Phototrophic plankton of siderotrophic meromictic Lake Kuznechikha (Republic Mari El,Russia)

During summer thermal stratification, a broad transition zone with hypoxic conditions is formed in meromictic ferruginous Lake Kuznechikha between the thermocline and the main gradient of water mineralization. In this zone, the chemical composition of water undergoes an ecologically significant transformation due to overlapping gradients of nutrient concentrations and redox conditions. We present an analysis of a strongly vertically structured community of prokaryotic and eukaryotic phototrophs developing in the lake as a whole and especially in the transition zone. In early summer 2009, a sequence of phototrophic organisms with depth in order Chlorophyceae → Chromatiales → Chloroflexales → Euglenales → Chlorobiales was observed in the transition zone, while cyanobacteria were almost completely absent. Biomass maximum of anoxygenic phototrophic bacteria was located between the peaks of phototrophic picoplankton and euglenoids. Such a coexistence of oxygenic and anoxygenic phototrophs in a wide range of depths is highly unusual and sharply distinguishes Lake Kuznechikha from waterbodies with sulphide-containing monimolimnion.     

Heterotrophic dinitrogen fixation (acetylene reduction) in phosphate-fertilised Microcoleus chthonoplastes microbial mat from the hypersaline inland lake ‘la Salada de Chiprana’ (NE Spain)

Microcoleus chthonoplastes dominated microbial mats are conspicuous along the shallow littoral zone in Lake Chiprana, a hypersaline lake located in the Ebro river basin in north-eastern Spain. Pigment data show that these mats included diatom species and anoxygenic phototrophs, Chloroflexus-type bacteria and purple bacteria. In situ, these mats showed low rates of dinitrogen fixation (acetylene reduction). Acetylene reduction was stimulated about 30-fold in excised mats after moderate phosphate fertilisation during 2 weeks incubation in a mesocosm. Pigment analyses showed that this treatment had little impact on the phototrophic community structure, except that it induced a decrease of Chloroflexus-type bacteria. The use of metabolic inhibitors indicated that methanogenic archaea and aerobic heterotrophic bacteria were the major dinitrogen fixers in this system. This is in agreement with the fact that the mat-building cyanobacterium M. chthonoplastes lacks the dinitrogenase reductase nifH gene and with the fact that acetylene reduction rates were strongly stimulated by additions of H₂/CO₂, methanol, fructose and sucrose, but not by lactate, acetate, formate and glucose. No significant differences where found for acetylene reduction rates when comparing light and dark incubations of these microbial mats. However, acetylene reduction rates were enhanced in the light when the near infrared (NIR) light was filtered out, which arrested anoxygenic photosynthesis. We suggest, therefore, that the chemoheterotrophic dinitrogen fixing bacteria were in competition with anoxygenic phototrophic bacteria for organic substrates, while the latter did not contribute to dinitrogen fixation in the mat.     

Ciliates from a fresh water sulfuretum

Ciliates were collected from a freshwater sulfuretum, Lake Cisó, which is part of a gypsum karstic area whose main feature is Lake Banyoles (Girona, Spain). Chromatium, Lamprocystis and Chlorobium are the major phototrophic sulfur bacteria in Lake Cisó. Blooms of a photosynthetic cryptomonad (up to 5 X 10(5) ind ml-1) were found at the metalimnion. The community of ciliates could be divided in three groups: aerobic, cosmopolitan, genera such as Stentor and Vorticella, in the epilimnion; a large population (up to 10(4) ind ml-1) of Coleps, adapted to low concentrations of both oxygen and sulfide, together with a few individuals of the equally sulfide-tolerant genus Paramecium, in the metalimnion, and anaerobic, true sulfide-loving genera such as Plagiopyla and Metopus, in the hypolimnion, where sulfide concentration was between 0.6 and 1.2 mM.     

