Spectral Irradiance and Distribution of Pigments in a Highly Layered Marine Microbial Mat

The spectral irradiance from 400 to 1,100 nm was measured with depth in the intertidal sand mats at Great Sippewissett Salt Marsh, Mass. These mats contained at least four distinct layers, composed of cyanobacteria, purple sulfur bacteria containing bacteriochlorophyll a (Bchl a), purple sulfur bacteria containing Bchl b, and green sulfur bacteria. Spectral irradiance was measured directly by layering sections of mat on a cosine receptor. Irradiance was also approximated by using a calibrated fiber-optic tip. With the tip, irradiance measurements could be obtained at depth intervals less than 250 μm. The irradiance spectra were correlated qualitatively and quantitatively with the distribution of the diverse chlorophyll pigments in this mat and were compared with spectra recorded in plain sand lacking pigmented phototrophs. We found that the shorter wavelengths (400 to 550 nm) were strongly attenuated in the top 2 mm of the mat. The longer wavelengths (red and near infrared) penetrated to much greater depths, where they were attenuated by Bchl a, b, and c-containing anoxygenic phototrophic bacteria. The specific attenuation bands in the irradiance spectra correlated with the specific in vivo absorption bands of the Bchl-protein complexes in the bacteria. We concluded that the pigments in the phototrophs had a profound affect on the light environment within the mat. It seems likely that the diverse Bchl-protein complexes found in the anoxygenic phototrophs evolved in dense mat environments as a result of competition for light.     

Characterization of a new strain of a purple nonsulfur bacterium from a thermal spring

A new budding purple nonsulfur bacterium of the genus Rhodobacter (strain Ku-2) was isolated from a mat of a moderately thermal spring (Baikal rift zone, Buryatia, Russia). The bacterium had lamellar photosynthetic membranes, which are typical of only one Rhodobacter species, Rba. blasticus. The cells contained spheroidene carotenoids and bacteriochlorophyll a (Bchl a). In vivo absorption spectrum of the cells, with the major maximum at 863 nm and an additional peak at 887 nm, is characteristic of the pigment-protein complexes of Bchl a-containing membranes. The previously described Rba. blasticus strains do not exhibit the 887-nm maximum. The new isolate was photoheterotrophic, with optimal growth occurring at 35°C, 3 g/L NaCl, and pH 7–8. The DNA G+C content was 64.4 mol %. The similarity between the 16S rRNA gene sequences of strain Ku-2 and the Rba. blasticus type strain was 98.7%. The PufM amino acid sequences of strain Ku-2 and the earlier studied Rba. blasticus type strain were 89.5 % identical. Thus, strain Ku-2 belongs to the genus Rhodobacter and is phylogenetically close to Rba. blasticus.     

‘Evolution of Photosynthesis’ (1970), re-examined thirty years later

I have re-examined my 1970 article ‘Evolution of Photosynthesis’ (Olson JM, Science 168: 438–446) to see whether any of my original proposals still survive. My original conviction that the evolution of photosynthesis was intimately connected with the origin of life has been replaced with the realization that photosynthesis may have been invented by the Bacteria after their divergence from the Archea. The common ancestor of all extant photosynthetic bacteria and cyanobacteria probably contained bacteriochlorophyll a, rather than chlorophyll a as originally proposed, and may have carried out CO₂ fixation instead of photoassimilation. The first electron donors were probably reduced sulfur compounds and later ferrous iron. The common ancestor of all extant reaction centers was probably similar to the homodimeric RC1 of present-day green sulfur bacteria (Chlorobiaceae) and heliobacteria. In the common ancestor of proteobacteria and cyanobacteria, the gene for the primordial RC1 was apparently duplicated and one copy split into two genes, one for RC2 and the other for a chlorophyll protein similar to CP43 and CP47 in extant cyanobacteria and chloroplasts. Homodimeric RC1 and homodimeric RC2 functioned in series as in the Z-scheme to deliver electrons from Fe(OH)⁺ to NADP⁺, while RC1 and/or RC2 separately drove cyclic electron flow for the production of ATP. In the line of evolution leading to proteobacteria, RC1 and the chlorophyll protein were lost, but RC2 was retained and became heterodimeric. In the line leading to cyanobacteria, both RC1 and RC2 replaced bacteriochlorophyll a with chlorophyll a and became heterodimeric. Heterodimeric RC2 further coevolved with a Mn-containing complex to utilize water as the electron donor for CO₂ fixation. The chlorophyll–protein was also retained and evolved into CP43 and CP47. Heliobacteria are the nearest photosynthetic relatives of cyanobacteria. The branching order of photosynthetic genes appears to be (1) proteobacteria, (2) green bacteria (Chlorobiaceae plus Chloroflexaceae), and (3) heliobacteria plus cyanobacteria. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of Growth Regulators and Herbicides on Photosynthetic Partial Reactions in Isolated Leaf Cells

