Fatty Acids Analysis of Photosynthetic Sulfur Bacteria by Gas Chromatography

The fatty acid profile of bacteria has been extensively studied for taxonomic classification purposes, as bacteria in general contain particular and rare fatty acids, compared with animal and plant tissues (Purcaro et al., 2010). Lipids and fatty acid composition are quite useful for characterization of different species and groups of photosynthetic bacteria. Fatty acids can be identified by gas chromatography using standard methods (Imhoff & Caumette, 2004) because physical and chemical factors (e.g., illumination) influence bacterial fatty acid composition, culture conditions, and the culture media used for bacterial growth. Fatty acid analysis as a chemotaxonomic tool has been applied in heterotrophic bacteria, but its use in photosynthetic sulfur bacteria it is very limited. Actually, most studies on photosynthetic purple and green sulfur bacteria fatty acid composition have been done with relatively few genera and have often given information of the major fatty acids.


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Microbial fatty acids can be completely saturated or contain several double bonds, although the presence of one or two double bonds is the most common. The site of these double bonds indicates their biosynthesis pathway. According to Scheuerbrandt & Bloch (1962), the aerobic route for the synthesis of unsaturated acids is the same for all living organisms, but the anaerobic pathway is for bacteria. These authors suggested that adopting the oxidative pathway to synthesize unsaturated fatty acids throughout evolution allowed living organisms to abandon the anaerobic route, which is the most "primitive" and yet, perhaps more complex The linear-chain fatty acids of bacteria are usually of the same class as those found in eukaryotic organisms, being part of membrane lipids with lengths that vary between 8 and 26 carbon atoms depending on the bacterial species. Bacterial monounsaturated fatty acids are primarily of type 7 (unsaturation is located on carbon 7 from the methyl terminal), although type 9, which is characteristic of higher organisms, can also be found.
In polyunsaturated fatty acids, the double bond is formed by dehydrogenation of one carbon-carbon bond, and is generally located in the center of the molecule (7 or 9). Both the length of the fatty acid chain and the incorporation of double bonds (Abbas & Card, 1980) participate in membrane fluidity. Hence the synthesis of longer-chain fatty acids causes the membrane to remain constant (Melchior, 1982cited in Jantzen & Bryan, 1985. Polyunsaturated fatty acids are common in mycobacteria containing phenolic acids of 36-38 carbons. They have also been observed in marine bacteria, such as Shewanella gelidimarina, Sh. hanedai, Sh. putrefaciens, Colwellia psychrerythraea, Flexibacter polymorphus (Russell & Fukunaga, 1990), and in isolates of the Antarctic, in which these chemical compounds have chains of 18 to 20 carbon atoms .
Dienoic fatty acids and others with a larger number of double bonds are more frequently found in eukaryotic organisms, but they have also been detected in some heterotrophic bacteria and cyanobacteria. The latter produce linoleic acid, which has also been found by Rabinowitch et al. (1993) in Escherichia coli, grown in both aerobic and anaerobic conditions. The C18:2 (octadecadienoic) polyunsaturated fatty acid is produced by Aerobacter aerogenes and Pseudomonas sp., in which it contributes 18.2% and 24%, respectively, to the total of their fatty acids, and in Serratia marcescens, contributing only 2.6-5.8% of the total extracted (Cho & Salton, 1966). Although the linolenic fatty acid, or C18:3 (6, 9, 12), is a rare fatty acid outside the animal kingdom, it is commonly found in the oils of some seeds. Initially it was described in mycelial lipids of Phycomyces blakesleeanus (Lechevalier & Lechevalier, 1988), and together with linolenic, or C18:3 (9, 12, 15), is part of the characteristic fatty acids of some cyanobacteria. The eicosapentaenoic acid (C20:53) is produced by several strains of Shewanella in percentages from 2 to 40% of the total fatty acids (Yazawa, 1996;Brown et al., 1997;Nichols et al., 1997). Yano et al. (1994) found docosahexaenoic acid (C22:63 or DHA) in a strain pertaining to the Vibrio genus and in the bacterial strain SCRC-21406 isolated from the gut of the deep-sea fish Glossanodon semifasciatus. Watanabe et al. (1997) reported the presence of this fatty acid in a proportion of 23%, and extremely small amounts (less than 3%) of other polyunsaturated fatty acids bound to their main phospholipids.
