John G. Ormerod

Quantitative and structural characteristics of lipids in Chlorobium and Chloroflexus

The lipid compositions of Chlorobium limicola (4 strains) and Chloroflexus aurantiacus (2 strains) have been compared. Both species contained straight-chain, saturated and monosaturated fatty acids as their main fatty acid constituents but the patterns were distinctly different. Chlorobium contained acids of chain-length essentally in the range C12−C18 with n-tetradecanoate, hexadecenoate and n-hexadecanoate predominating. Chloroflexus was characterized by the presence of significant amounts of C17 and C18−C20 fatty acids not detected in Chlorobium. The latter, on the other hand, contained hydroxylated and cyclopropane-substituted acids not detected in Chloroflexus. Simple wax esters (C28−C38) were found solely in Chloroflexus and accounted for 2.5–3.0% of the cell dry weight. Their fatty acid constituents ranged from C12−C19 (both saturated and monounsaturated isomers) whereas the alcohols were generally saturated and of chain-length C16−C19. Waxes in the range C34−C36 accounted for more than 60% of the total.The polar lipid patterns of the two genera also showed marked differences. All strains contained phosphatidyl-glycerol, monogalactosyl diglyceride and sulfoquinovosyldiglyceride. Chlorobium contained in addition cardiolipin, phosphatidylethanolamine, the unidentified “glycolipid II” and several other unidentified glycolipids, whereas phosphatidyl inositol and a diglycosyl diglyceride were specific for Chloroflexus. The latter lipid contained equimolar amounts of glucose and galactose.Phenol-water extraction yielded material comprising 14% of the dry cell weight for Chlorobium but only 2.5% for Chloroflexus. The Chlorobium material contained two 3-hydroxy fatty acids and several uncommon sugars (not identified). The analytical results were inconclusive regarding occurrence of 2-keto-3-deoxyoctonate. No typical lipopoly-saccharide constituents were found in Chloroflexus.

The bchU Gene of Chlorobium tepidum Encodes the C-20 Methyltransferase in Bacteriochlorophyll c Biosynthesis

Bacteriochlorophylls (BChls) c and d, two of the major light-harvesting pigments in photosynthetic green sulfur bacteria, differ only by the presence of a methyl group at the C-20 methine bridge position in BChl c. A gene potentially encoding the C-20 methyltransferase, bchU, was identified by comparative analysis of the Chlorobium tepidum and Chloroflexus aurantiacus genome sequences. Homologs of this gene were amplified and sequenced from Chlorobium phaeobacteroides strain 1549, Chlorobium vibrioforme strain 8327d, and C. vibrioforme strain 8327c, which produce BChls e, d, and c, respectively. A single nucleotide insertion in the bchU gene of C. vibrioforme strain 8327d was found to cause a premature, in-frame stop codon and thus the formation of a truncated, nonfunctional gene product. The spontaneous mutant of this strain that produces BChl c (strain 8327c) has a second frameshift mutation that restores the correct reading frame in bchU. The bchU gene was inactivated in C. tepidum, a BChl c-producing species, and the resulting mutant produced only BChl d. Growth rate measurements showed that BChl c- and d-producing strains of the same organism (C. tepidum or C. vibrioforme) have similar growth rates at high and intermediate light intensities but that strains producing BChl c grow faster than those with BChl d at low light intensities. Thus, the bchU gene encodes the C-20 methyltransferase for BChl c biosynthesis in Chlorobium species, and methylation at the C-20 position to produce BChl c rather than BChl d confers a significant competitive advantage to green sulfur bacteria living at limiting red and near-infrared light intensities.

Taxonomy, Physiology and Ecology of Heliobacteria

Heliobacteria are anoxygenic phototrophs that contain bacteriochlorophyll g as their sole chlorophyll pigment. These organisms are primarily soil residents and are phylogenetically related to Gram-positive bacteria, in particular to the endospore-forming Bacillus/Clostridium line. Some species of heliobacteria produce heat resistant endospores containing dipicolinic acid and elevated Ca2+ levels. Heliobacteria can grow photoheterotrophically on a limited group of organic substrates and chemotrophically (anaerobically) in darkness by pyruvate or lactate fermentation; they are also active nitrogen-fixers. Their photosynthetic system resembles that of photosystem I of green plants but is simpler, containing a small antenna closely associated with the reaction center located in the cytoplasmic membrane; no chlorosomes typical of the green sulfur bacteria or differentiated internal membranes typical of purple bacteria are found in the heliobacteria. Heliobacteria are apparently widely distributed in rice soils and occasionally found in other soils. The ecology of heliobacteria may be tightly linked to that of rice plants, and the ability of heliobacteria to produce endospores probably has significant survival value in the highly variable habitat of rice soils. The unique assemblage of properties shown by the heliobacteria has necessitated creation of a new taxonomic family of anoxygenic phototrophic bacteria, the Heliobacteriaceae, to accommodate organisms of this type.