The effect of indoleacetic acid, gibberellic acid, 2,4-dichloro-phenoxyacetic acid and amitrole at various concentration levels ranging from 0.05 to 1.0 mM on the photosynthetic evolution of O₂ and ¹⁴CO₂ fixation by isolated leaf cells was studied. The plant growth regulators enhanced O₂ evolution and ¹⁴CO₂ fixation at low concentrations and were inhibitory beyond a critical level. The amitrole had an inhibitory effect at all the concentration levels used. All the substances exhibited similar patterns of effect on the ferricyanide reduction by isolated chloroplasts and on the electron transport rates of sub-chloro-plast particles containing PS-I and PS-II independently, under non-phosphorylating conditions. As was seen from the response in all the three electron transport systems of the chloroplast studied, the electron transport chain connecting PS-II and PS-I could be considered as a possible site of action at least for the growth regulating substances as it is the only part that is common to all the three reactions. The phosphorylation associated with this part of the electron transport was “inhibited” by the substances even at the lowest concentration used. The stimulation of non-phosphorylating electron flow, with a simultaneous reduction in the rate of ATP synthesis, at low concentration levels indicated that these substances played a possible uncoupling role. The amitrole on the other hand appeared to have a generalized non-specific inhibitory action on all the partial reactions of photosynthesis.     

Primary Production in a Subtropical Stratified Coastal Lagoon—Contribution of Anoxygenic Phototrophic Bacteria

Anaerobic anoxygenic phototrophic bacteria can be found in the suboxic waters of shallow stratified coastal systems, and may play important roles in the total primary production of subtropical stratified coastal lagoons. We investigated the spatiotemporal variability of light CO₂ fixation and net oxygen production in the stratified Conceição Lagoon (Brazil) in summer and fall of 2007, as well as the contribution of bacteriochlorophyll a (BChl a)-containing bacteria to photosynthetically driven electron transfer. Both chlorophyll a (Chl a) and BChl a varied in space, while only BChl a varied in time (three-fold increase from summer to fall). In summer, net oxygen production and light CO₂ fixation were correlated, with both having higher rates with higher Chl a concentrations in the enclosed region of the lagoon. In fall, CO₂ fixation was decoupled from oxygen production. Denaturing gradient gel electrophoresis revealed that bacterial communities of oxic site 12 and suboxic site 33 formed one cluster, different from other oxic samples within the lagoon. In addition, BChl a/Chl a ratios at these sites were high, 40% and 45%, respectively. Light acted as the main factor controlling the BChl a concentration and CO₂ fixation rates. High turbidity within the enclosed area of the lagoon explained high BChl a and decoupling between CO₂ fixation and oxygen production in oxygenated waters. Contribution of purple sulfur bacteria to total bacterial density in suboxic waters was 1.2%, and their biomass contributed to a much higher percentage (12.2%) due to their large biovolume. Our results indicate a significant contribution of anaerobic anoxygenic bacteria to the primary production of the “dead zone” of Conceição Lagoon.     