Bacterial membranes of fatty acids consist of hydrophobic parts of glycerolipids and of lipid A. The hydrophobic parts exist in both Gram-positive and Gram-negative bacteria, whereas lipid A is only present in Gram-negative bacteria. This lipid usually contains several 3-OH fatty acid chains that can be used as a marker for detecting Gram-negative bacteria (Li et al., 2010).
Hydroxy fatty acids that carry an OH group, either in position two or three (2-OH-fatty acid or 3-OH-fatty acid), despite the fact that they are found in animal cells as D isomers, are frequently found in Gram-negative bacteria. They are generally a component of lipid A in their lipopolysaccharide, and, in some cases, they are also part of amino acids bound to lipids (Lechevalier & Moss, 1977). The presence of lipopolysaccharide in Gram-negative bacteria gives rise to the presence of hydroxy fatty acids; thus the presence of 10:0 3OH, 12:0 3OH, and/or 14:0 3OH fatty acids indicates that the organism is Gram-negative bacteria. Conversely, the absence of the lipopolysaccharide and hydroxy fatty acids indicates that the organism is Gram-positive (Kunistky et al., 2006).
Cyclopropane fatty acids are formed by the addition of a methyl group from Sadenosylmethionine in the double bond of a pre-existing fatty acid (Cronan et al., 1979;Grogan & Cronan Jr., 1997). In general, these fatty acids are part of the phospholipids of many bacterial species, and they have been related to the aging process of cells because they are produced in the stationary stage during their culture (Grogan & Cronan Jr., 1997). Because of their instability, Jantzen & Bryan (1985) recommend that, for their quantification, it is convenient to consider that their monounsaturated precursors are C16:1 for Cyc17:0, and C18:1 for Cyc19:0.
Fatty acids with alkyl branched chains are found in several bacterial groups. Methyl branches can be located in the last position (iso) or in the next-to-last (anti-iso), and, like unsaturated fatty acids their function is to regulate membrane fluidity (Jantzen & Bryan, 1985).
In Gram-negative bacteria, the most frequent saturated fatty acids are C16:0, C18:0, and C14:0; the most important unsaturated fatty acids are C16:1 cis or trans 9 and the C18:1 cis or trans 11, hydroxyl acids, which are part of their lipopolysaccharides (Janse, 1997). Grampositive bacteria are characterized by the presence of important amounts of branched fatty acids; Coryneforms and Actynomicetes contain tubeculostearic and mycolic acids, respectively, and are exclusive to both genera (Ratledge & Wilkinson, 1988).
It is important to mention that although branched fatty acids are characteristic of Grampositive bacteria, some Gram-negative can present them, such as Cytophaga, Flavobacterium, and Desulfobacter. It has been observed that they can make up almost 99% of the total fatty acid content in Flavobacterium thermophilum.
According to Janse (1997), in Gram-negative bacteria, the most frequent saturated fatty acids are C16:0, C18:0, and C14:0; the most common unsaturated are C16:1 cis or trans 9 and C18:1 cis or trans 11. The hydroxyl acids, which are part of lipid A that contains several 3-OH fatty acid chains, can be used as markers for detecting Gram-negative bacteria (Li et al., 2010). This fact has been reported only in Mch. indicum and analyzed using the MIS system by Sucharita et al. (2008).
It is evident that information available on fatty acids of purple photosynthetic sulfur bacteria (Chromatiaceae family) has serious limitations, but it is also clear that their main cellular components are C16:0, C16:1 and C18:1, as has been reported by Imhoff (2008). Table 3 includes information on species of Ectothiorhodospira, Halorhodospira, and Thiorhodospira genera, the last two have been analyzed very recently, and fatty acids with chains between 14 and 22 carbon atoms have been observed. Fatty acid C20:1 was only detected in Ec. shaposhnikovii and Halorhodospira halophila (DSM 244) and C22:0 in Hal. halophila BN 9626. This information reveals that major fatty acids for the Ectothiorhospiraceae family are C18:1 and C16:0 except for Thiorhodospira sibirica which has more C16:1 than C16:0.

Fatty acids in Chlorobiaceae family
Regarding the fatty acid composition of green sulfur bacteria pertaining to the Chlorobiaceae family, strains of the Chlorobium and Pelodyction genera have been analyzed (Table 4). It is important to note that Pelodyction was reclassified, and actually is a member of the Chlorobium genus (Imhoff & Thiel, 2010).

