Cloning and characterization of β-agarase AgaYT from Flammeovirga yaeyamensis strain YT

Diversity of Culturable Bacteria from the Coral Reef Areas in the South China Sea and Their Agar-Degrading Abilities

The South China Sea (SCS) is abundant in marine microbial resources with high primary productivity, which is crucial for sustaining the coral reef ecosystem and the carbon cycle. Currently, research on the diversity of culturable bacteria in the SCS is relatively extensive, yet the culturable bacteria in coral reefs has been poorly understood. In this study, we analyzed the bacterial community structure of seawater samples among Daya Bay (Fujian Province), Qionghai (Hainan Province), Xisha Islands, and the southern South China Sea based on culturable methods and detected their abilities for agar degradation. There were 441 bacterial strains, belonging to three phyla, five classes, 43 genera, and 101 species, which were isolated by marine agar 2216E (MA; Becton Dickinson). Strains within Gammaproteobacteria were the dominant group, accounting for 89.6% of the total bacterial isolates. To investigate vibrios, which usually correlated with coral health, 348 isolates were obtained from TCBS agar, and all isolates were identified into three phylum, three classes, 14 orders, 25 families, and 48 genera. Strains belonging to the genus Vibrio had the greatest number (294 strains), indicating the high selectivity of TCBS agar for vibrios. Furthermore, nineteen strains were identified as potentially novel species according to the low 16S rRNA gene similarity (<98.65%), and 28 strains (15 species) had agar-degrading ability. These results indicate a high diversity of culturable bacteria in the SCS and a huge possibility to find novel and agar-degrading species. Our study provides valuable microbial resources to maintain the stability of coral ecosystems and investigate their roles in the marine carbon cycle.

The Second Chromosome Promotes the Adaptation of the Genus Flammeovirga to Complex Environments

Approximately 10% of bacterial strains contain more than one chromosome; however, in contrast to the primary chromosomes, the mechanisms underlying the formation of the second chromosomes and the significance of their existence remain unclear. Species of the genus Flammeovirga are typical polysaccharide-degrading bacteria, and herein, we report complete genome maps of this genus. These genomes all had multireplicons and second chromosomes. The second chromosome, much larger than plasmids and even megaplasmids, had rRNA and a disparity of 1% relative to the main chromosome in guanine-cytosine (GC) content. The largest chromosomes carried core genes for cellular processes, while the second chromosomes were enriched with genes involved in the transport and metabolism of inorganic ions and carbohydrates, particularly genes encoding glycoside hydrolases and polysaccharide lyases, which constituted the genetic basis for the strains’ excellent capabilities to utilize polysaccharides. The second chromosomal evolution had a higher mutation rate than the primary chromosomes. Furthermore, the second chromosomes were also enriched in horizontal transfer genes and duplicated genes. The primary chromosomes were more evolutionarily conserved, while the second chromosomes were more plastic, which might be related to their different roles in the bacterial survival process. This study can be used as an example to explain possible formation mechanisms and functions of the second chromosomes, providing a reference for peer research on the second chromosomes. In particular, the second chromosomes were enriched in polysaccharide-degrading enzymes, which will provide theoretical support for using genomic data to mine tool-type carbohydrase resources.

IMPORTANCE For decades, the typical bacterial genome has been thought to contain a single chromosome and a few small plasmids carrying nonessential genes. However, an increasing number of secondary chromosomes have been identified in various bacteria (e.g., plant symbiotic bacteria and human pathogens). This study reported three complete genomes of the polysaccharide-degrading marine bacterial genus Flammeovirga, revealed that they harbor two chromosomes, and further identified that the presence of a multireplicon system is a characteristic of complete Flammeovirga genomes. These sequences will add to our knowledge on secondary chromosomes, especially within Bacteroidetes. This study indicated that the second chromosomes of the genus Flammeovirga initially originated from an ancestral plasmid and subsequently expanded by gene duplication or by obtaining heterologous genes with functions, thus promoting host strains to adapt to complex living environments (e.g., to degrade more diverse polysaccharides from marine environments). These findings will promote the understanding of the evolution and function of bacteria with multireplicon systems.