Bacteriochlorophyll-protein complexes of aerobic bacteria,Erythrobacter longus and Erythrobacter species OCh 114

Bacteriochlorophyll(Bchl)-protein complexes were isolated from obligate aerobic bacteria, Erythrobacter longus and Erythrobacter species OCh 114. The apparent molecular weights, absorption spectra and polypeptide compositions of the light-harvesting complexes were, in general, similar to those of the light-harvesting Bchl-protein complexes of purple photosynthetic bacteria. The reaction center complexes of these bacteria also showed similar properties to those of the purple bacteria except for slightly altered polypeptides. However, the following characteristic features of the light-harvesting systems were found in these aerobic bacteria. Major carotenoids were not bound to the Bchl-protein complex in E. longus. In Erythrobacter sp. OCh 114, a new type of Bchl-protein complex which showed a single absorption band in the near infrared region at 806 nm was obtained. The reaction center of strain OCh 114 was associated with a c-type cytochrome.Abbreviations Bchl bacteriochlorophyll a - RC reaction center - SDS sodium dodecylsulfate - PAGE polyacrylamide gel electrophoresis     

Molecular Characterization of Novel Red Green Nonsulfur Bacteria from Five Distinct Hot Spring Communities in Yellowstone National Park

We characterized and compared five geographically isolated hot springs with distinct red-layer communities in Yellowstone National Park. Individual red-layer communities were observed to thrive in temperatures ranging from 35 to 60°C and at pH 7 to 9. All communities were dominated by red filamentous bacteria and contained bacteriochlorophyll a (Bchl a), suggesting that they represented novel green nonsulfur (GNS) bacteria. The in vivo absorption spectra of individual sites were different, with two sites showing unusual Bchl a protein absorption bands beyond 900 nm. We prepared and analyzed 16S rRNA libraries from all of these sites by using a combination of general bacterial primers and new GNS-specific primers described here. These studies confirmed the presence of novel GNS-like bacteria in all five communities. All GNS-like clones were most similar to Roseiflexus castenholzii, a red filamentous bacterium from Japan that also contains only Bchl a. Phylogenies constructed by using GNS-like clones from Yellowstone red-layer communities suggest the presence of a moderately diverse new “red” cluster within the GNS lineage. Within this cluster, at least two well-supported subclusters emerged: YRL-A was most similar to Roseiflexus and YRL-B appeared to be novel, containing no known isolates. While these patterns showed some site specificity, they did not correlate with observed Bchl a spectrum differences or obvious features of the habitat.     

Antenna replacement in the evolutionary origin of chloroplasts

The endosymbiotic origin of chloroplasts from unicellular cyanobacteria is presently beyond doubt. Oxygenic photosynthesis is based on coordinated action of the two photosystems (PS), PS I and PS II, cooperating with several variants of the pigment antenna. In cyanobacteria, red algae, and glaucophytes, phycobilisomes (PBS) act as antennae, while in terrestrial plants, as well as in most macro- and microalgae, antennae are formed by chlorophyll a/b- and chlorophyll a/c-containing proteins. Advantages and disadvantages of the PBS antenna compared to other light-harvesting complexes form the basis for adaptive variations of the antenna in the course of development of eukaryotic photosynthesis. During the evolution of the “green” and “chromophyte” lineages of the chloroplasts, PBS, in spite of their optimal features of light absorption, were replaced by chlorophyll a/b- and chlorophyll a/c-containing light-harvesting complexes. Development of the cell wall associated with the limitation of motility and tissue formation in photosynthetic eukaryotes were the factors responsible for the antenna shift. The subsequent redistribution of cell resources in favor of cellulose biosynthesis required for increased CO₂ consumption, higher PS II levels, and greater number and density of the thylakoids in the chloroplasts, was incompatible with the energy-consuming and overly large PBS antenna.     