Carbon isotope fractionation by anoxygenic phototrophic bacteria in euxinic Lake Cadagno

Anoxygenic phototrophic bacteria utilize ancient metabolic pathways to link sulfur and iron metabolism to the reduction of CO₂. In meromictic Lake Cadagno, Switzerland, both purple sulfur (PSB) and green sulfur anoxygenic phototrophic bacteria (GSB) dominate the chemocline community and drive the sulfur cycle. PSB and GSB fix carbon utilizing different enzymatic pathways and these fractionate C‐isotopes to different extents. Here, these differences in C‐isotope fractionation are used to constrain the relative input of various anoxygenic phototrophs to the bulk community C‐isotope signal in the chemocline. We sought to determine whether a distinct isotopic signature of GSB and PSB in the chemocline persists in the settling fraction and in the sediment. To answer these questions, we also sought investigated C‐isotope fractionation in the water column, settling material, and sediment of Lake Cadagno, compared these values to C‐isotope fractionation of isolated anoxygenic phototroph cultures, and took a mass balance approach to investigate relative contributions to the bulk fractionation signature. We found a large C‐isotope fractionation between dissolved inorganic carbon (DIC) and particulate organic carbon (POC) in the Lake Cadagno chemocline. This large fractionation between the DIC and POC was also found in culture experiments carried out with anoxygenic phototrophic bacteria isolated from the lake. In the Lake Cadagno chemocline, anoxygenic phototrophic bacteria controlled the bulk C‐isotope fractionation, but the influence of GSB and PSB differed with season. Furthermore, the contribution of PSB and GSB to bulk C‐isotope fractionation in the chemocline could be traced in the settling fraction and in the sediment. Taken together with other studies, such as lipid biomarker analyzes and investigations of other stratified lakes, these results offer a firmer understanding of diagenetic influences on bacterial biomass.     

Discovery of the first light‐dependent protochlorophyllide oxidoreductase in anoxygenic phototrophic bacteria

In all photosynthetic organisms, chlorophylls function as light‐absorbing photopigments allowing the efficient harvesting of light energy. Chlorophyll biosynthesis recurs in similar ways in anoxygenic phototrophic proteobacteria as well as oxygenic phototrophic cyanobacteria and plants. Here, the biocatalytic conversion of protochlorophyllide to chlorophyllide is catalysed by evolutionary and structurally distinct protochlorophyllide reductases (PORs) in anoxygenic and oxygenic phototrophs. It is commonly assumed that anoxygenic phototrophs only contain oxygen‐sensitive dark‐operative PORs (DPORs), which catalyse protochlorophyllide reduction independent of the presence of light. In contrast, oxygenic phototrophs additionally (or exclusively) possess oxygen‐insensitive but light‐dependent PORs (LPORs). Based on this observation it was suggested that light‐dependent protochlorophyllide reduction first emerged as a consequence of increased atmospheric oxygen levels caused by oxygenic photosynthesis in cyanobacteria. Here, we provide experimental evidence for the presence of an LPOR in the anoxygenic phototrophic α‐proteobacterium Dinoroseobacter shibae DFL12^T. In vitro and in vivo functional assays unequivocally prove light‐dependent protochlorophyllide reduction by this enzyme and reveal that LPORs are not restricted to cyanobacteria and plants. Sequence‐based phylogenetic analyses reconcile our findings with current hypotheses about the evolution of LPORs by suggesting that the light‐dependent enzyme of D. shibae DFL12^T might have been obtained from cyanobacteria by horizontal gene transfer.     

Thymidine incorporation in Lake Cisó: Problems in estimating bacterial secondary production across oxic-anoxic interfaces

Abstract Heterotrophic bacterial activity was measured by means of the ³H-thymidine (³H-TdR) incorporation technique in Lake Cisó, a small holomictic lake with anoxic hypolimnion. We tested several methodological questions across the vertical profile: TdR concentration at which maximal incorporation is reached, linearity of incorporation and isotope dilution, during holomixis and stratification periods. The TdR concentration at which maximal incorporation is reached changed seasonally and vertically. During holomixis, maximal incorporation was not always reached at concentrations up to 40 nM. Uptake was always linear in short incubation times and decreased from epi- to hypolimnion. The isotope dilution technique indicated a degree of participation in DNA synthesis higher than 50%, although a linear relationship between the inverse of ³H-TdR incorporation and increasing ‘cold’ thymidine concentration was not always observed. Autoradiographic experiments showed a low percentage of bacteria taking up ³H-TdR in both aerobic and anaerobic samples. The percentage of total labeled bacteria seemed to be generally higher in the metalimnion (11% maximal value) than in the hypolimnion. Labeled Amoebobacter and Chromatium cells were detected in field samples. Amoebobacter cells photoassimilated TdR in culture. Therefore, our results show that ³H-TdR incorporation is not an appropriate technique to estimate bacterial secondary production in anaerobic systems and in oxic-anoxic interfaces.     

