Beverly K. Pierson

A phototrophic gliding filamentous bacterium of hot springs, Chloroflexus aurantiacus, gen. and sp. nov.

Chloroflexus aurantiacus, gen. and sp. n., is a filamentous phototrophic bacterium of hot springs. On an agar surface, holotype strain J-10-fl glides at 0.01–0.04 μm/sec. The filaments are 0.6–0.7 μm in width and indeterminate in length. Pigments include bacteriochlorophyll c and bacteriochlorophyll a (identified by spectrophotometry) in addition to β and γ-carotene and glycosides of the latter. Chlorobium vesicles are present. Photoheterotrophic growth occurs under anaerobic conditions. Aerobic chemoheterotrophic growth also occurs in darkness or light. Bacteriochlorophyll syntheses cease under aerobic conditions but some types of carotenoids continue to be made. The filament coloration is orange under all except anaerobic conditions in low light intensity where it is dull green. The pH optimum is near 8, the temperature optimum between 52° and 60°C. The DNA base composition for strain J-10-fl is 54.9 ± 1.0 moles % guanine + cytosine. Chloroflexus is unique in that there have been no previous reports of filamentous or gliding phototrophic bacteria. The combinations of bacteriochlorophylls a and c and the presence of chlorobium vesicles in a photoheterotroph and in an organism capable of aerobic growth are also unique. This metabolically versatile organism extends the taxonomic and phylogenetic limits of the “green line” of phototrophic bacteria.

Studies of pigments and growth in Chloroflexus aurantiacus, a phototrophic filamentous bacterium

Abstract1.The chlorophyll pigments of Chloroflexus aurantiacus were separated by column chromatography on powdered sugar and were identified by spectrophotometry in various solvents as BChl a and BChl c (chlorobium chlorophyll 660). The bacteriopheophytins were also prepared and characterized-spectrophotometrically. The identity of the BChl a is tentative because of its anomalous phase test behavior and because of changes in its absorption spectrum observed under different conditions of preparation.2.Growth rates of Chloroflexus at 55°C and synthesis of the 2 chlorophylls were compared in cells growing under anaerobic conditions at different light intensities. Growth rates increased with increasing light intensity to a saturation level of about 0.30 doublings/hr at 20 000 lux and above during the second exponential phase of growth. The rate of the first exponential phase continued to increase, at least up to 50 000 lux. The specific content of both chlorophylls decreased with increasing light intensity but to different extents. A linear relationship between specific chlorophyll content and growth rate for either chlorophyll was only observed over a limited range of growth rates. The ratio of BChl c/BChl a decreased with increasing light intensity. The greatest change occurred between 300 and 5000 lux. The differential responses of BChl c and BChl a to light intensity were also demonstrated by shifting highly pigmented cells (grown at low light intensity) to high light intensity. In these cases different rates of synthesis of the 2 pigments followed initial adjustments. Carotenoid synthesis did not decrease with increasing light intensity under anaerobic conditions.3.Chlorophyll synthesis was suppressed under fully aerobic conditions in dark-ness and light. In either case the pigments were diluted out by continued cell growth. At least 1 carotenoid pigment, however, was synthesized under aerobic conditions. Other carotenoids characteristic of anaerobic growth were not observed.4.The chemoheterotrophic aerobic growth rate at 55°C in darkness (0.14 to 0.22 d/hr) was less than the maximum second phase phototrophic rate under anaerobic conditions (0.30 d/hr). Aerobic growth rate in the light was the same as in darkness if chlorophylls were lacking, but was enhanced if these pigments were still present. The oxygen consumption rate was partially suppressed in the light only when chlorophylls were present in the cells.5.A light-minus-dark diffrence spectrum revealed the presence of a light-induced reversible decrease in absorbance of BChl a with a maximum effect at 860 nm, tentatively identifying a reaction center complex.

The Family Chloroflexaceae

The discovery of the photosynthetic flexibacteria was made by Pierson and Castenholz (1971), and Chloroflexus aurantiacus was the first genus and species described (Pierson and Castenholz, 1974a). The Chloroflexaceae was proposed as a family (Truper, 1976) with affinities to the Chlorobiaceae. The similarities with the green sulfur bacteria were signified by grouping both families under the suborder Chlorobiineae. The family was defined as follows: filamentous, phototrophic bacteria with gliding motility, Gram-negative, flexible cell walls, and bacteriochlorophyll (bchl) a and bchl c, d, or e. Included in the family were two other genera, “Oscillochloris” and Chloronema.

