NAOSH that was first reported in the GH2 family. SCO3479, a unique bifunctional enzyme with α-NAOSH and β-galactosidase activities, has many advantages for industrial applications. KEY POINTS: • SCO3481 is a dimeric α-neoagarooligosaccharide hydrolase belonging to GH117. • SCO3479 is a monomeric α-neoagarooligosaccharide hydrolase belonging to GH2. • SCO3479 is a novel and unique bifunctional enzyme that also acts as a β-galactosidase.

Optimal β-galactosidases for producing high-titer 3,6-anhydro-L-galactose from red-algal agarobiose

3,6-Anhydro-L-galactose (L-AHG) is a monomeric sugar in agarose derived from red macroalgae. Owing to its various physiological activities such as anti-inflammation, moisturizing, skin whitening, anti-colon cancer, and anti-cariogenicity, L-AHG is a potential functional ingredient. In our previous study, a simple and efficient two-step L-AHG production process was designed for high-titer L-AHG production, where a single enzyme was used after the liquefaction of agarose by acid prehydrolysis. However, the enzyme used did not completely hydrolyze agarobiose (AB). Therefore, in this study, for the efficient hydrolysis of AB and the high-titer production of L-AHG, various β-galactosidases belonging to glycoside hydrolase families 1, 2, 35, and 42 were compared by testing their substrate specificities and kinetic parameters. Among the five β-galactosidases, Bga42A, originating from Bifidobacterium longum ssp. infantis ATCC 15,697, showed the highest substrate specificity. Consequently, the two-step process utilizing Bga42A as a single enzyme resulted in a high-titer production of L-AHG at 85.9 g/L, demonstrating the feasibility of producing L-AHG from agarose. KEY POINTS: • L-AHG derived from red macroalgae has various physiological activities. • Various β-galactosidases were evaluated to efficiently hydrolyze agarobiose. • Bga42A showed the highest substrate specificity against agarobiose. • The highest amount of L-AHG with 85.9 g/L was simply produced.

Metabolic engineering enables Bacillus licheniformis to grow on the marine polysaccharide ulvan

Background

Marine algae are responsible for half of the global primary production, converting carbon dioxide into organic compounds like carbohydrates. Particularly in eutrophic waters, they can grow into massive algal blooms. This polysaccharide rich biomass represents a cheap and abundant renewable carbon source. In nature, the diverse group of polysaccharides is decomposed by highly specialized microbial catabolic systems. We elucidated the complete degradation pathway of the green algae-specific polysaccharide ulvan in previous studies using a toolbox of enzymes discovered in the marine flavobacterium Formosa agariphila and recombinantly expressed in Escherichia coli.

Results

In this study we show that ulvan from algal biomass can be used as feedstock for a biotechnological production strain using recombinantly expressed carbohydrate-active enzymes. We demonstrate that Bacillus licheniformis is able to grow on ulvan-derived xylose-containing oligosaccharides. Comparative growth experiments with different ulvan hydrolysates and physiological proteogenomic analyses indicated that analogues of the F. agariphila ulvan lyase and an unsaturated β-glucuronylhydrolase are missing in B. licheniformis. We reveal that the heterologous expression of these two marine enzymes in B. licheniformis enables an efficient conversion of the algal polysaccharide ulvan as carbon and energy source.

Conclusion

Our data demonstrate the physiological capability of the industrially relevant bacterium B. licheniformis to grow on ulvan. We present a metabolic engineering strategy to enable ulvan-based biorefinery processes using this bacterial cell factory. With this study, we provide a stepping stone for the development of future bioprocesses with Bacillus using the abundant marine renewable carbon source ulvan.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01931-0.

Emerging Research Topics in the Vibrionaceae and the Squid-Vibrio Symbiosis

The Vibrionaceae encompasses a cosmopolitan group that is mostly aquatic and possesses tremendous metabolic and genetic diversity. Given the importance of this taxon, it deserves continued and deeper research in a multitude of areas. This review outlines emerging topics of interest within the Vibrionaceae. Moreover, previously understudied research areas are highlighted that merit further exploration, including affiliations with marine plants (seagrasses), microbial predators, intracellular niches, and resistance to heavy metal toxicity. Agarases, phototrophy, phage shock protein response, and microbial experimental evolution are also fields discussed. The squid-Vibrio symbiosis is a stellar model system, which can be a useful guiding light on deeper expeditions and voyages traversing these "seas of interest". Where appropriate, the squid-Vibrio mutualism is mentioned in how it has or could facilitate the illumination of these various subjects. Additional research is warranted on the topics specified herein, since they have critical relevance for biomedical science, pharmaceuticals, and health care. There are also practical applications in agriculture, zymology, food science, and culinary use. The tractability of microbial experimental evolution is explained. Examples are given of how microbial selection studies can be used to examine the roles of chance, contingency, and determinism (natural selection) in shaping Earth's natural history.

