Global warming and the limitations of using fossil fuels are a main concern of all societies, and thus, the development of alternative fuel sources is crucial to improving the current global energy situation. Biofuels are known as the best alternatives of unrenewable fuels and justify increasing extensive research to develop new and less expensive methods for their production. The most frequent biofuels are bioethanol, biobutanol, biodiesel, and biogas. The production of these biofuels is the result of microbial activity on organic substrates like sugars, starch, oil crops, non-food biomasses, and agricultural and animal wastes. Several industrial production processes are carried out in the presence of high concentrations of NaCl and therefore, researchers have focused on halophiles for biofuel production. In this review, we focus on the role of halophilic microorganisms and their current utilization in the production of all types of biofuels. Also, the outstanding potential of them and their hydrolytic enzymes in the hydrolysis of different kind of biomasses and the production of biofuels are discussed.
Introduction
During the past years, the decrease in the availability of fossil fuels and environmental troubles have become major global problems, which has increased the focus and demand for alternative, environmentally friendly, and renewable energy resources. Biofuels are considered one of the best substitutes for fossil fuels and consequently, there is a growing interest in converting biomass to biofuel.
Some biofuels are produced directly from available food resources, like sugar, starch, and oil or from available crops, like sugar cane, corn, beets, wheat, sorghum, rapeseed, sunflower, soybean, palm, coconut, and Jatropha, which are recognized as first-generation biofuels, while second-generation biofuels are those which have been produced from raw materials with a difficult hydrolysis process, like lignocellulosic materials. Utilization of the second-generation biofuels from non-food biomasses is more attractive because, during the process, biofuel production would not compete with food production while it reduces some of the environmental issues such as land and water and also economic costs such as energy consumption. However, the first-generation biofuels still do have their own importance.
Biofuels are available in five types, including bioethanol, biobutanol, biogas, hydrogen, and biodiesel. Biodiesel and bioethanol are the main biofuels that are produced at industrial scales and >90% of total biofuel market is dedicated to them. Their success in this market is determined by various prerequisites defined by both chemical and physical properties. There are several chemical and thermo-chemical processes available for biofuel production but the biological conversion of biomass to biofuel by microorganisms is more cost-effective and has received great and extensive attention over the last years. During the biofuel synthesis process, harsh conditions like the increase in pH and salt concentration occur which make the environment at conditions similar to alkaline and saline environments. Thus, microorganisms thriving under these conditions can possibly be used for biomass breakdown and biofuel production.
Halophiles can be found in hypersaline environments which are widely distributed in various geographical areas on Earth, such as saline lakes, salt pans, salt marshes, or saline soils. These microorganisms can be found in all three domains of life including Archaea, Bacteria, and Eukarya, and are distinguished by their requirement of high salinity conditions for growth. They may be classified according to the amount of their salt (NaCl) requirement: slight, moderate, and extreme halophiles which grow optimally at 0.2–0.85 M (1–5%), 0.85–3.4 M (5–20%), and 3.4–5.1 M (20–30%) of NaCl, respectively. In contrast, non halophilic microorganism grow optimally at <0.2 M (1%) NaCl concentrations. Halotolerant microorganisms are those that can grow in the presence and absence of high concentrations of salt. Halophilic and halotolerant microorganisms can grow over a broad range of salt concentrations, while requirement or tolerance for salts sometimes depends on environmental and nutritional factors; hence, making them the best choice for industrial processes especially for biofuel production. Furthermore, enzymes produced by halophilic microorganisms have an optimal function under very high salt concentrations, like KCl concentrations of ∼4 M or NaCl concentrations higher than 5 M. These enzymes have some extra amino acids which provide an extensive negative charge on the surfaces of the enzymES. Thus, the enzymatic effect is enhanced during biofuel production. In this review, we discuss the different types of biofuels and the role of halophilic microorganisms in their production. Moreover, we also discuss the potential of halophiles in biofuel production.
Bioethanol/Biobutanol
Among all biofuels, bioethanol is frequently recognized as the most promising substitute and/or additive to gasoline which is why scientists have paid more attention to it. Production of bioethanol via different enzymes from different biomasses is more eco-friendly and more popular than other processes. Lignocellulosic biomass or other plant biomasses are renewable materials, comprising mainly cellulose, hemicelluloses, lignin, and starch. Production of bioethanol from biomass consists of four major steps, including biomass pretreatment, enzymatic hydrolysis, fermentation, and distillation.