Extracellular expression of agarase rAgaM1 in Bacillus subtilis and its ability for neoagaro-oligosaccharide production

An agarase gene (agaM1) was cloned, expressed and characterized by using Escherichia coli as host strain, revealing the outstanding properties of recombinant AgaM1 (rAgaM1) in agarose degradation and neoagaro-oligosaccharides (NAs) production in our previous work. In current study, agaM1 was extracellularly expressed in Bacillus subtilis, and we aim to assess the ability of the supernatant of recombinant B. subtilis fermentation broth containing rAgaM1 to degrade agarose without protein purification, which would save the cost of purification and avoid the activity loss during purification. The pH and temperature optima for the supernatant were 7.0 and 50 °C, respectively. The supernatant containing rAgaM1 has outstanding stability against 40 °C and 50 °C. Besides, we detailedly studied the possible influence factors of rAgaM1 expression in the supernatant, including pH, temperature, isopropyl β-D-thiogalactoside (IPTG) concentration, initial optical density at a wavelength of 600 nm (OD₆₀₀ ), and induction time, and the optimum conditions for rAgaM1 expression by B. subtilis were confirmed. Moreover, the supernatant was able to produce NAs by using the Gracilaria lemaneiformis, whose cells were broken by autoclaving, as substrate, and a total of 1.41 µmol ml^-1 of NA, including neoagarotetraose and neoagarohexaose, was produced after degradation for 48 h. This ability could save the cost of substrates in NA production, although the method requires a further study. Our results reveal that the NAs with great potential in food and pharmaceutical industries could be inexpensive to make by the supernatant containing rAgaM1 of B. subtilis fermentation broth in the foreseeable future.

Recombinant β-agarases: insights into molecular, biochemical, and physiochemical characteristics

Agarases (agarose 4-glycanohydrolase; EC 3.2.1.81) are class of enzymes that belong to glycoside hydrolase (GH) family capable of hydrolyzing agar. Their classification depends on hydrolysis pattern and product formation. Among all the agarases, β-agarases and the oligosaccharides formed by its action have fascinated quite a lot of industries. Ample of β-agarase genes have been endowed from marine sources such as algae, sea water, and marine sediments, and the expression of these genes into suitable host gives rise to recombinant β-agarases. These recombinant β-agarases have wide range of industrial applications due to its improved catalytic efficiency and stability in tough environments with ease of production on large scale. In this review, we have perused different types of recombinant β-agarases in consort with their molecular, physiochemical, and kinetic properties in detail and the significant features of those agarases are spotlighted. From the literature reviewed after 2010, we have found that the recombinant β-agarases belonged to the families GH16, GH39, GH50, GH86, and GH118. Among that, GH39, GH50, and GH86 belonged to clan GH-A, while the GH16 family belonged to clan GH-B. It was observed that GH16 is the largest polyspecific glycoside hydrolase family with ample number of β-agarases and the families GH50 and GH118 were found to be monospecific with only β-agarase activity. And, out of 84 non-catalytic carbohydrate-binding modules (CBMs), only CBM6 and CBM13 were professed in β-agarases. We witnessed a larger heterogeneity in molecular, physiochemical, and catalytic characteristics of the recombinant β-agarases including molecular mass: 32-132 kDa, optimum pH: 4.5-9, optimum temperature 16-60 °C, K _M: 0.68-59.8 mg/ml, and V _max: 0.781-11,400 U/mg. Owing to this extensive range of heterogeneity, they have lion's share in the multibillion dollar enzyme market. This review provides a holistic insight to a few aspects of recombinant β-agarases which can be referred by the upcoming explorers to this area.

Biochemical Characterization and Substrate Degradation Mode of a Novel Exotype β-Agarase from Agarivorans gilvus WH0801

Agarases are important hydrolytic enzymes for the biodegradation of agar. Understanding the degradation mode and hydrolysis products of agarases is essential for their utilization in oligosaccharide preparations. Herein, we cloned and expressed AgWH50B, a novel neoagarotetraose-forming β-agarase from Agarivorans gilvus WH0801 that has high specific activity and a fast reaction rate. AgWH50B consists of a C-terminal glycoside hydrolase family 50 catalytic domain with two tandem noncatalytic carbohydrate-binding modules (CBMs) in the N-terminus (residues 45-214 and 236-442). AgWH50B exhibited good enzymatic properties with high specific activity and catalytic efficiency (1523.2 U/mg and a V_max of 1700 μmol/min/mg) under optimal hydrolysis conditions of pH 7.0 and 40 °C. Analysis of the hydrolysis products revealed that this enzyme is an exotype β-agarase and that the dominant product of agarose or oligosaccharide degradation was neoagarotetraose. These findings suggest that AgWH50B could be utilized to yield abundant neoagarotetraose.