Multi-Step Enzymatic Production and Purification of 2-Keto-3-Deoxy-Galactonate from Red-Macroalgae-Derived Agarose

2-keto-3-deoxy sugar acids, which have potential as precursors in medicinal compound production, have gained attention in various fields. Among these acids, 2-keto-3-deoxy-l-galactonate (KDGal) has been biologically produced from D-galacturonate originating from plant-derived pectin. KDGal is also found in the catabolic pathway of 3,6-anhydro-l-galactose (AHG), the main component of red-algae-derived agarose. AHG is converted to 3,6-anhydrogalactonate by AHG dehydrogenase and subsequently isomerized to KDGal by 3,6-anhydrogalactonate cycloisomerase. Therefore, we used the above-described pathway to produce KDGal from agarose. Agarose was depolymerized to AHG and to agarotriose (AgaDP3) and agaropentaose (AgaDP5), both of which have significantly higher molecular weights than AHG. When only AHG was converted to KDGal, AgaDP3 and AgaDP5 remained unreacted. Finally, KDGal was effectively purified from the enzymatic products by size-exclusion chromatography based on the differences in molecular weights. These results show that KDGal can be enzymatically produced and purified from agarose for use as a precursor to high-value products.

Characterization of Agarolytic Pathway in a Terrestrial Bacterium Cohnella sp. LGH

Previously, we have reported that an endo-type β-agarase AgaW was responsible for the hydrolysis of agarose into the major product neoagarotetraose in a terrestrial agar-degrading bacterium Cohnella sp. LGH. Here, we identify and characterize the following depolymerization pathway in strain LGH through the genomic and enzymatic analysis. In the pathway, neoagarotetraose was depolymerized by a novel α-neoagarooligosaccharide (NAOS) hydrolase CL5012 into 3,6-anhydro-α-L-galactose (L-AHG) and agarotriose; Agarotriose was further depolymerized by a novel agarolytic β-galactosidase CL4994 into D-galactose and neoagarobiose; Neoagarobiose was finally depolymerized by CL5012 into L-AHG and D-galactose. Although α-agarase has not been identified in strain LGH, the combined action of CL5012 and CL4994 unexpectedly plays a critical role in the depolymerization of agarotetraose, one theoretical product of α-agarase hydrolysis of agarose. In this pathway, agarotetraose was depolymerized by CL4994 into D-galactose and neoagarotriose; Neoagarotriose was then depolymerized by CL5012 into L-AHG and agarobiose. Furthermore, another novel endo-type β-agarase CL5055 was identified as an isozyme of AgaW with different pH preference in the hydrolysis of agarose into α-NAOSs. Strain LGH seemed to lack a common exo-type β-agarase responsible for the direct depolymerization of agarose or neoagarooligosaccharide into neoagarobiose. These results highlight the diversity of agarolytic manner in bacteria and provide a novel insight on the diversity of agarolytic pathways.

A new carbohydrate-active oligosaccharide dehydratase is involved in the degradation of ulvan

Marine algae catalyze half of all global photosynthetic production of carbohydrates. Owing to their fast growth rates, Ulva spp. rapidly produce substantial amounts of carbohydrate-rich biomass and represent an emerging renewable energy and carbon resource. Their major cell wall polysaccharide is the anionic carbohydrate ulvan. Here, we describe a new enzymatic degradation pathway of the marine bacterium Formosa agariphila for ulvan oligosaccharides involving unsaturated uronic acid at the nonreducing end linked to rhamnose-3-sulfate and glucuronic or iduronic acid (Δ-Rha3S-GlcA/IdoA-Rha3S). Notably, we discovered a new dehydratase (P29_PDnc) acting on the nonreducing end of ulvan oligosaccharides, i.e., GlcA/IdoA-Rha3S, forming the aforementioned unsaturated uronic acid residue. This residue represents the substrate for GH105 glycoside hydrolases, which complements the enzymatic degradation pathway including one ulvan lyase, one multimodular sulfatase, three glycoside hydrolases, and the dehydratase P29_PDnc, the latter being described for the first time. Our research thus shows that the oligosaccharide dehydratase is involved in the degradation of carboxylated polysaccharides into monosaccharides.

Marine Polysaccharides: Occurrence, Enzymatic Degradation and Utilization

Macroalgae species are fast growing and their polysaccharides are already used as food ingredient due to their properties as hydrocolloids or they have potential high value bioactivity. The degradation of these valuable polysaccharides to access the sugar components has remained mostly unexplored so far. One reason is the high structural complexity of algal polysaccharides, but also the need for suitable enzyme cocktails to obtain oligo- and monosaccharides. Among them, there are several rare sugars with high value. Recently, considerable progress was made in the discovery of highly specific carbohydrate-active enzymes able to decompose complex marine carbohydrates such as carrageenan, laminarin, agar, porphyran and ulvan. This minireview summarizes these achievements and highlights potential applications of the now accessible abundant renewable resource of marine polysaccharides.