Chlorophylls <Emphasis Type="Italic">d</Emphasis> and <Emphasis Type="Italic">f</Emphasis> and their role in primary photosynthetic processes of cyanobacteria

The finding of unique Chl d- and Chl f-containing cyanobacteria in the last decade was a discovery in the area of biology of oxygenic photosynthetic organisms. Chl b, Chl c, and Chl f are considered to be accessory pigments found in antennae systems of photosynthetic organisms. They absorb energy and transfer it to the photosynthetic reaction center (RC), but do not participate in electron transport by the photosynthetic electron transport chain. However, Chl d as well as Chl a can operate not only in the light-harvesting complex, but also in the photosynthetic RC. The long-wavelength (Q_y) Chl d and Chl f absorption band is shifted to longer wavelength (to 750 nm) compared to Chl a, which suggests the possibility for oxygenic photosynthesis in this spectral range. Such expansion of the photosynthetically active light range is important for the survival of cyanobacteria when the intensity of light not exceeding 700 nm is attenuated due to absorption by Chl a and other pigments. At the same time, energy storage efficiency in photosystem 2 for cyanobacteria containing Chl d and Chl f is not lower than that of cyanobacteria containing Chl a. Despite great interest in these unique chlorophylls, many questions related to functioning of such pigments in primary photosynthetic processes are still not elucidated. This review describes the latest advances in the field of Chl d and Chl f research and their role in primary photosynthetic processes of cyanobacteria.     

Evolution of bacterial denitrification and denitrifier diversity

Little is known about the role of nitrate in evolution of bacterial energy-generating mechanisms. Denitrifying bacteria are commonly regarded to have evolved from nitrate-respiring bacteria. Some researchers regard denitrification to be the precursor of aerobic respiration; others feel the opposite is true. Currently recognized denitrifying bacteria such as Hyphomicrobium, Paracoccus, Pseudomonas and Thiobacillus form a very diverse group. However, inadequate testing procedures and uncertain taxonomic identification of many isolates may have overstated the number of genera with species capable of denitrification. Nitrate reductases are structurally similar among denitrifying bacteria, but distinct from the enzymes in other nitrate-reducing organisms. Denitryfying bacteria have one of two types of nitrite reductase, either a copper-containing enzyme or an enzyme containing a cytochrome cd moiety. Both types are distinct from other nitrate reductases. Organisms capable of dissimilatory nitrate reduction are widely distributed among eubacterial groups defined by 16S ribosomal RNA phylogeny. Indeed, nitrate reduction is an almost universal property of actinomycetes and enteric organisms. However, denitrification is restricted to genera within the purple photosynthetic group. Denitrification within the genus Pseudomonas is distributed in accordance with DNA and RNA homology complexes. Denitrifiers seem to have evolved from a common ancestor within the purple photosynthetic bacterial group, but not from a nitrate-reducing organism such as those found today. Although denitrification seems to have arisen at the same time as aerobic respiration, the evolutionary relationship between the two cannot be determined at this time.     

Isolation and characterization of bacteriochlorophyll-protein complexes from an aerobic bacterium,Pseudomonas radiora

Bacteriochlorophyll(Bchl)-protein complexes were isolated from a strictly aerobic and facultative methylotrophic bacterium Pseudomonas radiora strain MD-1. They were identified as the reaction center (RC)-B870 and the B870 complexes on the basis of their absorption spectra, light induced spectral changes and polypeptide compositions. The RC-B870 complex of this bacterium showed similar properties to those of typical purple photosynthetic bacteria, and contained c-type cytochrome which was oxidized upon illumination.Abbreviations Bchl bacteriochlorophyll - RC reaction center - SDS sodium dodecylsulfate - PAGE polyacrylamide gel electrophoresis     

The Formation and Dynamics of Deep Bacteriochlorophyll Maximum in the Temperate and Partly Meromictic Lake Verevi