Evolution of Reaction Centers in Photosynthetic Prokaryotes

Publisher Summary This chapter focuses on reaction centers (RCs) of photosynthetic prokaryotes and those phylogenetic characteristics that are most significant to reconstruct the ancestry of these bacteria. Photosynthesis is defined as the biological conversion of light energy to chemical energy in the form of electrochemical gradients and/or ATP. The diverse photosynthetic prokaryotes discussed are grouped into six categories: (1) filamentous photosynthetic bacteria, (2) green sulfur bacteria, (3) heliobacterium chlorum —the gram-positive line, (4) purple bacteria (including bacteriochlorophyll (BChl) a -containing nonphototrophic bacteria), (5) cyanobacteria (including Prochlorophytes), and (6) halobacteria. The cyanobacteria are the most diverse and ubiquitous of all the photosynthetic prokaryotes. In several species of cyanobacteria, the source of electrons for reduction of the oxidized RC-1 primary donor can be sulfide, as in the green sulfur bacteria. When photochemically generated electrons from RC-1 are not consumed for pyridine nucleotide reduction, they maybe cycled back to the primary donor and the energy recovered in the synthesis of ATP by cyclic photophosphorylation. In the absence of sulfide, all cyanobacteria provide electrons to reduce the primary donor to RC-1 from RC-2. RC-2 is similar to the RC in the purple bacteria.

Chloroflexus aurantiacus and ultraviolet radiation: Implications for archean shallow-water stromatolites

The phototrophic growth ofChloroflexus aurantiacus under anoxic conditions was determined as a function of continuous UV irradiance. Cultures grown under an irradiance of 0.01 Wm−2 exhibited a slightly depressed yield over the nonirradiated control. Yields decreased further with increasing irradiance. Inhibition was severe at an irradiance of 0.66 Wm−2. Growth ofE. coli cultures was severely depressed at UV-C irradiances that permitted good growth ofC. aurantiacus. Low levels of Fe3+ provided a very effective UV absorbing screen. The apparent UV resistance ofChloroflexus and the effectiveness of iron as a UV-absorbing screen in sediments and microbial mats are suggested to be likely mechanisms of survival of early phototrophs in the Precambrian in the absence of an ozone shield.

Heliothrix oregonensis, gen. nov., sp. nov., a phototrophic filamentous gliding bacterium containing bacteriochlorophyll a

An unusual filamentous, gliding bacterium was found in a few hot springs in Oregon where it formed a nearly unispecific top layer of microbial mats. It contained a bacteriochlorophyll a-like pigment and an abundance of carotenoids. There were no chlorosomes or additional chlorophylls. The organism was aerotolerant and appeared to be photoheterotrophic. It was successfully co-cultured with an aerobic chemoheterotroph in a medium containing glucose and casamino acids. Although it has many characteristics in common with the genus Chloroflexus, the lack of chlorosomes and bacteriochlorophyll c and the aerobic nature of this organism indicate that it should be placed in a new genus. This conclusion is supported by 5S rRNA nucleotide sequence data.

Complete genome sequence of the filamentous anoxygenic phototrophic bacterium Chloroflexus aurantiacus