Genome Sequencing Reveals the Complex Polysaccharide-Degrading Ability of Novel Deep-Sea Bacterium Flammeovirga pacifica WPAGA1

Flammeovirga pacifica strain WPAGA1 is a Gram-negative, polysaccharide-degrading bacterium isolated from the marine sediment of the West Pacific Ocean. This strain is a cosmopolitan marine bacterium that uses complex polysaccharides as exclusive source of carbon and energy and plays a key role in the marine carbon cycle. Genome sequence analysis of strain WPAGA1 revealed that the assembled fine genome contains 6,610,326 bp with 32.89% G+C content, 5036 open reading frames (ORFs) and abundant genomic elements. Amongst these ORFs, 1022 genes encoding carbohydrate enzymes were found in the F. pacifica WPAGA1 genome. In addition, abundant putative enzymes involved in degrading polysaccharide were found. These enzymes include amylase, xylosidase, cellulase, alginate lyase, pectate lyase, rhamnogalacturonan lyase, chitinase, carrageenase, heparinase and fucosidase. To further investigate the use of these polysaccharides in strain WPAGA1, a schematic of various polysaccharide-degrading metabolic pathways were deduced from the genome sequence. This study showed that strain WPAGA1 may serve as a potential candidate for research of glycometabolism and have potential biotechnological and industrial applications and play key roles in the marine carbon cycle.

Agarolytic bacterium Persicobacter sp. CCB-QB2 exhibited a diauxic growth involving galactose utilization pathway

The agarolytic bacterium Persicobacter sp. CCB‐QB2 was isolated from seaweed (genus Ulva) collected from a coastal area of Malaysia. Here, we report a high‐quality draft genome sequence for QB2. The Rapid Annotation using Subsystem Technology (RAST) annotation server identified four β‐agarases (PdAgaA, PdAgaB, PdAgaC, and PdAgaD) as well as galK, galE, and phosphoglucomutase, which are related to the Leloir pathway. Interestingly, QB2 exhibited a diauxic growth in the presence of two kinds of nutrients, such as tryptone and agar. In cells grown with agar, the profiles of agarase activity and growth rate were very similar. galK, galE, and phosphoglucomutase genes were highly expressed in the second growth phase of diauxic growth, indicating that QB2 cells use galactose hydrolyzed from agar by its agarases and exhibit nutrient prioritization. This is the first report describing diauxic growth for agarolytic bacteria. QB2 is a potential novel model organism for studying diauxic growth in environmental bacteria.

Expression and Characterization of a Novel Thermostable and pH-Stable β-Agarase from Deep-Sea Bacterium Flammeovirga Sp. OC4

A novel gene (aga4436), encoding a potential agarase of 456 amino acids, was identified in the genome of deep-sea bacterium Flammeovirga sp. OC4. Aga4436 belongs to the glycoside hydrolase 16 β-agarase family. Aga4436 was expressed in Escherichia coli as a fusion protein and purified. Recombinant Aga4436 showed an optimum agarase activity at 50-55 °C and pH 6.5, with a wide active range of temperatures (30-80 °C) and pHs (5.0-10.0). Notably, Aga4436 retained more than 90%, 80%, and 35% of its maximum activity after incubation at 30 °C, 40 °C, and 50 °C for 144 h, respectively, which exhibited an excellent thermostability in medium-high temperatures. Besides, Aga4436 displayed a remarkable tolerance to acid and alkaline environments, as it retained more than 70% of its maximum activity at a wide range of pHs from 3.0 to 10.0 after incubation in tested pHs for 60 min. These desirable properties of Aga4436 could make Aga4436 attractive in the food and nutraceutical industries.

Isolation and Characterization of a Glycosyl Hydrolase Family 16 β-Agarase from a Mangrove Soil Metagenomic Library

A mangrove soil metagenomic library was constructed and a β-agarase gene designated as AgaML was isolated by functional screening. The gene encoded for a 659-amino-acids polypeptide with an estimated molecular mass of 71.6 kDa. The deduced polypeptide sequences of AgaML showed the highest identity of 73% with the glycoside hydrolase family 16 β-agarase from Microbulbifer agarilyticus in the GenBank database. AgaML was cloned and highly expressed in Escherichia coli BL21(DE3). The purified recombinant protein, AgaML, showed optimal activity at 50 °C and pH 7.0. The kinetic parameters of Km and Vmax values toward agarose were 4.6 mg·mL(-1) and 967.5 μM·min(-1)·mg(-1), respectively. AgaML hydrolyzed the β-1,4-glycosidic linkages of agar to generate neoagarotetraose (NA4) and neoagarohexaose (NA6) as the main products. These characteristics suggest that AgaML has potential application in cosmetic, pharmaceuticals and food industries.