Community Shift from Phototrophic to Chemotrophic Sulfide Oxidation following Anoxic Holomixis in a Stratified Seawater Lake

Most stratified sulfidic holomictic lakes become oxygenated after annual turnover. In contrast, Lake Rogoznica, on the eastern Adriatic coast, has been observed to undergo a period of water column anoxia after water layer mixing and establishment of holomictic conditions. Although Lake Rogoznica''s chemistry and hydrography have been studied extensively, it is unclear how the microbial communities typically inhabiting the oxic epilimnion and a sulfidic hypolimnion respond to such a drastic shift in redox conditions. We investigated the impact of anoxic holomixis on microbial diversity and microbially mediated sulfur cycling in Lake Rogoznica with an array of culture-independent microbiological methods. Our data suggest a tight coupling between the lake''s chemistry and occurring microorganisms. During stratification, anoxygenic phototrophic sulfur bacteria were dominant at the chemocline and in the hypolimnion. After an anoxic mixing event, the anoxygenic phototrophic sulfur bacteria entirely disappeared, and the homogeneous, anoxic water column was dominated by a bloom of gammaproteobacterial sulfur oxidizers related to the GSO/SUP05 clade. This study is the first report of a community shift from phototrophic to chemotrophic sulfide oxidizers as a response to anoxic holomictic conditions in a seasonally stratified seawater lake.     

Microbial communities of the stratified soda Lake Doroninskoe (Transbaikal region)

The physicochemical properties, species composition, and vertical distribution of microorganisms in the water column, shoreline microbial mat, and small shoreline mud volcanoes of the stratified soda Lake Doroninskoe were investigated in September 2007. The lake is located in the Transbaikal region, in the permafrost zone (51°25′N; 112°28′E). The maximal depth of the contemporary lake is about 6 m, the pH value of the water is 9.72, and the water mineralization in the near-bottom horizon is 32.3 g l⁻¹. In summer, the surface oxygen-containing horizon of the water column becomes demineralized to 26.5 g l⁻¹; at a depth of 3.5–4.0 m, an abrupt transition occurs to the aerobic zone containing hydrosulfide (up to 12.56 g l⁻¹). Hydrosulfide was also detected in trace quantities in the upper water horizons. The density stratification of the water column usually ensures stable anaerobic conditions until the freezing period (November and December). The primary production of oxygenic phototrophs reached 176–230 μg l⁻¹. High rates of dark CO₂ assimilation (61–240 μg l⁻¹) were detected in the chemocline. Within this zone, an alkaliphilic species of sulfur-oxidizing bacteria of the genus Thioalkalivibrio was detected (10⁴ cells ml⁻¹). Lithoheterotrophic bacteria Halomonas spp., as well as bacteriochlorophyll a-containing aerobic anoxygenic phototrophic bacteria (AAP) Roseinatronobacter sp. capable of thiosulfate oxidation, were isolated from samples collected from the aerobic zone (0–3 m). The water transparency in September was extremely low; therefore, no visible clusters of anoxygenic phototrophic bacteria (APBs) were detected at the boundary of the hydrosulfide layer. However, purple sulfur bacteria which, according to the results of the 16S rRNA gene analysis, belong to the species Thioalkalicoccus limnaeus, Ectothiorhodospira variabilis, “Ect. magna,” and Ect. shaposhnikovii, were isolated from samples of deep silt sediments. Ect. variabilis and Ect. shaposhnikovii were the major APB species in the shoreline algo-bacterial mat. The halotolerant bacterium Ect. shaposhnikovii, purple nonsulfur bacteria of the genus Rhodobacter, and AAP of Roseococcus sp. were isolated from the samples collected from mud volcanoes. All these species are alkaliphiles, moderate halophiles, or halotolerant microorganisms.     