As cellulose, hemicelluloses and lignin are found in the rigid parts of plants and are highly resistant to biodegradation. To carry out fermentation reactions, it is necessary to pretreat this biomass at a high temperature or under extreme pH conditions. Alkali pretreatment, for example with lime, has been used to treat wheat straw, poplar wood, switch-grass, and corn stover. Lime can be substituted by alkaline salts during pretreatment. The resulting pH and salt concentration makes the surrounding environment similar to alkaline–saline lakes. Hydrolyzed biomass generates reduced sugars, which are then converted to ethanol by microbial action. Because of this, halophiles and their enzymes could play critical roles in the mentioned stages. There are some reports about sugar fermentation and direct production of ethanol and butanol by halophiles. For example, reported that the moderately halophilic bacterium, Nesterenkonia sp. strain F, isolated from Aran-Bidgol hypersaline lake in Iran, has the ability to produce butanol, and ethanol as well as acetone under aerobic and anaerobic conditions. This was the first report of butanol and ethanol production by a wild microorganism which does not belong to the class Clostridia. Also Nesterenkonia sp. strain F was the first halophilic strain shown to produce butanol under aerobic cultivation. Cultivation of Nesterenkonia sp. strain F under anaerobic conditions with 50 g/L of glucose for 72 h resulted in the production of 105 mg/L of butanol. Under aerobic conditions, through fermentation with 50 g/L initial glucose concentration, 66 mg/L of butanol and 291 mg/L of ethanol were produced. As shown in this study, the natural formation of butanol, which has been considered as exclusive to the Clostridia class, was also observed in Nesterenkonia sp. strain F, a halophile bacterium of the family Micrococcaceae of the order Actinomycetales under both, aerobic and anaerobic conditions. In another study, a marine yeast, which was identified as Candida sp. was isolated, characterized, and utilized for bioethanol production using Kappaphycus alvarezii, red algal biomass. In this report, first, the ability and efficiency of the isolated marine yeast to grow and ferment sugar to ethanol in the presence of 2.5–15% salt concentration was validated by fermenting galactose in the presence of different salts at various concentrations. They showed that the yeast produced 1.23% of the ethanol from undiluted hydrolysate, with 5.5% of reduced sugar content and 11.25% of salt concentration after 72 h of incubation, which represents 50% of conversion efficiency. However, in the case of 3:1 diluted hydrolysate having 3.77% reduced sugar and 9.0% of salt content, 1.76% ethanol was obtained after 48 h, with 100% conversion efficiency. Similarly, in the case of 2:1 as well as 1:1 dilution, 100% conversion was observed within 48 h. Although this yeast had grown in the presence of 13% salt, its fermentation efficiency was relatively low at 11.25% salt concentration, which revealed the inhibitory effect of high salt content on fermentation. However, in the presence of 6.25–11.25% salt, conversion of sugar to ethanol was 100%.
There are also shreds of evidence that show halophilic microorganisms could be used as non-food feedstocks for bioethanol production. For example, reported the direct conversion of the halophilic filamentous cyanobacterium Arthrospira platensis to ethanol without pretreatment or enzymatic hydrolysis processes. They indicated that A. platensis is a remarkable carbohydrate feedstock in the form of glycogen, which is a promising material for the production of bioethanol and other various commercially valuable chemicals. Prior to their study, ethanol was successfully produced at high yields (1.08 g/L per day) from non-pretreated cyanobacterial cells without adding any amylases, using an amylase-expressing strain of Saccharomyces cerevisiae and lysozyme. The total ethanol yield based on glycogen consumption was 86%, which is the highest yield of bioethanol from an oxygenic photosynthetic microorganism.
In addition, there are other feedstocks in which halophilic microorganisms could play roles in their hydrolysis process where the resulting products could be fermented to produce biofuels, especially bioethanol. Therefore, in the following sections, we described some of the important and frequent biomasses that could be used for bioethanol production.
Starch
Starch is the reserved form of carbohydrate in plants and therefore is one of the most frequent biomasses on earth as it is highly produced by plants annually. In industrial applications, starch is hydrolyzed to produce glucose, glucose syrups, and high-fructose corn syrups. The resulting glucose could be fermented further to bioethanol. Among all the industries which proceed by using enzymes, the starch industry consumes 15–20% of the total amount, and α-amylase, β-amylase, glucoamylase, and glucose isomerase are the main enzymes in this industrY.