Biochemical Characteristics and Substrate Degradation Pattern of a Novel Exo-Type β-Agarase from the Polysaccharide-Degrading Marine Bacterium Flammeovirga sp. Strain MY04

Exo-type agarases release disaccharide units (3,6-anhydro-l-galactopyranose-α-1,3-d-galactose) from the agarose chain and, in combination with endo-type agarases, play important roles in the processive degradation of agarose. Several exo-agarases have been identified. However, their substrate-degrading patterns and corresponding mechanisms are still unclear because of a lack of proper technologies for sugar chain analysis. Herein, we report the novel properties of AgaO, a disaccharide-producing agarase identified from the genus Flammeovirga AgaO is a 705-amino-acid protein that is unique to strain MY04. It shares sequence identities of less than 40% with reported GH50 β-agarases. Recombinant AgaO (rAgaO) yields disaccharides as the sole final product when degrading agarose and associated oligosaccharides. Its smallest substrate is a neoagarotetraose, and its disaccharide/agarose conversion ratio is 0.5. Using fluorescence labeling and two-stage mass spectrometry analysis, we demonstrate that the disaccharide products are neoagarobiose products instead of agarobiose products, as verified by (13)C nuclear magnetic resonance spectrum analysis. Therefore, we provide a useful oligosaccharide sequencing method to determine the patterns of enzyme cleavage of glycosidic bonds. Moreover, AgaO produces neoagarobiose products by gradually cleaving the units from the nonreducing end of fluorescently labeled sugar chains, and so our method represents a novel biochemical visualization of the exolytic pattern of an agarase. Various truncated AgaO proteins lost their disaccharide-producing capabilities, indicating a strict structure-function relationship for the whole enzyme. This study provides insights into the novel catalytic mechanism and enzymatic properties of an exo-type β-agarase for the benefit of potential future applications.

IMPORTANCE

Exo-type agarases can degrade agarose to yield disaccharides almost exclusively, and therefore, they are important tools for disaccharide preparation. However, their enzymatic mechanisms and agarose degradation patterns are still unclear due to the lack of proper technologies for sugar chain analysis. In this study, AgaO was identified as an exo-type agarase of agarose-degrading Flammeovirga bacteria, representing a novel branch of glycoside hydrolase family 50. Using fluorescence labeling, high-performance liquid chromatography, and mass spectrum analysis technologies, we provide a useful oligosaccharide sequencing method to determine the patterns of enzyme cleavage of glycosidic bonds. We also demonstrate that AgaO produces neoagarobiose by gradually cleaving disaccharides from the nonreducing end of fluorescently labeled sugars. This study will benefit future enzyme applications and oligosaccharide studies.

Novel Alginate Lyase (Aly5) from a Polysaccharide-Degrading Marine Bacterium, Flammeovirga sp. Strain MY04: Effects of Module Truncation on Biochemical Characteristics, Alginate Degradation Patterns, and Oligosaccharide-Yielding Properties

Alginate lyases are important tools for oligosaccharide preparation, medical treatment, and energy bioconversion. Numerous alginate lyases have been elucidated. However, relatively little is known about their substrate degradation patterns and product-yielding properties, which is a limit to wider enzymatic applications and further enzyme improvements. Herein, we report the characterization and module truncation of Aly5, the first alginate lyase obtained from the polysaccharide-degrading bacterium Flammeovirga. Aly5 is a 566-amino-acid protein and belongs to a novel branch of the polysaccharide lyase 7 (PL7) superfamily. The protein rAly5 is an endolytic enzyme of alginate and associated oligosaccharides. It prefers guluronate (G) to mannuronate (M). Its smallest substrate is an unsaturated pentasaccharide, and its minimum product is an unsaturated disaccharide. The final alginate digests contain unsaturated oligosaccharides that generally range from disaccharides to heptasaccharides, with the tetrasaccharide fraction constituting the highest mass concentration. The disaccharide products are identified as ΔG units. While interestingly, the tri- and tetrasaccharide fractions each contain higher proportions of ΔG to ΔM ends, the larger final products contain only ΔM ends, which constitute a novel oligosaccharide-yielding property of guluronate lyases. The deletion of the noncatalytic region of Aly5 does not alter its M/G preference but significantly decreases the enzymatic activity and enzyme stability. Notably, the truncated protein accumulates large final oligosaccharide products but yields fewer small final products than Aly5, which are codetermined by its M/G preference to and size enlargement of degradable oligosaccharides. This study provides novel enzymatic properties and catalytic mechanisms of a guluronate lyase for potential uses and improvements.