A heterotrophic bacterium inhibits growth of several species of the genus Chlorobium

A gram-negative bacterium (designed as strain BF 9500) causing growth inhibition zones on cell lawns of the anoxygenic phototrophic bacterium Chlorobium limicola BF 8000 was isolated from Lake Cisó (Spain). Strain BF 9500, identified as Stenotrophomonas maltophilia, caused growth inhibition zones on cell lawns of several strains of C. limicola except C. limicola DSM 245. It also inhibited other Chlorobium species and several heterotrophic bacteria. However, it had no effect on the growth of the eleven strains of Chromatiaceae tested. Strain BF 9500 caused the lysis of C. limicola BF 8000, whose cells formed “ghosts”. To date, this is the first report of a bacterium causing the lysis of species of the genus Chlorobium. Received: 15 August 1996 / Accepted: 15 January 1997     

Primary Production of Phytoplankton in a Strongly Stratified Temperate Lake

Lake Verevi (12.6 ha, maximum depth 11.0 m, mean depth 3.6 m) is a strongly eutrophic and stratified lake. Planktothrix agardhii is the most characteristic phytoplankton species in summer and autumn, while photosynthesizing sulphur bacteria can occur massively in the metalimnion. Primary production (PP) and chlorophyll a concentration (Chl a) were seasonally studied in 1991, 1993, 2000, and 2001. Vertical distribution of PP was rather complex, having usually two peaks, one at or near the surface (0–1 m), and another deeper (at 3–7 m) in the metalimnion. The values of dark fixation of CO₂ in the metalimnion were in most cases higher than those in the upper water layer. Considering the average daily PP 896 mg C m⁻² and yearly PP 162 mg C m⁻², Secchi depth 2.34 m, and epilimnetic concentrations of chlorophyll a (19.6 mg m⁻³), total nitrogen and total phosphorus (TP, 52 mg m⁻³) in 2000, L. Verevi is a eutrophic lake of a ‘good’ status. Considering the total amounts of nutrients stored in the hypolimnion, the average potential concentrations in the whole water column could achieve 1885 mg m⁻³ of TN and 170 mg m⁻³ of TP reflecting hypertrophic conditions and a ‚bad’ status. Improvement of the epilimnetic water quality from the 1990s to the 2000s may have resulted from incomplete spring mixing and might not reflect the real improvement. A decreased nutrient concentration in the epilimnion has supported the establishment of a ‘clear epilimnion state’ allowing light to penetrate into the nutrient-rich metalimnion and sustaining a high production of cyanobacteria and phototrophic sulphur bacteria.     

Benthic phototrophic community from Kiran soda lake,south-eastern Siberia

Phototrophic bacterial mats from Kiran soda lake (south-eastern Siberia) were studied using integrated approach including analysis of the ion composition of water, pigments composition, bacterial diversity and the vertical distribution of phototrophic microorganisms in the mats. Bacterial diversity was investigated using microscopic examination, 16S rRNA gene Illumina sequencing and culturing methods. The mats were formed as a result of decomposition of sedimented planktonic microorganisms, among which cyanobacteria of the genus Arthrospira predominated. Cyanobacteria were the largest part of phototrophs in the mats, but anoxygenic phototrophs were significant fraction. The prevailing species of the anoxygenic phototrophic bacteria are typical for soda lakes. The mats harbored aerobic anoxygenic phototrophic bacteria, purple sulfur and non-sulfur bacteria, as well as new filamentous phototrophic Chloroflexi. New strains of Thiocapsa sp. Kir-1, Ectothiorhodospira sp. Kir-2 and Kir-4, Thiorhodospira sp. Kir-3 and novel phototrophic Chloroflexi bacterium Kir15-3F were isolated and identified.