Fatty acids as a chemotaxonomic tool in purple and green sulfur bacteria
With the development of techniques and equipment to analyze the chemical properties of living organisms, new tools arose that markedly helped choose those properties that would enable the distinction, separation, and, finally, the classification of microorganisms (Stanier, 1968), providing data for more in-depth studies on their phylogenetic relations.
One of the first authors to propose the use of the chemical characteristics of microorganisms for taxonomic purposes was Wolochow (1959 cited in Abel et al., 1963). He suggested that microorganisms could be differentiated from "higher" organisms based on their chemical properties, as these properties are unique, providing a better theoretical basis for their classification. This concept was later amplified by Abel et al. (1963), who pointed out the importance of qualitative and quantitative analyses of several selected compounds, which would lead to a method for differentiation among species. These authors suggested that the chemical composition of living organisms could be regulated by natural or evolutionary relations and that this information could be obtained whenever culturing bacteria under defined conditions.
Based on previous studies performed by Scheuerbrandt & Bloch (1962) using liquid gas chromatography, the anaerobic and aerobic routes for the biosynthesis of fatty acids were proposed, enabling the distinction between prokaryotic and eukaryotic microorganisms to be made. It was set forth that no microorganisms can use both routes at the same time and that the presence of polyunsaturated fatty acids is an evolutionary event. This idea was later revised by Erwin & Bloch (1964), who stated that the composition of lipid molecules is determined not only genetically but also by the conditions of the environment; hence, factors such as temperature, nutritional and culture conditions (aerobic, anaerobic, etc.) can modify the lipidic pattern of living organisms.
From the start of bacterial classification it was recognized that structural characteristics by themselves were not enough for successful classification (Cohn 1872, cited in Stanier, 1968. Variations in the composition of fatty acids of bacterial cells have been used for their identification since the introduction of gas chromatography more than 50 years ago by Abel et al. (1963). Both the qualitative and quantitative composition was useful in differentiating microorganisms (Sasser, 2001). In bacteria, these cellular components have the advantage of not being present in their plasmids (Boom & Cronan Jr., 1989).
According to De Gelder (2008), chemotaxonomic techniques provide phenotypic properties, one of which is the fatty acid methyl ester (FAME) analysis that determines the fatty acid composition of the membrane and outer membrane of cells. Using this technique, fatty acids are extracted from cell hydrolysates and derivatized to volatile methyl esters which can be detected by gas chromatography. FAME allows grouping of many organisms according to the similarity of their fatty acid pattern, which is useful for identification. Quantitative data on bacterial fatty acid composition have allowed for the distinction and separation of families, genera, and even species.
In heterotrophic bacteria, the fatty acid composition has been widely studied for chemotaxonomic purposes. However, for purple and green photosynthetic sulfur bacteria, the use of this taxonomic tool is limited; most of the available data focus on major, aside from having been performed in limited species. Some of the data have been included in the description of new species, dealing mostly with the major fatty acids and with no attention given to the minor ones, despite that the latter could provide more information to separate the genera, and perhaps their species, providing more power to the use of this chemotaxonomic tool.
Currently published data on photosynthetic bacteria fatty acid composition are incomplete; data do not mention aspects like age of the bacterial culture and incubation conditions (temperature, light, etc). They neither specify the analytical techniques such as the extraction method, the total area of chromatogram integration, size of the samples, etc. nor how data were handled (normalization or standardization of data, etc.).
These deficiencies have made it quite difficult to compare information between diverse laboratories; consequently, the use of this tool becomes limited for chemotaxonomic classification and for its possible application to the phylogeny study of these ancestral photosynthetic microorganisms.
The advancement of knowledge on fatty acids in photosynthetic bacteria seems to have slowed down in the last decade, and with some exceptions, species descriptions do not include their fatty acid pattern. This is due to the fact that it is stated in the standard recommendations for the description of new species of anoxygenic photosynthetic bacteria, proposed by Imhoff & Caumette (2004), which were established in accordance with 30b of the International Code of Nomenclature of Bacteria and the support of the International Committee on Systematics of Prokaryotes, that fatty acid analysis is optional for the identification of photosynthetic bacteria.
In this respect, Tindall et al. (2010) presented a series of tests that must be used to determine t h e l o c a t i o n o f a s t r a i n a s a m e m b e r o f a n e x i s t i n g t a x o n , a n d t h e y p r o v i d e t h e methodologies and how those methodologies should be used and implemented; among these is the chemical characterization of cells with consideration of fatty acid composition for this objective. As a general recommendation, they point out the need to study the chemotaxonomic features of the most closed taxa for their comparison, particularly when new genera are being proposed.
With the aim of achieving reproducible profiles, Janse (1997) proposed a series of elements not only to apply a determined technique, but also to analyze the generated information. These are as follows: 1) use of cells of the same age, 2) a standard culture medium for both aerobic and anaerobic bacteria, 3) a gas chromatograph with a flame ionization detector (FID), an integrator, and a computer with a printer, 4) fatty acid solutions of known composition (standard), and 5) a statistical program for cluster analysis and analysis of its main components. Aside from the aforementioned, De Gelder (2010) points out the need to count with an extensive FAME database. With the Microbial Identification System (MIS), the techniques and methods for the analysis and identification of fatty acids, and their statistical analysis are well standardized and automated for heterotrophic bacteria. This system enables researchers to have, in a short time, accurate information on the composition and distribution of these chemical compounds in different microorganisms. By means of MIS, more than 300 fatty acids and related compounds (aldehydes, hydrocarbons, dimethyl acetyls) have been identified (Sasser, 2001).