Characterization of a thermostable and alkali-stable α-amylase from deep-sea bacterium Flammeovirga pacifica

A thermostable α-amylase (designated as Amy16) has been previously identified in Flammeovirga pacifica isolated from deep-sea sediments. The DNA sequence of Amy16 exhibited no significant similarity with those of any known protein, including the glycoside hydrolases. Amino acid sequence analysis revealed that Amy16 belonged to GH13 family and possessed a conserved DXEXD motif, which was essential for its hydrolysis activities. The recombinant Amy16 purified with Ni(+) affinity column after its heterologous expression in Escherichia coli cells was most active at 50 °C and retained more than 81% of its initial activity after incubation at 60 °C for 20 min. The optimal pH for Amy16 was determined to be 6.5, and a good tolerance to alkaline environment was observed. Low concentration of Mg(2+), Sr(2+), Na(+) and K(+) slightly increased the activity of Amy16. Results of thin layer chromatography experiments revealed that Amy16 was able to hydrolyse starch into maltose in a time-dependent manner, suggesting that Amy16 is a liquid-type endoenzyme with starch hydrolysis activities. Therefore, our study presented thermostable and alkali-stable Amy16, which may be suitable for use as an additive in detergents.

Extracellular production of a novel endo-β-agarase AgaA from Pseudomonas vesicularis MA103 that cleaves agarose into neoagarotetraose and neoagarohexaose

The gene agaA, of the isolated marine bacterium Pseudomonas vesicularis MA103, comprised 2958-bp nucleotides encoding a putative agarase AgaA of 985 amino acids, which was predicted to contain a signal peptide of 29 amino acids in the N-terminus, a catalytic domain of glycoside hydrolase 16 (GH16) family, a bacterial immunoglobulin group 2 (Big 2), and three carbohydrate binding modules 6 (CBM 6). The gene agaA was cloned and overexpressed in Escherichia coli, and the optimum temperatures for AgaA overexpression were 16, 20 and 24 °C. The agaA was cloned without its signal peptide for cytosolic production overexpression, whereas it was cloned with the heterologous signal peptide PelB and its endogenous signal peptide for periplasmic and extracellular productions, respectively. Extracellular and periplasmic rAgaA showed greater activity than that of cytosolic rAgaA, indicating that membrane translocation of AgaA may encourage proper protein folding. Time-course hydrolysis of agarose by rAgaA was accomplished and the products were analyzed using thin layer chromatography and matrix-assisted laser desorption inoization-time of flight mass spectrometry, indicating that AgaA from P. vesicularis was an endo-type β-1,4 agarase that cleaved agarose into neoagarotetraose and neoagarohexaose as the final products.

Draft genome sequence of an agar-degrading marine bacterium Flammeovirga pacifica WPAGA1

Flammeovirga pacifica WPAGA1(T), which was isolated from sediment of the west Pacific Ocean in 2009 has the ability to produce agar-oligosaccharides from Gracilaria lemaneiformis directly by enzyme-degradation. The draft genome sequence of this strain was sequenced and annotated. Its draft genome contained 6,507,364 bp with a G+C content of 33.8%. Genome sequence information provided a basis for analyzing the digestion of G. lemaneiformis.

Purification and characterization of a novel β-agarase of Paenibacillus sp. SSG-1 isolated from soil

Agar is a polysaccharide polymer material, generally extracted from seaweed. Most agar degradation strains were isolated from seawater. In order to find new species resources and novel agarase from soil, an agar-degrading bacterium Paenibacillus sp. SSG-1 was isolated from soil. Agarase SSG-1a was purified to homogeneity by 30.2 fold with a yield of 4.8% through ammonium sulfate precipitation, DEAE FF chromatography and native-PAGE separation. The tandem mass spectrometry (MS/MS) results indicated that purified SSG-1a should be a novel β-agarase. The molecular mass of SSG-1a was estimated to be 77 kDa. The optimal temperature and pH for SSG-1a were 50°C and pH 6.0, respectively. Moreover, SSG-1a was stable in pH range of 4.0-10.0 and at temperature up to 40°C. It could hydrolyze the β-1,4 linkage of agarose to produce neoagarohexaose (95 mol%) and neoagarooctaose (5 mol%). Metal ion Mn(2+) and reducing reagents (β-Me and DTT) could increase its activity by 150% and 60%, respectively.