     

Anoxygenic phototrophic bacteria from gypsum karst lakes of Lithuania

The spatial distribution and composition of anoxygenic phototrophic bacteria in the enriched bacterial communities from different depths of karst lakes Kirkilai and Ramunelis was studied using spectrophotometric analysis, as well as microbiological and molecular methods. In Lake Kirkilai, the highest bacterial abundance was measured in the metalimnion and near the bottom (up to 10.7 × 10⁶ cell/mL); in Lake Ramunelis it was in the anoxic hypolimnion (up to 22.4 × 10⁶ cell/mL). Increased water mineralization (0.5–1.2 g/L) with the domination of SO ₄ ^2? and Ca²⁺ ions created favorable conditions for the development of sulfate-reducing bacteria; hydrogen sulfide produced as a result of their life activity facilitated the development of sulfur-oxidizing bacteria. The pigment analysis of phototrophic green and purple sulfur bacteria showed the domination of green sulfur bacteria in the enrichment culture. The results of phylogenetic analysis showed that Chlorobium limicola dominated in the enrichment culture for the green sulfur bacteria, whereas purple nonsulfur bacteria of the genus Rhodopseudomonas dominated in the enrichment culture for the purple sulfur bacteria.     

Phytoplankton, bacterial production and protozoan bacterivory in stratified, brackish-water Lake Shira (Khakasia, Siberia)

Rates of oxygenic and anoxygenic photosynthesis, chemoautotrophic and heterotrophic bacterial production and protozoan bacterivory were measured in the pelagic zone of the stratified brackish-water lake with the purpose to determine the vertical distribution of these processes and to estimate their significance in the functioning of planktonic community of the lake. In midsummer, total daily primary productivity was about 1.3 g C m^–2, of which 72% was produced by the phytoplankton, 24% by the chemoautotrophic bacteria, and only 4% by the phototrophic sulphur bacteria. Thus anoxygenic photosynthesis is a negligible source of organic matter in the lake. The production of heterotrophic bacteria averaged 1.5 g C m^–2 d^–1 and exceeded the total photosynthesis of phytoplankton and photosynthetic bacteria by a factor of 1.5. The estimated total primary production was too low to sustain the bacterial production. Probably the carbon cycle in the lake is dependent on the input of allochthonous organic matter. As a rule, the maximal rates of primary production and heterotrophic bacterial production were found in the chemocline or at the upper boundary of the chemocline. Heterotrophic flagellates dominated among the protozoan populations and were the major consumers of the bacterioplankton production in the lake. They showed maximal ingestion rates from 2.3 to 2.9 mg C m^–3 h^–1 at the upper boundary of the chemocline, where they consumed from 50 to 54% of the production of heterotrophic bacteria. Data obtained indicate that in Lake Shira the oxic-anoxic interface is the site of the most intensive production and mineralization of organic matter.     

Adaptation to Hydrogen Sulfide of Oxygenic and Anoxygenic Photosynthesis among Cyanobacteria

Four different types of adaptation to sulfide among cyanobacteria are described based on the differential toxicity to sulfide of photosystems I and II and the capacity for the induction of anoxygenic photosynthesis. Most cyanobacteria are highly sensitive to sulfide toxicity, and brief exposures to low concentrations cause complete and irreversible cessation of CO₂ photoassimilation. Resistance of photosystem II to sulfide toxicity, allowing for oxygenic photosynthesis under sulfide, is found in cyanobacteria exposed to low H₂S concentrations in various hot springs. When H₂S levels exceed 200 μM another type of adaptation involving partial induction of anoxygenic photosynthesis, operating in concert with partially inhibited oxygenic photosynthesis, is found in cyanobacterial strains isolated from both hot springs and hypersaline cyanobacterial mats. The fourth type of adaptation to sulfide is found at H₂S concentrations higher than 1 mM and involves a complete replacement of oxygenic photosynthesis by an effective sulfide-dependent, photosystem II-independent anoxygenic photosynthesis. The ecophysiology of the various sulfide-adapted cyanobacteria may point to their uniqueness within the division of cyanobacteria.     