The MIS system (MIDi Co) has been used for fatty acid analysis of non-photosynthetic bacteria and the description of species as in Rheinheimera soli (Ryu et al., 2008) and
Alcalilimnicola halodurans (Yakimov et al., 2001), which are phylogenetically related to the Chromatiaceae and Ectothiorhodospiraceae families. Additionally, fatty acid profiles of purple non-sulfur bacteria have been obtained with the MIS system, an example being Rhodovulum phaeolacus (Lakshmi et al., 2011).
In photosynthetic purple sulfur bacteria, MIS has been applied to fatty acid analysis of different species, for example in different strains of Marichromatium (Sucharita et al., 2010) and Ectothiorhodospira (Bryantseva et al., 2010) genera. Nevertheless, one of the limitations of the MIS system is that it can only detect fatty acids with fewer than 20 carbons, and according to Tindall et al. (2010), the MIS system provides a comprehensive, but incomplete, database with some discrepancies that need to be clarified or compounds that are currently not included in the database.
Due to the variability in fatty acid analysis, the use of chromatographic techniques as taxonomic tools requires standardization of the culture conditions, the physiological age of cells, and the analysis techniques to obtain reproducible results. The need for a standard technique for fatty acid analysis of sulfur photosynthetic bacteria is real, and a technique for this purpose is included at the present chapter. For the Isolation of wild strains of phototrophic purple sulfur bacteria, water samples from Tampamachoco lagoon (Veracruz, México) were collected using a 5-L van Dorn sampler. Test tubes containing Chromatiaceae and Chlorobiaceae specific culture media (Pfennig & Trüper, 1981;Nuñez-Cardona et al., 2008)  30.0 mL sodium bicarbonate solution (5% distilled water), 6.0 mL Na 2 S . 9H 2 O (6% distilled water), and distilled water (ca. 950 mL) up to 1 L, and adjusted to pH 7.3. For Chlorobium species, the same medium was used but Na 2 S . 9H 2 O was increased to 10.0 mL; solution SL10 and pH 6.8 were used. Test tubes were kept at room temperature under constant illumination by 60-W incandescent and fluorescent light bulbs placed at 20-25 cm, were used for growth of Chromatiaceae and Chlorobiaceae, respectively (van Niel, 1971). Agar shake technique on semisolid medium sealed with oil paraffin was used to isolate and purify photosynthetic purple and green sulfur bacteria (Pfennig & Trüper, 1981). Pure cultures were verified by optical microscope observations. Table 5 contains the list of the cultures and collection strains used for fatty acid analysis, general characteristics (main pigments and cell forms), and optimal salinity requirements for each one. At the present document, all the bacteria strains are cited as it is in the original papers or as they were handled. This is because constantly photosynthetic bacteria are reclassified and their names also change.