Microorganisms living on macroalgae: diversity, interactions, and biotechnological applications

Marine microorganisms play key roles in every marine ecological process, hence the growing interest in studying their populations and functions. Microbial communities on algae remain underexplored, however, despite their huge biodiversity and the fact that they differ markedly from those living freely in seawater. The study of this microbiota and of its relationships with algal hosts should provide crucial information for ecological investigations on algae and aquatic ecosystems. Furthermore, because these microorganisms interact with algae in multiple, complex ways, they constitute an interesting source of novel bioactive compounds with biotechnological potential, such as dehalogenases, antimicrobials, and alga-specific polysaccharidases (e.g., agarases, carrageenases, and alginate lyases). Here, to demonstrate the huge potential of alga-associated organisms and their metabolites in developing future biotechnological applications, we first describe the immense diversity and density of these microbial biofilms. We further describe their complex interactions with algae, leading to the production of specific bioactive compounds and hydrolytic enzymes of biotechnological interest. We end with a glance at their potential use in medical and industrial applications.

An extra peptide within the catalytic module of a β-agarase affects the agarose degradation pattern

Agarase hydrolyzes agarose into a series of oligosaccharides with repeating disaccharide units. The glycoside hydrolase (GH) module of agarase is known to be responsible for its catalytic activity. However, variations in the composition of the GH module and its effects on enzymatic functions have been minimally elucidated. The agaG4 gene, cloned from the genome of the agarolytic Flammeovirga strain MY04, encodes a 503-amino acid protein, AgaG4. Compared with elucidated agarases, AgaG4 contains an extra peptide (Asn(246)-Gly(302)) within its GH module. Heterologously expressed AgaG4 (recombinant AgaG4; rAgaG4) was determined to be an endo-type β-agarase. The protein degraded agarose into neoagarotetraose and neoagarohexaose at a final molar ratio of 1.5:1. Neoagarooctaose was the smallest substrate for rAgaG4, whereas neoagarotetraose was the minimal degradation product. Removing the extra fragment from the GH module led to the inability of the mutant (rAgaG4-T57) to degrade neoagarooctaose, and the final degradation products of agarose by the truncated protein were neoagarotetraose, neoagarohexaose, and neoagarooctaose at a final molar ratio of 2.7:2.8:1. The optimal temperature for agarose degradation also decreased to 40 °C for this mutant. Bioinformatic analysis suggested that tyrosine 276 within the extra fragment was a candidate active site residue for the enzymatic activity. Site-swapping experiments of Tyr(276) to 19 various other amino acids demonstrated that the characteristics of this residue were crucial for the AgaG4 degradation of agarose and the cleavage pattern of substrate.

Agar degradation by microorganisms and agar-degrading enzymes

Agar is a mixture of heterogeneous galactans, mainly composed of 3,6-anhydro-L-galactoses (or L-galactose-6-sulfates) D-galactoses and L-galactoses (routinely in the forms of 3,6-anhydro-L-galactoses or L-galactose-6-sulfates) alternately linked by β-(1,4) and α-(1,3) linkages. It is a major component of the cell walls of red algae and has been used in a variety of laboratory and industrial applications, owing to its jellifying properties. Many microorganisms that can hydrolyze and metabolize agar as a carbon and energy source have been identified in seawater and marine sediments. Agarolytic microorganisms commonly produce agarases, which catalyze the hydrolysis of agar. Numerous agarases have been identified in microorganisms of various genera. They are classified according to their cleavage pattern into three types-α-agarase, β-agarase, and β-porphyranase. Although, in a broad sense, many other agarases are involved in complete hydrolysis of agar, most of those identified are β-agarases. In this article we review agarolytic microorganisms and their agar-hydrolyzing systems, covering β-agarases as well as α-agarases, α-neoagarobiose hydrolases, and β-porphyranases, with emphasis on the recent discoveries. We also present an overview of the biochemical and structural characteristics of the various types of agarases. Further, we summarize and compare the agar-hydrolyzing systems of two specific microorganisms: Gram-negative Saccharophagus degradans 2-40 and Gram-positive Streptomyces coelicolor A3(2). We conclude with a brief discussion of the importance of agarases and their possible future application in producing oligosaccharides with various nutraceutical activities and in sustainably generating stock chemicals for biorefinement and bioenergy.