Advances in agaro-oligosaccharides preparation and bioactivities for revealing the structure-function relationship

Agaro-oligosaccharides originating from red algae have attracted increasing attention in both basic theoretical research and applied fields due to their excellent bioactivities, which indicates the wide prospects of agaro-oligosaccharides for application in the food, pharmaceutical and cosmetic industries. Thus, a considerable number of studies regarding functional agaro-oligosaccharides preparation as well as the bioactivities exploration have been carried out. Based on these studies, this review first introduced different methods that have been used in agar extraction from red algae, and further provided research progress on arylsulfatase. Then, different methods used for agaro-oligosaccharides production were summarized. Moreover, the abundant bioactivities of agaro-oligosaccharides were described in detail. Finally, this review has discussed current research problems and further provided critical aspects, which may be helpful for revealing the structure-function relationship of agaro-oligosaccharide.

Molecular Characterization of a Novel 1,3-α-3,6-Anhydro-L-Galactosidase, Ahg943, with Cold- and High-Salt-Tolerance from Gayadomonas joobiniege G7

1,3-α-3,6-anhydro-L-galactosidase (α-neoagarooligosaccharide hydrolase) catalyzes the last step of agar degradation by hydrolyzing neoagarobiose into monomers, D-galactose, and 3,6-anhydro-Lgalactose, which is important for the bioindustrial application of algal biomass. Ahg943, from the agarolytic marine bacterium Gayadomonas joobiniege G7, is composed of 423 amino acids (47.96 kDa), including a 22-amino acid signal peptide. It was found to have 67% identity with the α-neoagarooligosaccharide hydrolase ZgAhgA, from Zobellia galactanivorans, but low identity (< 40%) with the other α-neoagarooligosaccharide hydrolases reported. The recombinant Ahg943 (rAhg943, 47.89 kDa), purified from Escherichia coli, was estimated to be a monomer upon gel filtration chromatography, making it quite distinct from other α-neoagarooligosaccharide hydrolases. The rAhg943 hydrolyzed neoagarobiose, neoagarotetraose, and neoagarohexaose into D-galactose, neoagarotriose, and neoagaropentaose, respectively, with a common product, 3,6- anhydro-L-galactose, indicating that it is an exo-acting α-neoagarooligosaccharide hydrolase that releases 3,6-anhydro-L-galactose by hydrolyzing α-1,3 glycosidic bonds from the nonreducing ends of neoagarooligosaccharides. The optimum pH and temperature of Ahg943 activity were 6.0 and 20°C, respectively. In particular, rAhg943 could maintain enzyme activity at 10°C (71% of the maximum). Complete inhibition of rAhg943 activity by 0.5 mM EDTA was restored and even, remarkably, enhanced by Ca²⁺ ions. rAhg943 activity was at maximum at 0.5 M NaCl and maintained above 73% of the maximum at 3M NaCl. K_m and V_max of rAhg943 toward neoagarobiose were 9.7 mg/ml and 250 μM/min (3 U/mg), respectively. Therefore, Ahg943 is a unique α-neoagarooligosaccharide hydrolase that has cold- and high-salt-adapted features, and possibly exists as a monomer.

Agarose degradation for utilization: Enzymes, pathways, metabolic engineering methods and products

Red algae are important renewable bioresources with very large annual outputs. Agarose is the major carbohydrate component of many red algae and has potential to be of value in the production of agaro-oligosaccharides, biofuels and other chemicals. In this review, we summarize the degradation pathway of agarose, which includes an upstream part involving transformation of agarose into its two monomers, D-galactose (D-Gal) and 3,6-anhydro-α-L-galactose (L-AHG), and a downstream part involving monosaccharide degradation pathways. The upstream part involves agarolytic enzymes such as α-agarase, β-agarase, α-neoagarobiose hydrolase, and agarolytic β-galactosidase. The downstream part includes the degradation pathways of D-Gal and L-AHG. In addition, the production of functional agaro-oligosaccharides such as neoagarobiose and monosaccharides such as L-AHG with different agarolytic enzymes is reviewed. Third, techniques for the setup, regulation and optimization of agarose degradation to increase utilization efficiency of agarose are summarized. Although heterologous construction of the whole agarose degradation pathway in an engineered strain has not been reported, biotechnologies applied to improve D-Gal utilization efficiency and construct L-AHG catalytic routes are reviewed. Finally, critical aspects that may aid in the construction of engineered microorganisms that can fully utilize agarose to produce agaro-oligosaccharides or as carbon sources for production of biofuels or other value-adding chemicals are discussed.