α-Amylase
α-Amylase (also known as endo-1, 4-α-D-glucan glucanohydrolase EC 3.2.1.1) cleaves α-1,4 linkages between adjacent glucose units of starch and produces glucose, maltose, and maltotriose to form linear amylose chains. As mentioned above, the produced glucose could be a substrate for bioethanol production. The presence of this extracellular endoenzyme has been reported in several halophile microorganisms which are summarized in. This enzyme has been found in various groups of halophilic microorganisms, including mainly bacteria, archaea, fungi , marine bacteria, and Actinobacteria. The molecular weight of these α-amylase enzymes varies from 30 to 140 kDa. Most of them can work properly in the presence of high concentrations of salt while some of them are active in a broad range of temperature and pH values. Among them, the α-amylase from Nesterenkonia sp. strain F was interesting. Three of the amylase enzymes produced by this strain have been purified with molecular masses of 57, 100 and 110 kDa. These enzymes had a maximum activity at pH 6.5–7.5 and 40°C and in a broad range of NaCl concentrations (0–4 M), with optimal activity at 0.25 M NaCl. One of these amylases had the ability to metabolize starch, which makes it very important in Biotechnology. The activity of the enzymes was not hindered by Ca2+, Rb+, Li+, Cs+, Mg2+, and Hg2+, whereas Fe3+, Cu2+, Zn2+, and Al3+ strongly inhibited the enzyme activity. This α-amylase was inhibited by EDTA, though PMFS and β-mercaptoethanol had no inhibitory effect. This enzyme was also the first and only reported to increase the microbial α-amylase activity in the presence of organic solventS. Furthermore, as mentioned above, Nesterenkonia sp. strain F also has the ability to produce ethanol and butanol directly from glucose. This suggests that with some modifications ethanol and butanol could be produced from starch only by utilizing this strain.
β-Amylase
β-Amylase is an exoenzyme that hydrolyzes starch by removing maltose from the no reducing end of the starch. Unlike α-amylase, β-amylases are very rare. This enzyme is secreted by several species of the genus Bacillus, including B. polymyxa, B. cereus, and B. megaterium, and by Clostridium thermosulfuroge. Up to now, the presence of β-amylase has been reported only in two halophile bacteria. These two β-amylases are from two moderately halophilic bacteria, Halobacillus sp. strain LY9 and Salimicrobium halophilum strain LY20. These enzymes showed activity under high temperatures and pH values and the optimal activity of both enzymes was at 1.7 M NaCl which revealed the potential of these β-amylases in industrial processes.
Biodiesel
During the past decades, biodiesel was regarded as an important alternative energy source because of the suitable properties and environmental benefits of it, and also because it is derived from biological resources. According to the catalysts engaged in the process, either chemical or enzymatic methods could produce biodiesel while the enzymatic process using lipases is more effective than the chemical methods. Today, >95% of biodiesel is produced from edible oils, such as soybean oil, palm oil, and rapeseed oil, which may lead to the global imbalance of food supply and also may increase the cost of biodiesel production. Thus, biodiesel production from non-edible oils is a more logical approach. These non-edible oils include Jatropha oil and oils from halophilic microalgae. In the following sections, we have discussed the role of halophilic biomasses in biodiesel production; then, we review the role of halophilic microorganisms with lipase activity in biodiesel production.
Halophiles as Biomass
Microalgae
Microalgae are known as the largest primary biomass that could be a safe and clean source of energy production in order to decrease global warming and environmental pollutioN. Their high lipid content, in some cases up to 80% of their weight, high efficiency, fast growth, bio fixation of waste CO2, contribution to greenhouse preventing effects, and the possibility of being cultured on inappropriate farmlands, led to an increasing research interest on their use for biodiesel synthesis. Dunaliella salina is a halophilic green microalga found in saline environments such as saline lakes, salt ponds, and marine waters. D. salina produces high amounts of carotenoids which makes it a good source of food and antioxidant agents. Besides, it plays an important role in biodiesel production. Because of its high lipid content, especially linoleic and palmitic acids, Dunaliella is recognized as a good feedstock for biodiesel production. These fatty acids from Dunaliella would further get methylated to produce biodieseL. In a recent study, the ability of 21 halophilic microalgae, isolated from the hypersaline Bardawil lagoon, was evaluated in order to induce lipid production. Among all the isolates, a green microalga Tetraselmis elliptica, having the high lipid production capacity and predominant fatty acid contents of palmitic acid (C16:0) and oleic acid (C18:1n-9), was suggested as a potential source for biodiesel productION.