Culture conditions
All strains were grown anaerobically by triplicate in 1500-mL bottles with metal screw caps and autoclavable rubber seals containing the specific culture medium described before; pH between 7.0-7.5 for Chromatiacea and 6.8 for green sulfur bacteria. Liquid cultures of purple sulfur bacteria were illuminated constantly by incandescent light bulbs (2000 lux) and green sulfur bacteria were exposed to fluorescent light bulbs. Incubation temperature was 23 ºC except for the thermophilic strain Chr. tepidum (43 ºC). Enough biomass of each culture was attained with a sodium sulfide solution neutralized with hydrochloric acid, added almost daily when sulfur was not present in cultures the latter was detected by the use of lead acetate paper (Merck).
After 15 days of incubation and without sulfur in the liquid cultures, the cells were concentrated by centrifugation (5000-10,000 rpm at 5 °C ). Cultures of Amoebobacter and Thiocapsa were centrifuged at 10,000 rpm since it was ratter difficult to concentrate the cells.
Once the cultures were concentrated, the cells were frozen at -20 °C, for further lyophilization.

Sampling conditions
Temperature, age of the cells, as well as the composition of the medium, affect cells fatty acid composition and limits the comparison of the results among different laboratories and the application of these cellular components as a chemotaxonomic tool. Few reports deal with the importance of the size and sample conditions (dry or wet) for fatty acids analysis. It has been proposed that for heterotrophic bacteria, less than 40.0 mg of wet weight were enough (Sasser, 2001). It has also been indicated that 1.0 mg of dry weight (Brondz & Olzen, 1986) or a single colony could be enough for fatty acids analysis (Buyer, 2002). In the case of phototrophic sulfur bacteria, Tacks and Holt (1971) used 10-15 g of wet weight of Thiocapsa floridiana (Thiocapsa roseopersicina).
At the present work, and for the sake of reproducibility it was used cells of the same age (Janse, 1997;Sasser, 2001); grown in standard conditions (as previously described) and fatty

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Fatty Acids Analysis of Photosynthetic Sulfur Bacteria by Gas Chromatography 129 acid analyses were done weighing 500 mg and 250 mg of dry cells, for purple and green photosynthetic sulfur bacteria, respectively.

Fatty acid analysis
For fatty acid analysis, fat present in the freeze-dried cells (three different samples of the same strain) was saponified, followed by diazomethane derivatization (Sigma N-nitrosomethylurea), and evaporation with nitrogen gas. Identification of fatty acids was accomplished by comparison of the retention times with standard mixtures of fatty acid methyl esters. Fatty acids final identification was done by gas chromatography and mass spectrometry (Muñoz et al., 1996;Núñez-Cardona et al., 2008). Details of these techniques are described below.

Extraction of lipids
Each sample was treated with 2.0 mL benzene and 8.0 mL of KOH solution (5.0 g/100 mL methanol) and containing 5% (w/v) potassium hydroxide in a screw-cap test tube (14X120 mm) fitted with a Teflon-lined cap and heated in a cover bath at 80 °C during 4 hours. The samples were cooled at room temperature and were acidified with a solution of H 2 SO 4 (20% v/v) to pH 1, this was verified with pH paper test. Diethyl ether was added to extract lipids and it was added water to wash the extracts until neutral and the clean separated ether was evaporated using a nitrogen gas atmosphere.

Preparation of methyl esters
The lipids were methylated using diazomethane, each sample was prepared with 2.0 g N-Nitroso-N-methylurea (Sigma) dissolved in a precooled solution containing 30.0 mL of diethyl ether, 2.0 g KOH and 6.0 mL distilled water. The mixture was stirred for 5 to 10 minutes, it was removed the supernatant and placed in a new tube cooled in ice containing potassium hydroxide pellets. This solution must be conserved to -80 °C and handled in a hood because diazomethane is very toxic. At the end, 10 ml of diazomethane was added into each tube with the dried lipids and methylation was achieved within 10-20 minutes; after this time, the content was evaporated at 40 °C in a water bath.

Fatty acid methyl ester analysis by gas chromatography
N-hexane (0.04-1.0 mL) was added to the dried extracts and 2.0 L were injected in a gas chromatograph supported by a silica capillary column (15m x 0.25 mm I.D.) with crosslinked methyl silicone (HP-1, Hewlett-Packard) as stationary phase. The column was inserted in an HP-5890A gas chromatograph equipped with a flame ionization detector. The column was programmed at 175 to 300 ºC for 15 minutes. Injector and detector temperatures were 275 and 300 ºC, respectively. The carrier gas was helium with a flow rate of approximately 1.0 mL/min and the split ratio was approximately 1:50. The chromatograms were integrated by using a HP3396 Series II Integrator. Identification of fatty acids was accomplished by comparison of the retention times with standard mixtures of fatty acid methyl esters.