Biological upgrading of 3,6-anhydro-L-galactose from agarose to a new platform chemical

Recently, the utilization of renewable biomass instead of fossil fuels for producing fuels and chemicals is receiving much attention due to the global climate change. Among renewable biomass, marine algae are gaining importance as third generation biomass feedstocks owing to their advantages over lignocellulose. Particularly, red macroalgae have higher carbohydrate contents and simpler carbohydrate compositions than other marine algae. In red macroalgal carbphydrates, 3,6-anhydro-L-galactose (AHG) is the main sugar composing agarose along with D-galactose. However, AHG is not a common sugar and is chemically unstable. Thus, not only AHG but also red macroalgal biomass itself cannot be efficiently converted or utilized. Here, we biologically upgraded AHG to a new platform chemical, its sugar alcohol form, 3,6-anhydro-l-galactitol (AHGol), an anhydrohexitol. To accomplish this, we devised an integrated process encompassing a chemical hydrolysis process for producing agarobiose (AB) from agarose and a biological process for converting AB to AHGol using metabolically engineered Saccharomyces cerevisiae to efficiently produce AHGol from agarose with high titers and yields. AHGol was also converted to an intermediate chemical for plastics, isosorbide. To our knowledge, this is the first demonstration of upgrading a red macroalgal biomass component to a platform chemical via a new biological route, by using an engineered microorganism.

Recent trends on seaweed fractionation for liquid biofuels production

Concerns about fossil fuels depletion has led to seek for new sources of energy. The use of marine biomass (seaweed) to produce biofuels presents widely recognized advantages over terrestrial biomasses such as higher production ratio, higher photosynthetic efficiency or carbon-neutral emissions. In here, interesting seaweed sources as a whole or as a residue from seaweed processing industries for biofuel production were identified and their diverse composition and availability compiled. In addition, the pretreatments used for seaweed fractionation were thoroughly revised as this step is pivotal in a seaweed biorefinery for integral biomass valorization and for enabling biomass-to-biofuel economic feasibility processes. Traditional and emerging technologies were revised, with particular emphasis on green technologies, relating pretreatment not only with the type of biomass but also with the final target product(s) and yields. Current hurdles of marine biomass-to-biofuel processes were pinpointed and discussed and future perspectives on the development of these processes given.

Implications of agar and agarase in industrial applications of sustainable marine biomass

Agar, a major component of the cell wall of red algae, is an interesting heteropolysaccharide containing an unusual sugar, 3,6-anhydro-L-galactose. It is widely used as a valuable material in various industrial and experimental applications due to its characteristic gelling and stabilizing properties. Agar-derived oligosaccharides or mono-sugars produced by various agarases have become a promising subject for research owing to their unique biological activities, including anti-obesity, anti-diabetic, immunomodulatory, anti-tumor, antioxidant, skin-whitening, skin-moisturizing, anti-fatigue, and anti-cariogenic activities. Agar is also considered as an alternative sustainable source of biomass for chemical feedstock and biofuel production to substitute for the fossil resource. In this review, we summarize various biochemically characterized agarases, which are useful for industrial applications, such as neoagarooligosaccharide or agarooligosaccharide production and saccharification of agar. Additionally, we succinctly discuss various recent studies that have been conducted to investigate the versatile biological activities of agar-derived saccharides and biofuel production from agar biomass. This review provides a basic framework for understanding the importance of agarases and agar-derived saccharides with broad applications in pharmaceutical, cosmetic, food, and bioenergy industries.

Characterization of a Novel α-Neoagarobiose Hydrolase Capable of Preparation of Medium- and Long-Chain Agarooligosaccharides

α-Neoagarobiose hydrolase plays an important role in saccharification processes of marine biomass. In this study, an α-neoagarobiose hydrolase from Streptomyces coelicolor A3(2), designated as ScJC117, was identified, purified, and characterized. It has a sequence of 370 amino acids and belongs to the GH117 family. ScJC117 exhibited good activity under optimal hydrolysis conditions of pH 6.0 and 30°C, where it showed the K_m and k_cat for neoagarobiose of 11.57 mM and 0.48 s^–1, respectively. ScJC117 showed the ability to hydrolyze neoagarooligosaccharides with the polymerization degrees of 2–14. A basis of catalytic activity toward the first α-1,3-glycosidic bond of the neoagarooligosaccharides from the non-reducing end, ScJC117 can be classified as an exo-type α-neoagarobiose hydrolase. These results suggested that ScJC117 could be used in the preparation of odd agarooligosaccharides (especially agaroheptaose-agaroundecaose) and 3,6-anhydro-L-galactose, which has a functional food additive potential. Moreover, ScJC117 can be used for comprehensive utilization of red algae.