Succession processes in the anoxygenic phototrophic bacterial community in Lake Kislo-Sladkoe (Kandalaksha Bay,White Sea)

The community of anoxygenic phototrophic bacteria (APB) in the water column of Lake Kislo- Sladkoe (Kandalaksha Bay, White Sea), which has recently become separated from the sea, was investigated in March?April 2012, March?April 2013, and in September 2013. The lake, which was previously considered meromictic, was in fact mixed and was strongly affected by the sea. In winter the lake is sometimes washed off with seawater, and this together with the seasonal cycles of succession processes determines the succession of the community. The consequences of the mixing in autumn 2011 could be observed in the APB community as late as autumn 2013. Green-colored green sulfur bacteria (GSB) usually predominated in the chemocline. In winter 2013 stagnation resulted in turbidity of water under the ice, which was responsible for both predominance of the brown GSB forms and the changes ratio of the species of purple sulfur bacteria (PSB) in anoxic water layers. Production of anoxygenic photosynthesis in the lake was at least 240 mg C m^-2 day^-1 in September and 0–20 mg C m^–2 day^–1 in March—April, which corresponded to 40 and 69%, respectively, of oxygenic photosynthesis. Okenone-containing purple sulfur bacteria, strain TcakPS12, were isolated in 2012 from lake water. The ells of this strain form filaments of not separated cells. Strain TcakPS12 exhibited 98% similarity with the type strains of Thiocapsa pendens DSM 236 and Thiocapsa bogorovii BBS, as well as with the strains AmPS10 and TcyrPS10, which were isolated from Lake Kislo-Sladkoe in 2010.     

Biological fractionation of stable carbon isotopes at the aerobic/anaerobic water interface of meromictic water bodies

Mass-spectrometric investigation of carbon isotope composition (δ¹³C) was carried out for suspended organic matter and dissolved mineral compounds for the water column of some meromictic water bodies differing in salinity and trophic state. As a rule, a more pronounced carbon isotope fractionation (resulting from the metabolism of phytoplankton and anoxygenic phototrophic bacteria) was revealed in the zones of enhanced oxygenic and anoxygenic photosynthesis. Carbon isotope fractionation at the border between oxidized and reduced waters depends both on the activity of microbial communities and on the dominant species of phototrophic microorganisms. Analysis of the distribution profiles of the isotopic composition of suspended organic matter and dissolved mineral carbon revealed active mineralization of the organic matter newly formed via anoxygenic photosynthesis in the monimolimnion by microbial communities, resulting in the release of isotopically light carbon dioxide. Mineral carbon in the anaerobic zones of highly productive meromictic water bodies is therefore enriched with the light ¹²C isotope.     

The Phototrophic Community Found in Lake Khilganta (an Alkaline Saline Lake Located in the Southeastern Transbaikal Region)

The structure of the phototrophic community found in Lake Khilganta (the Agin-Buryat Autonomous Area), a shallow saline soda lake (depth, 35–45 cm; water mineralization, 45 g/l; alkalinity, 30 mg-equiv/l; pH 9.5) has been studied. The bottom of the lake is covered with a 10- to 15-mm microbial mat, whose basis is formed by the filamentous cyanobacterium Microcoleus chthonoplastes. The mat exhibits pronounced layering and contains a significant amount of minerals. Six zones, which have characteristic colors and consistencies and are composed of intermittent layers, have been identified along the vertical profile. Live phototrophic bacteria have been found in the three upper zones. The bulk of the cyanobacteria is concentrated in the upper zone. In the lower zones, the development of purple bacteria has been observed. The diurnal dynamics of the vertical distribution of phototrophic microorganisms, which results from variations in the physicochemical environmental parameters, is described. Ectothiorhodospira sp. are dominant among the anoxyphotobacteria present. Their number, determined according to the inoculation method, is 10⁶–10⁷ cells/ml. The purple bacteria of the genera Allochromatium, Thiocapsa, and Rhodovulum are also present. Experiments with isolated pure cultures have shown that the anoxygenic photosynthetic bacteria of Lake Khilganta are halotolerant and alkalitolerant or alkaliphilic. In liquid enrichment cultures, at pH 9.5, the ratio of anoxyphotobacteria species is close to that observed in the lake. When the pH is increased to 10.4, it is Ectothiorhodospira, which is the most adapted to life under increased mineralization and alkalinity, that predominantly develops. Photosynthetic activity has been observed in the three upper mat zones and constitutes, on average, 1.5 g C/(m²h); the share of anoxygenic photosynthesis accounts for 75–95% of the total productivity. The main role in sulfide oxidation belongs to the phototrophic anoxyphotobacteria and cyanobacteria. In terms of the physicochemical conditions and structure of the phototrophic community, Lake Khilganta is similar to shallow saline water bodies of marine origin. The main differences consist in the increased alkalinity and in the consequent prevalence of alkaliphilic and alkalitolerant microorganisms and in the absence of representatives of the neutrophilic group of green sulfur bacteria.__________Translated from Mikrobiologiya, Vol. 74, No. 3, 2005, pp. 410–419.Original Russian Text Copyright © 2005 by Kompantseva, Sorokin, Gorlenko, Namsaraev.     