Vertical distribution of phytoplankton and the formation of deep chlorophyll maximum (DCM) in the metalimnion of a small stratified and partly meromictic temperate lake was studied in 1999 and 2000. During summer DCM usually occurred on the borderline of H₂S and oxygen-containing waters. At the depths where the bacteriochlorophyll (Bchl) maxima were observed, the sulphide concentration was usually relatively low compared to the bottom layers, where its concentration reached as high as possible saturation level. In April 2000, DCM was formed at the depth of 3.5 m, and lowered thereafter slowly to 6.5 m by October. The concentration of Bchl d reached the highest values (over 1000 μg l⁻¹) just before the water column was mixed up in autumn. In December and April Bchl d was detectable only near the bottom of the lake. The concentration of chlorophyll a yielded by the spectrophotometric phaeopigment corrected method and by HPLC (high pressure liquid chromatography), fit rather well in the upper layers. In deeper water layers chlorophyll a concentration (Chl a) measured by spectrophotometry was overestimated about 47 times if compared to HPLC values because of the high Bchl d in that layer. In most cases vertical profiles of primary production (PP) did not coincide with the vertical distribution of the pigment content; the maximum values of PP were found in the epilimnion. In some cases PP had notably high values also at the depth of DCM. In the upper layers Chl a usually did not exceeded 20 μg l⁻¹ in spring and 10 μg l⁻¹ in summer_. The moderately high Chl a in the epilimnion in spring was significantly reduced after the formation of thermocline most probably because of the establishment of the nutrient limitation in epilimnion. Decreasing Chl a concentration in the epilimnion led to increased water transparency and better light conditions for photosynthetic bacteria in metalimnion.     

Isolation and characterization of polysaccharides of a hybrid mushroom (backcross mating between PfloVv12 and Volvariella volvacea)

Three polysaccharide fractions (PS-I, PS-II, and PS-III) were isolated from the aqueous extract of a hybrid mushroom obtained through backcross mating of a somatic hybrid mushroom PfloVv12 (Sterile line) with Volvariellavolvacea. PfloVv12 was obtained through protoplast fusion of Pleurotusflorida and V. volvacea. PS-I was identified as 1,6-β glucan. PS-II and PS-III were identified as mannoglucogalactan but differing in molecular weights only. On the basis of total acid hydrolysis, methylation analysis, periodate oxidation, and NMR experiment (¹H, ¹³C, DEPT-135, DQF-COSY, TOCSY, NOESY, ROESY, HMQC, and HMBC) the structures of these polysaccharides were established as;     

Bio-optical Characteristics and the Vertical Distribution of Photosynthetic Pigments and Photosynthesis in an Artificial Cyanobacterial Mat

Abstract Zonations of photosynthesis and photopigments in artificial cyanobacterial mats were studied with (i) oxygen and pH microsensors, (ii) fiber-optic microprobes for field radiance, scalar irradiance, and PSII fluorescence, and (iii) a light microscope equipped with a spectrometer for spectral absorbance and fluorescence measurements. Our analysis revealed the presence of several distinct 1–2 mm thick cyanobacterial layers mixed with patches of anoxygenic photosynthetic bacteria. Strong attenuation of visible light confined the euphotic zone to the uppermost 3 mm of the mat, where oxygen levels of 3–4 times air saturation and a pH peak of up to pH 8.8 were observed under saturating irradiance (413 μmol photon m⁻² s⁻¹). Oxygen penetration was 5 mm in light and decreased to 1 mm in darkness. Volumetric oxygen consumption in the photic and aphotic zones of illuminated mat was 5.5 and 2.9 times higher, respectively, than oxygen consumption in dark incubated mats. Scalar irradiance reached 100–150% of incident irradiance in the upper 0.5 mm of the mat due to intense scattering in the matrix of cells, exopolymers, and carbonate precipitates. In deeper mat layers scalar irradiance decreased nearly exponentially, and highest attenuation coefficients of 6–7 mm⁻¹ were found in cyanobacterial layers, where photosynthesis and photopigment fluorescence also peaked. Visible light was attenuated >100 times more strongly than near infrared light. Microscope spectrometry on thin sections of mats allowed detailed spectral absorbance and fluorescence measurements at defined positions relative to the mat surface. Besides strong spectral signals of cyanobacterial photopigments (Chl a and phycobiliproteins), the presence of both green and purple photosynthetic bacteria was evident from spectral signals of Bchl a and Bchl c. Microprofiles of photopigment absorbance correlated well with microdistributions of phototrophs determined in an accompanying study. Received: 20 December 1999; Accepted: 10 June 2000; Online Publication: 28 August 2000     

Single-Molecule Spectroscopy Reveals that Individual Low-Light LH2 Complexes from Rhodopseudomonas palustris 2.1.6. Have a Heterogeneous Polypeptide Composition