BackgroundChloroflexus aurantiacus is a thermophilic filamentous anoxygenic phototrophic (FAP) bacterium, and can grow phototrophically under anaerobic conditions or chemotrophically under aerobic and dark conditions. According to 16S rRNA analysis, Chloroflexi species are the earliest branching bacteria capable of photosynthesis, and Cfl. aurantiacus has been long regarded as a key organism to resolve the obscurity of the origin and early evolution of photosynthesis. Cfl. aurantiacus contains a chimeric photosystem that comprises some characters of green sulfur bacteria and purple photosynthetic bacteria, and also has some unique electron transport proteins compared to other photosynthetic bacteria.MethodsThe complete genomic sequence of Cfl. aurantiacus has been determined, analyzed and compared to the genomes of other photosynthetic bacteria.ResultsAbundant genomic evidence suggests that there have been numerous gene adaptations/replacements in Cfl. aurantiacus to facilitate life under both anaerobic and aerobic conditions, including duplicate genes and gene clusters for the alternative complex III (ACIII), auracyanin and NADH:quinone oxidoreductase; and several aerobic/anaerobic enzyme pairs in central carbon metabolism and tetrapyrroles and nucleic acids biosynthesis. Overall, genomic information is consistent with a high tolerance for oxygen that has been reported in the growth of Cfl. aurantiacus. Genes for the chimeric photosystem, photosynthetic electron transport chain, the 3-hydroxypropionate autotrophic carbon fixation cycle, CO2-anaplerotic pathways, glyoxylate cycle, and sulfur reduction pathway are present. The central carbon metabolism and sulfur assimilation pathways in Cfl. aurantiacus are discussed. Some features of the Cfl. aurantiacus genome are compared with those of the Roseiflexus castenholzii genome. Roseiflexus castenholzii is a recently characterized FAP bacterium and phylogenetically closely related to Cfl. aurantiacus. According to previous reports and the genomic information, perspectives of Cfl. aurantiacus in the evolution of photosynthesis are also discussed.ConclusionsThe genomic analyses presented in this report, along with previous physiological, ecological and biochemical studies, indicate that the anoxygenic phototroph Cfl. aurantiacus has many interesting and certain unique features in its metabolic pathways. The complete genome may also shed light on possible evolutionary connections of photosynthesis.

Photosynthesis 3.5 thousand million years ago.

The recent discovery of stromatolites and microfossils in 3.5-Ga-old sedimentary rock formations is evidence for the existence of phototrophic prokaryotes at that time. Values of δ13C for sedimentary organic carbon strongly suggest autotrophic CO2 fixation, and the existence of large deposits of sedimentary sulfate is consistent with a photosynthesis dependent on reduced sulfur compounds for reducing power. The ancient photoautotrophs are though to have contained only one kind of reaction center with either chlorophyll a or bacteriochlorophyll a as primary electron donor and with one or more iron-sulfur centers as secondary electron acceptors. Light-harvesting pigments might have been chlorophyll a, bacteriochlorophyll a, or possibly bacteriochlorophyll c.A new proposal is made to explain how these organisms could have survived an intense UV flux at the earths surface in the absence of an ozone layer. Photochemically produced ferric iron was abundant in sediments, and the UV-absorption of this ferric iron would have been sufficient to shield those organisms living below the watersediment interface.

Origin and evolution of photosynthetic reaction centers

The prototype reaction center may have used protoporphyrin-IX associated with small peptides to transfer electrons or protons across the primitive cell membrane. The precursor of all contemporary reaction centers contained chlorophylla molecules as both primary electron donor and initial electron acceptor and an Fe-S center as secondary acceptor (RC-1 type). The biosynthetic pathway for chlorophylla evolved along with the evolution of a better organized reaction center associated with cytochromes and quinones in a primitive cyclic electron transport system. This reaction center probably functioned initially in photoassimilation, but was easily adapted to CO2 fixation using H2 and H2S as reductants. During this phase bacteriochlorophyllg may have evolved from chlorophylla in response to competition for light, and thereby initiated the gram-positive line of eubacteria. A second reaction center (RC-2) evolved from RC-1 between 3.5 and 2.5 Ga ago in response to the competition for reductants for CO2 fixation. The new organism containing RC-2 in series with RC-1 would have been able to use poor reducing agents such as the abundant aqueous ferrous ion in place of H2 and H2S. This new organism is proposed to be the common ancestor of all phototrophic eubacteria except those related to the gram-positive bacteria. All organisms containing bacteriochlorophylla lost either RC-1 or RC-2, while those organisms containing chlorophylla (ancestors of cyanobacteria) added a water-splitting enzyme to RC-2 between 3.0 and 2.5 Ga ago in order to use H2O in place of hydrated ferrous ion as electron donor for autotrophic photosynthesis.

Phylum BVI. Chloroflexi phy. nov.

The phylum Chloroflexi is a deep branching lineage of Bacteria The single class within Chloroflexi subdivides into two orders: the “Chloroflexales” and the “Herpetosiphonales”. Gram-negative, filamentous Bacteria. exhibiting gliding motility. Peptidoglycan contains L-ornithine as the diamino acid. Lipopolysaccharide-containing outer membrane not present.