Anticariogenic Activity of Agarobiose and Agarooligosaccharides Derived from Red Macroalgae

3,6-Anhydro-l-galactose (AHG) produced from agarose in red macroalgae was recently suggested as an anticariogenic sugar to replace widely used xylitol. However, the multi-step process for obtaining monomeric sugar AHG from agarose may be expensive. Generally, it is easier to obtain oligosaccharides than monosaccharides from polysaccharides. Therefore, a one-step process to obtain agarobiose (AB) from agarose was recently developed, and here, we suggest AB as a new anticariogenic agent, owing to its anticariogenic activity against Streptococcus mutans. Among AHG-containing oligosaccharides, AB, neoagarobiose (NAB), agarooligosaccharides (AOSs), and neoagarooligosaccharides (NAOSs), AB showed higher inhibitory activity than AOSs against the growth and lactic acid production of S. mutans; no such inhibitory activity was observed for NAB and NAOSs. This inhibitory effect of AB was comparable to the previously reported inhibitory activity of AHG against S. mutans. These results suggest that AB, which can be more economically and simply produced than AHG, may serve as an anticariogenic sugar.

Novel Two-Step Process Utilizing a Single Enzyme for the Production of High-Titer 3,6-Anhydro-l-galactose from Agarose Derived from Red Macroalgae

3,6-Anhydro-l-galactose (l-AHG), a major component of agarose derived from red macroalgae, has excellent potential for industrial applications based on its physiological activities such as skin whitening, moisturizing, anticariogenicity, and anti-inflammation. However, l-AHG is not yet commercially available due to the complexity, inefficiency, and high cost of the current processes for producing l-AHG. Currently, l-AHG production depends on a multistep process requiring several enzymes. Here, we designed and tested a novel two-step process for obtaining high-titer l-AHG by using a single enzyme. First, to depolymerize agarose preferentially into agarobiose (AB) at a high titer, the agarose prehydrolysis using phosphoric acid as a catalyst was optimized at a 30.7% (w/v) agarose loading, which is the highest agarose or agar loading reported so far. Then AB produced by the prehydrolysis was hydrolyzed into l-AHG and d-galactose (d-Gal) by using a recently discovered enzyme, Bgl1B. We suggest that this simple and efficient process could be a feasible solution for the commercialization and mass production of l-AHG.

Metagenomics Investigation of Agarlytic Genes and Genomes in Mangrove Sediments in China: A Potential Repertory for Carbohydrate-Active Enzymes

Monosaccharides and oligosaccharides produced by agarose degradation exhibit potential in the fields of bioenergy, medicine, and cosmetics. Mangrove sediments (MGSs) provide a special environment to enrich enzymes for agarose degradation. However, representative investigations of the agarlytic genes in MGSs have been rarely reported. In this study, agarlytic genes in MGSs were researched in detail from the aspects of diversity, abundance, activity, and location through deep metagenomics sequencing. Functional genes in MGSs were usually incomplete but were shown as results, which could cause virtually high number of results in previous studies because multiple fragmented sequences could originate from the same genes. In our work, only complete and nonredundant (CNR) genes were analyzed to avoid virtually high amount of the results. The number of CNR agarlytic genes in our datasets was significantly higher than that in the datasets of previous studies. Twenty-one recombinant agarases with agarose-degrading activity were detected using heterologous expression based on numerous complete open-reading frames, which are rarely obtained in metagenomics sequencing of samples with complex microbial communities, such as MGSs. Aga2, which had the highest crude enzyme activity among the 21 recombinant agarases, was further purified and subjected to enzymatic characterization. With its high agarose-degrading activity, resistance to temperature changes and chemical agents, Aga2 could be a suitable option for industrial production. The agarase ratio with signal peptides to that without signal peptides in our MGS datasets was lower than that of other reported agarases. Six draft genomes, namely, Clusters 1-6, were recovered from the datasets. The taxonomic annotation of these genomes revealed that Clusters 1, 3, 5, and 6 were annotated as Desulfuromonas sp., Treponema sp., Ignavibacteriales spp., and Polyangiaceae spp., respectively. Meanwhile, Clusters 2 and 4 were potential new species. All these genomes were first reported and found to have abilities of degrading various important polysaccharides. The metabolic pathway of agarose in Cluster 4 was also speculated. Our results showed the capacity and activity of agarases in the MGS microbiome, and MGSs exert potential as a repertory for mining not only agarlytic genes but also almost all genes of the carbohydrate-active enzyme family.