Rhodopseudomonas palustris belongs to the group of purple bacteria that have the ability to produce LH2 complexes with unusual absorption spectra when they are grown at low-light intensity. This ability is often related to the presence of multiple genes encoding the antenna apoproteins. Here we report, for the first time to our knowledge, direct evidence that individual low-light LH2 complexes have a heterogeneous αβ-apoprotein composition that modulates the site energies of Bchl a molecules, producing absorption bands at 800, 820, and 850 nm. The arrangement of the Bchl a molecules in the “tightly coupled ring” can be modeled by nine αβ-Bchls dimers, such that the Bchls bound to six αβ-pairs have B820-like site energies and the remaining Bchl a molecules have B850-like site energies. Furthermore, the experimental data can only be satisfactorily modeled when these six αβ-pairs with B820 Bchl a molecules are distributed such that the symmetry of the assembly is reduced to C₃. It is also clear from the measured single-molecule spectra that the energies of the electronically excited states in the mixed B820/850 ring are mainly influenced by diagonal disorder.     

Seasonal Changes in the Structure of the Anoxygenic Phototrophic Bacterial Community in Lake Mogilnoe,a Relict Lake on Kil'din Island in the Barents Sea

An anaerobic phototrophic bacterial community in Lake Mogilnoe, a relict lake on Kil'din Island in the Barents Sea, was studied in June 1999 and September 2001. Irrespective of the season, the upper layer of the anaerobic zone of this lake had a specific species composition of sulfur phototrophic bacteria, which were dominated by the brown-colored green sulfur bacterium Chlorobium phaeovibrioides. The maximum number of sulfur phototrophic bacteria was observed in June 1999 at a depth of 9 m, which corresponded to a concentration of bacteriochlorophyll (Bchl) e equal to 4.6 mg/l. In September 2001, the maximum concentration of this pigment (3.4 mg/l) was found at a depth of 10 m. In both seasons, the concentration of Bchl a did not exceed 3 μg/l. Purple sulfur bacteria were low in number, which can be explained by their poor adaptation to the hydrochemical and optical conditions of the Lake Mogilnoe water. In June 1999, the water contained a considerable number of Pelodictyon phaeum microcolonies and Prosthecochloris phaeoasteroides cell chains, which was not the case in September 2001. A 16S rDNA-based phylogenetic analysis of pure cultures of phototrophic bacteria isolated from the lake water confirmed that the bacterial community is dominated by Chl. phaeovibrioides and showed the presence of three minor species, Thiocystis gelatinosa, Thiocapsa sp., and Thiorhodococcus sp., the last of which is specific to Lake Mogilnoe.     

Competitive Inhibitions of the Chlorophyll Synthase of Synechocystis sp. Strain PCC 6803 by Bacteriochlorophyllide a and the Bacteriochlorophyll Synthase of Rhodobacter sphaeroides by Chlorophyllide a