Gas chromatography-mass spectrometry analysis
Fatty acids were subjected to gas chromatography-mass spectrometry, using an HP-5890 gas chromatograph attached to a HP5989X quadripole mass spectrometer with a dimethylpolysiloxane column TRB1 (30 m). Injector and detector temperatures were 225 ºC, and two ramps were used, one from 10 ºC/min to 240 ºC and the other one from 40 ºC/min to 270 ºC. The injection mode was splitless, ca. 1:50.

Reproducibility of fatty acid pattern
With the aim of reaching reproducible data, the integration area of the fatty acid peaks was fixed to 1800 as minimal area and the extract of each sample was injected as many times to get a total area between 14 × 10 6 and 18 × 10 6 for purple sulfur and 7 × 10 6 for green photosynthetic sulfur bacteria.
To assess the reproducibility of the fatty acids pattern, the coefficient of variation (CV) was calculated; for this, it was used the peaks of each fatty acid generated in the chromatograms of the three independent samples of each strain. The standard deviation (of each peak that corresponds to one fatty acid) was calculated and divided by the mean and multiplied by 100, this was made following the recommendations of Bounsfield et al. (1983) and Rainey et al. (1994).

Results and discussion
With the technique applied for fatty acid analysis, it was possible to determine at least 90-99% of the total fatty acid composition from purple sulfur bacteria of the Chromatiaceae family. Data are the results of three different chromatograms from three different cultures of the same strain. The variation coefficient calculated for each fatty acid was less than 10%; therefore, the results here presented have a high degree of reproducibility. Major and minor fatty acids for photosynthetic sulfur bacteria are given in Table 6.  Table 6. Fatty acid composition of purple sulfur bacteria (Chromatiaceae) identified by Gas chromatography-Mass spectrometry. NI=no identified, X(%)=average (%).
Results showed the presence of saturated and unsaturated fatty acids with chains from 12 to C20:1 long; C13:0 as well as aC17:0 were almost present only in phototrophic purple sulfur bacteria with spherical shapes, such as DSM 4197, DSM 5811, DSM 6210, and isolates T1f6, T11s and T11rosa. These results agree with those of Núñez-Cardona et al. (2008), who found the same fatty acids in Amoebobacter sp. (Bk18) and Amoebobacter roseus (DSM 235), which are also spherical. C13:0 fatty acid accounted for less than 1% (0.42-0.66) and aC17:0 for between 0.11 and 2.93% of the fatty acids present. Tindall et al. (2010) recommend that all components constituting 1% or more of the fatty acids must be reported, but according to these results, the identification of C13:0 and aC17:0 are specific components of two genera (Amoebobacter and Thiocapsa). The importance of reporting minor fatty acids for taxonomic purposes is evident even when the fatty acids are present in percentages below 1% because minor fatty acids point out the differences among genera and probably among species too.
Imhoff & Bias Imhoff (1996) expressed that photosynthetic purple sulfur bacteria have equilibrated proportions of C16:0, C16:1 and C18:1, pointing out C18:1 as the most abundant, but not as a major fatty acid. Table 6 shows that, in the conditions described above, C18:1 could account for more than 50% of the total fatty acids from strains like Chr. salexigens DSM 4395, Tc. halophila DSM 6210 and some Chromatium spp. from marine origin (e.g., T9s64, T9s62, T9s642, T9s69). The order of abundance of fatty acids from purple sulfur bacteria is as follows: C18:0, C16:1, and C16:0. With exception of Chr. tepidum DSM 3771, Chr. salexigens DSM 4395 and Tc. halophila DSM 6210 (extremophilic microorganisms) that present some variations in response to their own specific requirements for growth within the systems they inhabit.
With the exception of the data reported by Imhoff and Thiemann (1991) for Chr. purpuratum BN5500, Ec. halophila, and Ec. mobilis, there is no available information on the influence of salinity on fatty acid composition in the Chromatiaceae family. Nevertheless, Tc. halophila DSM 6210, as in Chr. salexigens DSM 4395, grown in salt concentrations of 7 and 10%, respectively, showed the same behavior as Chr. purpuratum and Ectothiorohospira species; i.e., when there are higher salt concentrations in the culture media, these bacteria produce more saturated fatty acids, especially C16:0 and C18:0, and there is a consequent decrease of C18:1 (Imhoff & Thiemann, 1991).