Model-Based Complete Enzymatic Production of 3,6-Anhydro-l-galactose from Red Algal Biomass

3,6-Anhydro-l-galactose (l-AHG) is a bioactive constituent of agar polysaccharides. To be used as a cosmetic or pharmaceutical ingredient, l-AHG is more favorably prepared by enzymatic saccharification of agar using a combination of agarolytic enzymes. Determining the optimum enzyme combination from the natural repertoire is a bottleneck for designing an efficient enzymatic-hydrolysis process. We consider all theoretical enzymatic-saccharification routes in the natural agarolytic pathway of a marine bacterium, Saccharophagus degradans 2-40. Among these routes, three representative routes were determined by removing redundant enzymatic reactions. We simulated each l-AHG production route with simple kinetic models and validated the reaction feasibility with an experimental procedure. The optimal enzyme mixture (with 67.3% maximum saccharification yield) was composed of endotype β-agarase, exotype β-agarase, agarooligosaccharolytic β-galactosidase, and α-neoagarobiose hydrolase. This approach will reduce the time and effort needed for developing a coherent enzymatic process to produce l-AHG on a mass scale.

Biochemical characterization of an ulvan lyase from the marine flavobacterium Formosa agariphila KMM 3901T

Carbohydrates are the product of carbon dioxide fixation by algae in the ocean. Their polysaccharides are depolymerized by marine bacteria, with a vast array of carbohydrate-active enzymes. These enzymes are important tools to establish biotechnological processes based on algal biomass. Green tides, which cover coastal areas with huge amounts of algae from the genus Ulva, represent a globally rising problem, but also an opportunity because their biomass could be used in biorefinery processes. One major component of their cell walls is the anionic polysaccharide ulvan for which the enzymatic depolymerization remains largely unknown. Ulvan lyases catalyze the initial depolymerization step of this polysaccharide, but only a few of these enzymes have been described. Here, we report the cloning, overexpression, purification, and detailed biochemical characterization of the endolytic ulvan lyase from Formosa agariphila KMM 3901^T which is a member of the polysaccharide lyase family PL28. The identified biochemical parameters of the ulvan lyase reflect adaptation to the temperate ocean where the bacterium was isolated from a macroalgal surface. The NaCl concentration has a high influence on the turnover number of the enzyme and the affinity to ulvan. Divalent cations were shown to be essential for enzyme activity with Ca²⁺ likely being the native cofactor of the ulvan lyase. This study contributes to the understanding of ulvan lyases, which will be useful for future biorefinery applications of the abundant marine polysaccharide ulvan.

Oxidative demethylation of algal carbohydrates by cytochrome P450 monooxygenases

Sugar O-methylation shields algal polysaccharides against microbial hydrolytic enzymes. Here, we describe cytochrome P450 monooxygenases from marine bacteria that, together with appropriate redox-partner proteins, catalyze the oxidative demethylation of 6-O-methyl-D-galactose, which is an abundant monosaccharide of the algal polysaccharides agarose and porphyran. This previously unknown biological function extends the group of carbohydrate-active enzymes to include the class of cytochrome P450 monooxygenases.

Synergistic effect of the enzyme complexes comprising agarase, carrageenase and neoagarobiose hydrolase on degradation of the red algae

In the practice of converting red algae biomass into biofuel or valuable biomaterials, the critical step is the decomposition process of the agarose to give fermentable monomeric sugars. In this study, we selected three enzymes such as agarase, carrageenase and neoagarobiose hydrolase to inducible the simultaneous hydrolysis of the major substrates such as agar and carrageenan constituting the pretreated red algae, and expressed the chimeric enzymes and formed a complexes through optimization of addition ratio. As a result, hydrolysis by enzyme complexes showed a maximum sugar release of 679 mg L^-1 with 67.9% saccharification yield from G. verrucosa natural substrate. The difference in the reducing sugar by the enzyme complexes was 3.6-fold higher than that of the monomer enzyme (cAgaB yield 188.6 mg L^-1). The synergistic effect of producing sugars from red algae biomass through these enzyme complexes can be a very important biological tools aimed at bioenergy production.

Statistical Optimization of Culture Variables for Enhancing Agarase Production by Dendryphiella arenaria Utilizing Palisada perforata (Rhodophyta) and Enzymatic Saccharification of the Macroalgal Biomass

Agarase is a promising biocatalyst for several industrial applications. Agarase production was evaluated by the marine fungus Dendryphiella arenaria utilizing Palisada perforata as a basal substrate in semi-solid state fermentation. Seaweed biomass, glucose, and sucrose were the most significant parameters affecting agarase production, and their levels were further optimized using Box-Behnken design. The maximum agarase activity was 7.69 U/mL. Agarase showed a degree of thermostability with half-life of 99 min at 40 °C, and declining to 44.72 min at 80 °C. Thermodynamics suggested an important process of protein aggregation during thermal inactivation. Additionally, the enzymatic saccharification of the seaweed biomass using crude agarase was optimized with respect to biomass particle size, solid/liquid ratio, and enzyme loadings. The amount of biosugars obtained after optimization was 26.15 ± 1.43 mg/g. To the best of our knowledge, this is the first report on optimization of agarase in D. arenaria.