On the other hand, halophilic microalgae Dunaliella sp. has been employed as a favorable feedstock for bioethanol production. The data indicated that the acidic pretreatment of the microalgal biomass of Dunaliella sp. using diluted sulfuric acid (1%) enhanced the bioethanol production level up to 7.26 g/L, which was 10.7 times higher than the level obtained from untreated biomass. Several studies have shown the effect of different chemical factors on growth and lipid accumulation in microalgae. Among them, salinity is one of the most important factors and microalgae cells are directed toward energy storage, particularly lipid synthesis rather than an active growth under salt stress. examined the growth and lipid synthesis of marine microalgae, Nannochloropsis salina along with deleterious algae opponents within an open culture system at different percentages of salinity. They observed that the highest algal growth and biomass occurred at the salinities of 22 and 34 PSU, while the minimum density of harmful opponent organisms was achieved at 22 PSU. In order to determine whether lipid synthesis reaches the maximum level under salinity stress, N. salina was cultivated at a concentration of 22 PSU, allowing the cells to reach the stationary growth phase and then increased the salinity to 34, 46, and 58 PSU. Interestingly, they found that lipids accumulate at higher salinities and with a maximum at 34 PSU (36% dry mass).
While it is still unclear how microalgae cells develop their selective response to salt stress on cellular and molecular levels, what matters the most now is to find a convenient and reliable approach for enhancing lipid production in marine microalgae after exposure to salt stress. Some simple examples of such adaptive responses of microalgae cells may include the changes in their morphology, physiology, and biochemical processes, as they occur only with salt-induced stress. It has been revealed that hypersaline cyanobacterial species accumulate glycine betaine as the compatible solute for osmoregulation. In contrast, freshwater cyanobacteria such as Synechococcus species can develop an adaptive strategy following an increase in external salt concentration, arising the soluble sugars accumulation. The osmolyte glycine betaine is highly consistent in cytoplasmic activities, enabling the cells to preserve membrane elasticity against denaturation by Na+ and other antagonistic ions in high osmolality conditions. Efforts have also been made to generate a genetically engineered freshwater cyanobacterium, Synechococcus sp. PCC 7942, with glycine betaine synthesis ability, allowing it to grow faster under high saline conditions compared to the untransformed cells. As for another example, overexpression of BetT protein, known as the betaine transporter from a halotolerant alkaliphilic cyanobacterium Aphanothece halophytica, in Synechococcus cells resulted in NaCl-activated betaine uptake activities with enhanced salt tolerance. Previously, it has been shown that salt-stress tolerance in A. halophytica, as a halotolerant alkaliphilic cyanobacterium, could be related to the existence of an additional Na+-dependent F1F0-ATPase in the cytoplasmic membrane have recently reported that the halophilic microalgae A. halophytica can be a promising feedstock for bioenergy generation. This cyanobacterium with adaptive plasticity in response to various environmental conditions could grow maximally at 60 ppt salinity, 0.05 g L–1 (N), 0.5 g L–1 (P), and 0.5 g L–1 (K). The favorable and general properties of this strain, like the high capacity to produce lipids with a low free fatty acid content, are the most potent and promising reasons for it be considered as an alternative way of clean energy production.