Carbon Dioxide-Fixation in Photosynthetic Green Sulfur Bacteria

The main products of carbon dioxide-fixation in washed suspensions of Chlorobium thiosulfatophilum are a polyglucose, α-ketoglutarate, and α-keto-β-methylvalerate. All of these can be formed by a mechanism involving the reductive carboxylic acid cycle. The reductive pentose phosphate cycle appears to play a quantitatively minor role in carbon dioxide-fixation under these conditions.

Glycolipids and the structure of chlorosomes in green bacteria

Abstract1.The cellular content of galactolipids in Chlorobium and Chloroflexus is not related to bacteriochlorophyll content nor to the total amount of chlorosome material in the cells.2.Chlorosomes of both bacteria were agglutinated by Ricinus lectin and the agglutination was increased after treatment of the chlorosomes with trypsin.3.When cell free preparations of both bacteria were treated with trypsin prior to centrifugation on sucrose gradients, the resulting chlorosome fractions were less contaminated with material derived from the cytoplasmic membrane than when trypsin was not employed.

Synthesis, storage and degradation of polyglucose in Chlorobium thiosulfatophilum.

Cultures of Chlorobium thiosulfatophilum form polyglucose during growth. The polyglucose is laid down within the cells as rosette-like granules, which are made up from smaller grains. The size of each granule appears to be limited to less than 30 nm, since an increase in polyglucose content leads to more granules being formed rather than an increase in granule size.The polyglucose in washed cells is fermented in the dark to acetate, propionate, caproate and succinate, of which acetate by far comprises the largest fraction (68%). During incubation of washed cells without hydrogen donor, the level of polyglucose decreases regardless of whether the cells are incubated in the dark or in the light. Since the products formed from polyglucose under the two different conditions are not the same, it is suggested that polyglucose in the dark serves as an energy source, whereas when in the light the role of polyglucose is mainly to provide the cell with reducing power.

Effect of light internsity on vesicle formation in Chlorobium

Abstract1. Chlorobium limicola forma sp. thiosulfatophilum was cultivated at 22 and 22000 lux. 2. The content of bchl d on a protein basis in the low light intensity cultures was about twice that of the high light intensity cultures. 3. After growth at 22 lux the red bchl d peak was at c. 743 nm, while at the higher intensity this peak was at c. 732 nm. 4. Electron microscopy of thin sections of Chlorobium revealed that vesicle size was greater at the low light intensity than at the high. 5. This was confirmed by sucrose density gradient centrifugation of differentially 14C-labelled vesicles from cultures grown at the two intensities. 6. The optimum temperature for growth was about 35°C. Incubation at the optimum temperature was particularly beneficial at high light intensity.

Phototrophic Bacteria that Form Heat Resistant Endospores

The Heliobacteriaceae are a newly discovered family of green anaerobic phototrophs which contain bacteriochlorophyll (Bchl) g (absorption spectrum, Fig. l) and have reaction centres probably similar to those of PS I (1, 2).

Biosynthesis of chlorosome proteins is not inhibited in acetylene-treated cultures of Chlorobium vibrioforme

The composition, abundance and apparent molecular masses of chlorosome polypeptides from Chlorobium tepidum and Chlorobium vibrioforme 8327 were compared. The most abundant, low-molecular-mass chlorosome polypeptides of both strains had similar electrophoretic mobilities and abundances, but several of the larger proteins were different in both apparent mass and abundance. Polyclonal antisera raised against recombinant chlorosome proteins of Cb. tepidum recognized the homologous proteins in Cb. vibrioforme, and a one-to-one correspondence between the chlorosome proteins of the two species was confirmed. As previously shown [Ormerod et al. (1990) J Bacteriol 172: 1352–1360], acetylene strongly suppressed the synthesis of bacteriochlorophyll c in Cb. vibrioforme strain 8327. No correlation was found between the bacteriochlorophyll c content of cells and the cellular content of chlorosome proteins. Nine of ten chlorosome proteins were detected in acetylene-treated cultures, and the chlorosome proteins were generally present in similar amounts in control and acetylene-treated cells. These results suggest that the synthesis of chlorosome proteins and the assembly of the chlorosome envelope is constitutive. It remains possible that the synthesis of bacteriochlorophyll c and its insertion into chlorosomes might be regulated by environmental parameters such as light intensity.

Physiology of the Photosynthetic Prokaryotes

Living organisms grow by synthesizing in an ordered fashion the complex macromolecules of their own cells from simpler molecules. In general, the energy requirements for this can be met either by degrading part of the nutritional substrate for respiration (heterotrophic organisms) or by converting light energy into chemical energy as in the phototrophic organisms. The proportions of these two types of organisms on the earth are difficult to estimate, but their activities balance each other. In the long term both types are dependent on each other for major nutrients—heterotrophs must have the oxygen and organic molecules produced by photosynthesis; the phototrophs depend on the heterotrophs for keeping the oxygen content of the atmosphere at a tolerable level and for carbon dioxide, produced by respiration. The phototrophs also depend on sunlight, which is the driving force for the whole system. The two modes of life, heterotrophy and photototrophy, must have existed side by side on the surface of the earth for thousands of millions of years.