Synergistic enzymatic saccharification and fermentation of agar for biohydrogen production

Nowadays, marine biomass is gradually considered as another utilizable material for the sustainable bioenergy development. In the present study, galactose, the main component of agar polysaccharide, was utilized for the biohydrogen production by Enterobacter sp. CN1. The highest hydrogen yield of 303.2mL/g was obtained in the cultivation media containing 5.87g/L of galactose, together with initial pH of 7.3 and incubation temperature of 36°C, after the response surface methodology (RSM) analysis. After the saccharification process by the agarase (AgaXa) and neoagarobiose hydrolase (NH852), the agar hydrolysate obtained was further applied to generate biohydrogen by strain CN1. Under the synergistic enzymatic saccharification and fermentation process, the production of biohydrogen was obtained to be 5047±228mL/L from 50g/L of agar, resulting in 3.86-fold higher than the control without enzymatic pretreatment.

Enzymatic Processes in Marine Biotechnology

In previous review articles the attention of the biocatalytically oriented scientific community towards the marine environment as a source of biocatalysts focused on the habitat-related properties of marine enzymes. Updates have already appeared in the literature, including marine examples of oxidoreductases, hydrolases, transferases, isomerases, ligases, and lyases ready for food and pharmaceutical applications. Here a new approach for searching the literature and presenting a more refined analysis is adopted with respect to previous surveys, centering the attention on the enzymatic process rather than on a single novel activity. Fields of applications are easily individuated: (i) the biorefinery value-chain, where the provision of biomass is one of the most important aspects, with aquaculture as the prominent sector; (ii) the food industry, where the interest in the marine domain is similarly developed to deal with the enzymatic procedures adopted in food manipulation; (iii) the selective and easy extraction/modification of structurally complex marine molecules, where enzymatic treatments are a recognized tool to improve efficiency and selectivity; and (iv) marine biomarkers and derived applications (bioremediation) in pollution monitoring are also included in that these studies could be of high significance for the appreciation of marine bioprocesses.

Fusion of agarase and neoagarobiose hydrolase for mono-sugar production from agar

In enzymatic saccharification of agar, endo- and exo-agarases together with neoagarobiose hydrolase (NABH) are important key enzymes for the sequential hydrolysis reactions. In this study, a bifunctional endo/exo-agarase was fused with NABH for production of mono-sugars (D-galactose and 3,6-anhydro-L-galactose) from agar using only one fusion enzyme. Two fusion enzymes with either bifunctional agarase (Sco3476) or NABH (Zg4663) at the N-terminus, Sco3476-Zg4663 (SZ) and Zg4663-Sco3476 (ZS), were constructed. Both fusion enzymes exhibited their optimal agarase and NABH activities at 40 and 35 °C, respectively. Fusions SZ and ZS enhanced the thermostability of the NABH activity, while only fusion SZ showed a slight enhancement in the NABH catalytic efficiency (K _cat/K _M) from 14.8 (mg/mL)^-1 s^-1 to 15.8 (mg/mL)^-1 s^-1. Saccharification of agar using fusion SZ resulted in 2-fold higher mono-sugar production and 3-fold lower neoagarobiose accumulation when compared to the physical mixture of Sco3476 and Zg4663. Therefore, this fusion has the potential to reduce enzyme production cost, decrease intermediate accumulation, and increase mono-sugar yield in agar saccharification.

3,6-Anhydro-l-galactose, a rare sugar from agar, a new anticariogenic sugar to replace xylitol

The significance for anticariogenic sugar substitutes is growing due to increasing demands for dietary sugars and rising concerns of dental caries. Xylitol is widely used as an anticariogenic sugar substitute, but the inhibitory effects of xylitol on Streptococcus mutans, the main cause of tooth decay, are exhibited only at high concentrations. Here, the inhibitory effects of 3,6-anhydro-l-galactose (AHG), a rare sugar from red macroalgae, were evaluated on S. mutans, in comparison with those of xylitol. In the presence of 5g/l of AHG, the growth of S. mutans was retarded. At 10g/l of AHG, the growth and acid production by S. mutans were completely inhibited. However, in the presence of xylitol, at a much higher concentration (i.e., 40g/l), the growth of S. mutans still occurred. These results suggest that AHG can be used as a new anticariogenic sugar substitute for preventing dental caries.