Facultative anoxygenic photosynthesis in the cyanobacterium Oscillatoria limnetica.

An isolate from H2S-rich layers of the Solar Lake, the cyanobacterium Oscillatoria limnetica, exhibits both oxygenic and anoxygenic photosynthesis. It can use Na2S as an electron donor for CO2 photoassimilation (photosystem I supplies the energy) in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea or 700-nm light. A stoichiometric ratio of approximately 2 is observed between the Na2S consumed and the photoassimilated CO2. The anoxygenic phototrophic capability of this cyanobacterium explains its growth in nature in high sulfide concentrations and indicates a selective advantage.     

Development of a real-time PCR method for the detection of fossil 16S rDNA fragments of phototrophic sulfur bacteria in the sediments of Lake Cadagno

Lake Cadagno is a crenogenic meromictic lake situated in the southern range of the Swiss Alps characterized by a compact chemocline that has been the object of many ecological studies. The population dynamics of phototrophic sulfur bacteria in the chemocline has been monitored since 1994 with molecular methods such as 16S rRNA gene clone library analysis. To reconstruct paleo-microbial community dynamics, we developed a quantitative real-time PCR methodology for specific detection of 16S rRNA gene sequences of purple and green sulfur bacteria populations from sediment samples. We detected fossil 16S rDNA of nine populations of phototrophic sulfur bacteria down to 9-m sediment depth, corresponding to about 9500 years of the lake's biogeological history. These results provide the first evidence for the presence of 16S rDNA of anoxygenic phototrophic bacteria in Holocene sediments of an alpine meromictic lake and indicate that the water column stratification and the bacterial plume were already present in Lake Cadagno thousands of years ago. The finding of Chlorobium clathratiforme remains in all the samples analyzed shows that this population, identified in the water column only in 2001, was already a part of the lake's biota in the past.     

Isolation and characterization of aerobic anoxygenic phototrophs from exposed soils from the Sør Rondane Mountains,East Antarctica

This study investigated the culturable aerobic phototrophic bacteria present in soil samples collected in the proximity of the Belgian Princess Elisabeth Station in the Sør Rondane Mountains, East Antarctica. Until recently, only oxygenic phototrophic bacteria (Cyanobacteria) were well known from Antarctic soils. However, more recent non-cultivation-based studies have demonstrated the presence of anoxygenic phototrophs and, particularly, aerobic anoxygenic phototrophic bacteria in these areas. Approximately 1000 isolates obtained after prolonged incubation under different growth conditions were studied and characterized by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Representative strains were identified by sequence analysis of 16S rRNA genes. More than half of the isolates grouped among known aerobic anoxygenic phototrophic taxa, particularly with Sphingomonadaceae, Methylobacterium and Brevundimonas. In addition, a total of 330 isolates were tested for the presence of key phototrophy genes. While rhodopsin genes were not detected, multiple isolates possessed key genes of the bacteriochlorophyll synthesis pathway. The majority of these potential aerobic anoxygenic phototrophic strains grouped with Alphaproteobacteria (Sphingomonas, Methylobacterium, Brevundimonas and Polymorphobacter).