Fatty acid C20:1 was detected in almost all strains of the Chromatiaceae family here analyzed; in general, it was not higher than 1% (0.3-0.83%) but in the thermophilic Chr. tepidum, C20:1 accounts for 2.44% of the total fatty acids. This cellular component was also identified in phospholipids of Thiocapsa spp. (5811 and BF6400) by Macarrón (1998) and in intact cells of Chr. purpuratum BN 5500 (Imhoff & Bias-Imhoff, 1995). It is important to mention that C20:1 was not recorded by Sucharita et al. (2010), using the MIS system, perhaps because MIS is designed to identify fatty acids among 9-20 carbons in length (Sasser, 2001), is not possible to detect the presence of this fatty acid with this system. It is also evident that the results of these authors showed the presence of main fatty acids in four strains of Marichromatium, but only in Mch. indicum, they detailed minor and major fatty acids.
The differences recorded here in the fatty acid compositions of purple sulfur bacteria are in agreement with molecular studies as have been reported by Imhoff & Caumette (2004). Analysis of 16S rDNA revealed genetic differences among marine, halophilic, and the freshwater photosynthetic bacteria. These results led to the taxonomic reclassification and description of new genera of purple photosynthetic sulfur bacteria (Imhoff et al., 1998).
Tree strains of green sulfur bacteria were used to validate the standardized technique for fatty acid analysis of photosynthetic sulfur bacteria and to know the fatty acid profile of another photosynthetic group; results of this analysis are shown in  Table 7. Fatty acid detected in three strains of green sulfur bacteria.
As indicated before, for green sulfur bacteria only 250 mg of dried cells was used because of the difficulties to clean the samples. As with purple sulfur bacteria, each green sulfur bacterium was assayed using independent cultures in triplicate. Invariably, after fatty acid extraction and analysis by gas chromatography, the results showed the presence of 12 fatty acids with chains between 12 and 18 carbon atoms in all Chlorobium strains assayed. According to these results, a fatty acid profile for these strains could be C16:1>C16:0 and C14:0. It is also clear that fatty acid composition contains minor components including C12:0, 12-Me-C14:0 (aC15:0), C15:0, C14-Me-C16:0(aC17:0), 15-Me-C16:1, C17:0, C18:1, C18:0, and a cyclic fatty acid (Cyc17:0) The two hydroxy-acids (C14:0 3OH and C16:0 3:OH) were not detected as reported by Knudsen et al. (1982). According to Boon et al. (1997), hydroxyl acids are part of the cellular wall of Gram-negative bacteria and bounded to lipid A and cannot be extracted with soft organic compounds; a special procedure for analysis and extraction is necessary. Nevertheless, in the Marichromatium indicum strain (Sucharita et al., 2010) it was possible to identify hydroxyl fatty acids using the MIS system Unbranched fatty acids are characteristic in Gram-positive bacteria; although they have been detected in the Clorobium strains analyzed here. The fatty acids aC15:0 and an aC17:0 were detected previously linked to phospholipids in both Chlorobium limicola and Cb. phaeobacteroides in quantities less than 0.5% (Macarrón, 1998). Additionally, C15:0 was identified as a fatty acid component of the profile of Chlorobium sp. by Frederickson et al. (1986). 15-Me-C16:1 was the only fatty acid not reported before for green sulfur bacteria.

Conclusions
Information on fatty acids of photosynthetic purple and green sulfur bacteria is scarce, and the studies of these cellular components are limited to a very low number of collection strains; there is no detailed information about the techniques applied for fatty acid analysis and the genera most studied being Chlorobium, Chromatium and Ectothiorhodospira. With the technique assayed in members of the Chromatiaceae and Chlorobiaceae families it was possible to detect 12 fatty acids both major and minor components. There is clear evidence that extremophilic strains conserve the same fatty acid profile, but there are quantitative differences. The profile of fatty acids in sulfur and green photosynthetic sulfur bacteria is consistent among the strains analyzed, and almost all the fatty acids recorded in these bacteria were previously reported. Fatty acid analysis using standard conditions offers a valuable tool to utilize this information for chemotaxonomic purposes and minor components must be taken into account. Also, it is necessary intensify the study of fatty acids in photosynthetic purple and green sulfur bacteria.