Enhanced catalytic efficiency of endo-β-agarase I by fusion of carbohydrate-binding modules for agar prehydrolysis

Recently, Microbulbifer thermotolerans JAMB-A94 endo-β-agarase I was expressed as catalytic domain (GH16) without a carbohydrate-binding module (CBM). In this study, we successfully constructed different fusions of GH16 with its original CBM6 and CBM13 derived from Catenovulum agarivorans. The optimum temperature and pH for fusions GH16-CBM6, GH16-CBM13, GH16-CBM6-CBM13 and GH16-CBM13-CBM6 were similar to GH16, at 55°C and pH 7. All the constructed fusions significantly enhanced the GH16 affinity (Km) and the catalytic efficiency (Kcat/Km) toward agar. Among them, GH16-CBM6-CBM13 exhibited the highest agarolytic activity, for which Km decreased from 3.67 to 2.11mg/mL and Kcat/Km increased from 98.6 (mg/mL)^-1sec^-1 to 400.6 (mg/mL)^-1sec^-1. Moreover, all fusions selectively increased GH16 binding ability to agar, in which the highest binding ability of 95% was obtained with fusion GH16-CBM6-CBM13. Melted agar was prehydrolyzed with GH16-CBM6-CBM13, resulting in a degree of liquefaction of 45.3% and reducing sugar yield of 14.2%. Further addition of Saccharophagus degradans agarolytic enzymes resulted in mono-sugar yields of 35.4% for galactose and 31.5% for 3,6-anhydro-l-galactose. There was no pH neutralization step required and no 5-hydroxymethylfurfural detected, suggesting the potential of a new enzymatic prehydrolysis process for efficient production of bio-products such as biofuels.

Efficacy of acidic pretreatment for the saccharification and fermentation of alginate from brown macroalgae

This study was performed to evaluate the effectiveness of acidic pretreatment in increasing the enzymatic digestibility of alginate from brown macroalgae. Pretreatment with 1 % (w/v) sulfuric acid at 120 °C for 30 min produced oligosaccharides, mannuronic acid, and guluronic acid. Enzymatic saccharification of pretreated alginate by alginate lyases produced 52.2 % of the theoretical maximal sugar yield, which was only 7.5 % higher than the sugar yield obtained with unpretreated alginate. Mass spectrometric analyses of products of the two reactions revealed that acidic pretreatment and enzymatic saccharification produced saturated monomers (i.e., mannuronic and guluronic acid) with saturated oligosaccharides and unsaturated monomers (i.e., 4-deoxy-L-erythro-5-hexoseulose uronic acid; DEH), respectively. While DEH is further metabolized by microorganisms, mannuronic acid and guluronic acid are not metabolizable. Because of the poor efficacy in increasing enzymatic digestibility and owing to the formation of non-fermentable saturated monomers, acidic pretreatment cannot be recommended for enzymatic saccharification and fermentation of alginate.

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

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.

Pretreatment and saccharification of red macroalgae to produce fermentable sugars

Red macroalgae are currently considered as renewable resources owing to their high carbohydrate and low lignin and hemicellulose contents. However, utilization of red macroalgae has been limited owing to the lack of established methods for pretreatment and an effective saccharification system. Furthermore, marine red macroalgae consist of the non-favorable mixed sugars for industrial microorganisms. In this review, we suggest strategies for converting red macroalgae to bio-based products, focusing on the pretreatment and saccharification of red macroalgae to produce fermentable sugars and the microbial fermentation of these sugars by industrial microorganisms. In particular, some recent breakthroughs for the efficient utilization of red macroalgae include the discovery of key enzymes for the complete monomerization of red macroalgal carbohydrate and the catabolic pathway of 3,6-anhydro-l-galactose, the most abundant sugar in red macroalgae. This review provides a comprehensive perspective for the efficient utilization of red macroalgae as sustainable resources to produce bio-based products.

Molecular cloning and expression of a new α-neoagarobiose hydrolase from Agarivorans gilvus WH0801 and enzymatic production of 3,6-anhydro-l-galactose

A new α-neoagarobiose hydrolase (NABH) called AgaWH117 was cloned from Agarivorans gilvus WH0801. The gene encoding this hydrolase consists of 1,086 bp and encodes a protein containing 361 amino acids. This new NABH showed 74% amino acid sequence identity with other known NABHs. The molecular mass of the recombinant AgaWH117 was estimated to be 41 kDa. Purified AgaWH117 showed endolytic activity during neoagarobiose degradation, yielding 3,6-anhydro-l-galactose (l-AHG) and d-galactose as products. It showed a maximum activity at a temperature of 30 °C and a pH of 6.0 and was stable at temperatures below 30 °C. Its Km and Vmax values were 2.094 mg/mL and 6.982 U/mg, respectively. The cloning strategy used and AgaWH117 isolated in this study will provide information on the saccharification process of marine biomass. This study provides a method to produce l-AHG from agarose by using AgaWH117 without an acid and describes its one-step purification by using Bio-Gel P2 chromatography.

Draft Genome Sequence of the Nonmarine Agarolytic Bacterium Cellvibrio sp. OA-2007

Cellvibrio sp. OA-2007 is a Gram-negative, aerobic, and agarolytic bacterium isolated from activated sludge. We present the draft genome sequence of strain OA-2007, composed of 97 contigs, totaling 4,595,379 bp in size, and containing 4,094 open reading frames, with a G+C content of 47.71%.