Lipase From Halophiles
As a ubiquitous hydrolytic enzyme, lipases (EC 3.1.1.3) have several applications in biotechnological and industrial fields, especially in biodiesel production. These enzymes catalyze the reverse reactions in non-aqueous solvent systems and along with the oil–water interface, they hydrolyze triglycerides into glycerol and fatty acids. Bacteria and fungi are the main producers of lipases in the industry. Lipases from halophilic microorganisms have their own valuable characteristics. These enzymes could work properly in the harsh conditions in most industrial processes. Thus, screening for novel lipases from halophiles may be a proper approach. So far, only two lipases from halophiles have ever been used for biodiesel production. One of these lipases was purified from the halophilic bacterium Idiomarina sp. W33. The molecular mass of this organic solvent-tolerant extracellular lipase was about 67 kDa. A substrate specificity test indicated that this enzyme preferentially hydrolyzes the long-chained p-nitrophenyl esters. The lipase from strain W33 had optimal activity at 60°C, pH 7.0–9.0, and 10% NaCl and could remain stable over a broad range of temperatures (30–90°C), pH values (7.0–11.0), and NaCl concentrations (0–25%). This lipase was thermostable, alkali-stable, and halotolerant. Diethyl pyrocarbonate and phenylarsine oxide inhibited the enzyme activity. Therefore, histidine and cysteine residues are important in the active site of it. In the presence of hydrophobic organic solvents, this lipase exhibited high stability and activity with log Pow ≥ 2.13. Lipase from strain W33 also was used for biodiesel production from Jatropha oil. The yield of the free and immobilized form of lipase was 84 and 91%, respectively. Usually, free lipases have low biodiesel production because they get aggregated in low water media and have caused mass transfer problems. Biodiesel synthesis from immobilized lipases might be due to their larger surface area. Another lipase from halophiles was purified from Haloarcula sp. G41. This haloarchaeal strain was isolated from the saline soil of Yuncheng Salt Lake, China. The molecular mass of the purified lipase was 45 kDa where the salinity of the medium strongly affected the production of lipase. Maximum production of lipase was achieved in the presence of 20% NaCl or 15% Na2SO4. It preferred long-chained p-nitrophenyl esters. The lipase from the strain G41 showed thermostable, alkali-stable, and halostable properties and also high activity and stability over a broad range of temperature (30–80°C), pH (6.0–11.0), and NaCl concentration (10–25%), with an optimum at 70°C, pH 8.0, and 15% NaCl. Like the lipase from Idiomarina sp. W33, this lipase is a metalloenzyme where serine and cysteine residues are essential for its function. In the presence of hydrophobic organic solvents, this enzyme showed high stability and activity with log Pow ≥ 2.73. Application of this lipase was assayed in biodiesel production. Free and immobilized forms of the lipase from the strain G41 reached in yield 80.5 and 89.2%, respectivelY. In addition to these lipases, several other lipase enzymes from halophilic microorganisms were isolated and characterized. Finally, several screenings have been carried out in order to isolate new halophiles with lipase activity.
Biogas
Hydrogen
Among different energy sources, hydrogen (H2) has attracted great attention. Because of its easy conversion to electricity and clean combust, biological ways of hydrogen production are the new interest of many scientists. The favorite biological producers of H2 are the photosynthetic microorganisms (green algae, cyanobacteria, and photosynthetic bacteria) and non-photosynthetic bacteria (nitrogen-fixing bacteria and anaerobic bacteria). Several studies have reported the production of H2 by halophilic photosynthetic and non-photosynthetic bacteria. In the case of photosynthetic bacteria, showed that a community of halophilic bacteria originated from night soil treatment sludge, strongly produced H2 from raw starch in the light and in the presence of 3% NaCl. The effective H2 producing strains of the community were Vibrio fluvialis, Rhodobium marinum, and Proteus vulgaris. The levels of H2 produced from starch by co-culture of V. fluvialis and R. marinum were nearly equal to the bacterial community, indicating the major role of these two halophile bacteria on H2 production from starch. Further observations have demonstrated that in pure culture, V. fluvialis produced acetic acid and ethanol from the degradation of starch and it appeared like that the strain R. marinum used this material for H2 production in bacterial communities or co-cultures. However, the pure culture of R. marinum in a synthetic medium containing acetic acid and ethanol couldn’t produce H2, suggesting that V. fluvialis supplied both substrates and some unknown factors for H2 production by R. marinum. Furthermore, these co-cultures were used for H2 production from two microalgae, Chlamydomonas reinhardtii and Dunaliella tertiolecta. The results showed a high yield of H2 production from these starch-rich biomasses. The advantage of the photosynthetic bacteria in H2 production is their low cost as they are able to produce it in the presence of the light from non-food biomasses or agricultural wastes. This study has shown the role of halophiles in this low-cost procESS. In another study, a new species of the family Vibrionaceae, Vibrio sp. showed the highest hydrogen yield (with 90% efficacy) at the highest NaCl concentrations (7.5%) under dark conditions. This hydrogen production occurred in a microbial community under moderate salinity conditions, suggesting new possibilities of technological development for treating saline effluents and producing biohydrogen. Vibrio tritonius strain AM2 is the other halophilic strain with the ability to produce hydrogen from glucose and mannitol and powdered brown macroalgae containing 31.1% dry weight of mannitol. This strain was isolated from the gut of a marine invertebrate and yielded 1.7 mol H2/mol mannitol at pH 6 and 37°C. Compared to glucose, mannitol might be a better substrate for bioH2 production using strain AM2, which showed its ability to produce hydrogen from non-food feedstocks. Fermentation product profiling showed that this strain might be utilizing the formate-hydrogen pathway for hydrogen production. In a study, the production of biohydrogen by soil bacteria was observed under high salt concentrations (26% NaCl). This finding is important as it indicates that H2 producing bacteria can be found in hypersaline environments. The requirements to Cl– ions were also observed in these bacteria.