The photosynthetic growth of Synechocystis sp. strain PCC 6803 is hampered by exogenously added bacteriochlorophyllide a (Bchlide a) in a dose-dependent manner. The growth inhibition caused by Bchlide a, however, is relieved by an increased level of exogenously added chlorophyllide a (Chlide a). The results are explained by the competitive inhibition of chlorophyll synthase by Bchlide a, with inhibition constants (K_Is) of 0.3 mM and 1.14 mM in the presence of sufficient geranylgeranyl pyrophosphate (GGPP) and phytyl pyrophosphate (PPP), respectively. Surprisingly, the bacteriochlorophyll synthase of Rhodobacter sphaeroides is inhibited competitively by Chlide a, with K_Is of 0.54 mM and 0.77 mM in the presence of sufficient GGPP and PPP, respectively. Consistently, exogenously added Chlide a inhibits the metabolic conversion of exogenously added Bchlide a to bacteriochlorophyll a by an R. sphaeroides bchFNB-bchZ mutant that neither synthesizes nor metabolizes Chlide a. The metabolic inhibition by Chlide a, however, is relieved by the elevated level of Bchlide a. Thus, the chlorophyll synthase of Synechocystis sp. PCC 6803 and the bacteriochlorophyll synthase of R. sphaeroides, both of which perform ping-pong-type reactions, are inhibited by Bchlide a and Chlide a, respectively. Although neither inhibitor is catalyzed by the target enzyme, inhibitions in the competitive mode suggest a structural similarity between their active sites.The biosynthetic pathways for bacteriochlorophyll a (Bchl a) and chlorophyll a (Chl a) share the metabolic steps from protoporphyrin IX to chlorophyllide a (Chlide a) (Fig. ​(Fig.1).1). The C₂₀ moiety from geranylgeranyl pyrophosphate (GGPP) can be directly esterified to ring D of Chlide a by chlorophyll synthase (ChlG) to yield geranylgeranylated Chl a (Chl a_gg), which is subsequently reduced (at positions 6, 10, and 14 of GG) by chlorophyll reductase (ChlP) to yield phytylated Chl a (Chl a_p, but it is usually abbreviated as Chl a) (2, 7). The chlorophyll synthase of Avena sativa has a broad substrate specificity for C₂₀, and it may accept either GGPP or phytyl pyrophosphate (PPP) as the first substrate in its ping-pong-type reaction (27). Either a geranylgeranylated or phytylated enzyme esterifies the second substrate Chlide a, yielding Chl a_gg or Chl a, respectively (5, 24). Chlorophyll reductase reduces either the GG moiety of Chl a_gg or free GGPP, yielding Chl a or free PPP, respectively (25).Open in a separate windowFIG. 1.Chl a and Bchl a biosynthetic pathways (6, 7). The chemical structures of Chlide a and Bchlide a are shown. bchF codes for 3-vinyl bacteriochlorophyllide hydratase; bchXYZ for three subunits comprising COR; bchC for 3-hydroxyethyl bacteriochlorophyllide dehydrogenase; chlG and bchG for chlorophyll synthase and bacteriochlorophyll synthase, respectively; and chlP and bchP for chlorophyll reductase and bacteriochlorophyll reductase, respectively.Chlide a may be further metabolized to bacteriochlorophyllide a (Bchlide a) (Fig. ​(Fig.1).1). Chlide a reductase (COR) reduces ring B of Chlide a to form 3-vinyl bacteriochlorophyllide a, whose C-3-vinyl group on ring A is then converted into an acetyl group through the activities of hydratase (BchF) and dehydrogenase (BchC) to form Bchlide a (6). The hydratase reaction may alternatively precede that of COR (Fig. ​(Fig.1).1). Once Bchlide a is formed, its ring D is esterified with the C₂₀ geranylgeranyl moiety by bacteriochlorophyll synthase (BchG), yielding geranylgeranylated Bchl a (Bchl a_gg) (3). The C₂₀ moiety is subsequently reduced by bacteriochlorophyll reductase (BchP), yielding the phytylated Bchl a (Bchl a_p, but it is usually abbreviated as Bchl a) (1).The biosynthesis of Chl a has been regarded as a metabolism that evolved after Bchl a (33). ChlG and ChlP have been thought to emerge through the gene duplication of BchG and BchP, respectively. Recently, we found that the COR reaction, which is specific to Bchl a biosynthesis, generates superoxide at low levels of oxygen (16), and we further proposed that the degeneration of the superoxide-generating COR step may be associated with the emergence of cyanobacterium-based Chl a biosynthesis (15).The predicted sequence of ChlG of Synechocystis sp. strain PCC 6803 bears 35% identity with that of Rhodobacter sphaeroides BchG. Nonetheless, chlorophyll synthase and bacteriochlorophyll synthase exhibit a high degree of substrate specificity to distinguish their own Mg-tetrapyrrole substrates from that of the other enzyme (23, 28). We further examined whether chlorophyll synthase and bacteriochlorophyll synthase are affected by Bchlide a and Chlide a, respectively, which are structurally similar to each other. In this work, we found that the chlorophyll synthase of Synechocystis sp. PCC 6803 is competitively inhibited by Bchlide a. We further found that the bacteriochlorophyll synthase of R. sphaeroides is competitively inhibited by Chlide a. Thus, the active site of chlorophyll synthase is recognized by Bchlide a, while that of bacteriochlorophyll synthase is recognized by Chlide a. The results suggest a structural similarity between the active sites of the two enzymes.     

Photosynthetic and Phylogenetic Primers for Detection of Anoxygenic Phototrophs in Natural Environments

Primer sets were designed to target specific 16S ribosomal DNA (rDNA) sequences of photosynthetic bacteria, including the green sulfur bacteria, the green nonsulfur bacteria, and the members of the Heliobacteriaceae (a gram-positive phylum). Due to the phylogenetic diversity of purple sulfur and purple nonsulfur phototrophs, the 16S rDNA gene was not an appropriate target for phylogenetic rDNA primers. Thus, a primer set was designed that targets the pufM gene, encoding the M subunit of the photosynthetic reaction center, which is universally distributed among purple phototrophic bacteria. The pufM primer set amplified DNAs not only from purple sulfur and purple nonsulfur phototrophs but also from Chloroflexus species, which also produce a reaction center like that of the purple bacteria. Although the purple bacterial reaction center structurally resembles green plant photosystem II, the pufM primers did not amplify cyanobacterial DNA, further indicating their specificity for purple anoxyphototrophs. This combination of phylogenetic- and photosynthesis-specific primers covers all groups of known anoxygenic phototrophs and as such shows promise as a molecular tool for the rapid assessment of natural samples in ecological studies of these organisms.     

Pigment carbon and nitrogen isotopic signatures in euxinic basins

The carbon and nitrogen isotopic signatures of chloropigments and porphyrins from the sediments of redox‐stratified lakes and marine basins reveal details of past biogeochemical nutrient cycling. Such interpretations are strengthened by modern calibration studies, and here, we report on the C and N isotopic composition of pigments and nutrients in the water column and surface sediment of redox‐stratified Fayetteville Green Lake (FGL; New York). We also report δ¹³C and δ¹⁵N values for pyropheophytin a (Pphe a) and bacteriochlorophyll e (Bchl e) deposited in the Black Sea during its transition to a redox‐stratified basin ca. 7.8 ka. We propose a model for evolving nutrient cycling in the Black Sea from 7.8 to 6.4 ka, informed by the new pigment data from FGL. The seasonal study of water column nutrients and pigments at FGL revealed population dynamics in surface and deep waters that were also captured in the sediments. Biomass was greatest near the chemocline, where cyanobacteria, purple sulfur bacteria (PSB), and green sulfur bacteria (GSB) had seasonally variable populations. Bulk organic matter in the surface sediment, however, was derived mainly from the oxygenated surface waters. Surface sediment pigment δ¹³C and δ¹⁵N values indicate intact chlorophyll a (Chl a) was derived from near the chemocline, but its degradation product pheophytin a (Phe a) was derived primarily from surface waters. Bacteriopheophytin a (Bphe a) and Bchl e in the sediments came from chemocline populations of PSB and GSB, respectively. The distinctive δ¹³C and δ¹⁵N values for Chl a, Phe a, and Bphe a in the surface sediment are inputs to an isotopic mixing model that shows their decomposition to a common porphyrin derivative can produce non‐specific sedimentary isotope signatures. This model serves as a caveat for paleobiogeochemical interpretations in basins that had diverse populations near a shallow chemocline.     

A cyanobacterium that contains chlorophyll f - a red-absorbing photopigment

A Chl f-containing filamentous cyanobacterium was purified from stromatolites and named as Halomicronema hongdechloris gen., sp. nov. after its phylogenetic classification and the morphological characteristics. Hongdechloris contains four main carotenoids and two chlorophylls, a and f. The ratio of Chl f to Chl a is reversibly changed from 1:8 under red light to an undetectable level of Chl f under white-light culture conditions. Phycobiliproteins were induced under white light growth conditions. A fluorescence emission peak of 748 nm was identified as due to Chl f. The results suggest that Chl f is a red-light inducible chlorophyll.