Glycerol and Biohydrogen
In hypersaline environments, glycerol plays an important role. Some halophiles accumulate organic solutes like glycerol in their cytoplasm to overcome the pressure of high salt concentration in their surroundings. For example, the green algae of the genus Dunaliella are the main producers of glycerol in hypersaline environments worldwide. Some member of the order Halanaerobiales has the ability to metabolize glycerol and produce important productS. In the biodiesel industry, glycerol is produced as a by-product which often contains inhibitory factors for microorganisms such as heavy metals and salts. The advantage of the halophilic bacteria, in this case, is that contamination like this is not a major problem for them, because they have been reported to be heavy metal resistant and high salt concentrations cause no problem for their growth. On the other hand, growing halophiles in high salt concentrations could lower the sterilization costs in the glycerol production process as several non-halophilic microorganisms cannot live under such high salt conditions.
Glycerol-based hydrogen production by halophilic bacteria has been reported from Halanaerobium saccharolyticum subspecies saccharolyticum and senegalensis. These anaerobic, Gram-negative strains were isolated from the sediments of hypersaline lakes and belong to the order Halanaerobia. Hydrogen, carbon dioxide, and acetate were the main metabolites of glycerol fermentation of both strains. The highest hydrogen yields were achieved with 2.5 g/L glycerol and 150 g/L salt at pH 7–7.4. To improve the hydrogen yield of H. saccharolyticum subsp. saccharolyticum, the genome of this strain has been sequenced. Following the genome sequence analysis, the glycerol fermentation pathways of this bacterium were reconstructed. This reconstruction revealed that the putative fermentation products were hydrogen, carbon dioxide, acetate, butyrate, butanol, ethanol, lactate, malate, and 1,3-propanediol (a vitamin B12-dependent route) and four [FeFe]-hydrogenases, two of them putative bifurcating hydrogenases requiring both reduced ferredoxin and NADH, were identified. The putative bifurcating hydrogenases are suggested to be involved in the high-yielded H2 production. Furthermore, the genes for a multidrug efflux pump (Acr type), β-lactamase, mercuric reductase, a copper-translocating ATPase, and a cobalt–zinc–cadmium-resistant protein were identified which means that H. saccharolyticum should be resistant to a wide variety of antibiotics and toxic compounds, including heavy metals.
Methane
Methane (CH4) biogas is assumed as a renewable fuel for generating heat and electricity. A diverse community of microbes (mainly bacteria and methanogens) could convert the biomasses like livestock manure, crop residues, food wastes, food-processing wastes, municipal sludge, and municipal solid wastes to methane via anaerobic digestion. In the absence of oxygen, microorganisms hydrolyze the polymers of biomasses and the resulting hydrolysis products further get fermented to short chain fatty acids (SCFA), H2, and CO2, then archaeal methanogens ultimately convert them to methane biogas (a mixture of CH4 and CO2). Marine macroalgae can be used as biomass for the production of biomethane. But the disadvantage is that marine macroalgae contain salts which leads to inhibition of microbial activity in macroalgae-based methane production. By diluting the salinity, some studies have reported the production of methane. Albeit, it seems that the production of methane under non-diluted conditions is more advantageous. At high salt concentrations, the production of methane is stronger and lower amounts of water are needeD. Using this method, have reported a series of experiments in which they modified the production of methane from brown algae by the methanogenic microbial community from marine sediments under high salinity . In one of their studies, the predominant bacterial and archaeal strains with methane production activity belonged to the family Fusobacteriaceae and the genus Methanosaeta, respectively .
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