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21 pages; International audience; In modern stromatolites, mineralization results from a complex interplay between microbial metabolisms, the organic matrix, and environmental parameters. Here, we combined biogeochemical, mineralogical, and microscopic analyses with measurements of metabolic activity to characterize the mineralization processes and products in an emergent (

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.

Summary ‘Photoarsenotrophy’, the use of arsenite as an electron donor for anoxygenic photosynthesis, is thought to be an ancient form of phototrophy along with the photosynthetic oxidation of Fe(II), H2S, H2 and NO2−. Photoarsenotrophy was recently identified from Paoha Island's (Mono Lake, CA) arsenic-rich hot springs. The genomes of several photoarsenotrophs revealed a gene cluster, arxB2AB1CD, where arxA is predicted to encode for the sole arsenite oxidase. The role of arxA in photosynthetic arsenite oxidation was confirmed by disrupting the gene in a representative photoarsenotrophic bacterium, resulting in the loss of light-dependent arsenite oxidation. In situ evidence of active photoarsenotrophic microbes was supported by arxA mRNA detection for the first time, in red-pigmented microbial mats within the hot springs of Paoha Island. This work expands on the genetics for photosynthesis coupled to new electron donors and elaborates on known mechanisms for arsenic metabolism, thereby highlighting the complexities of arsenic biogeochemical cycling.

Introduction. The green sulfur bacteria are predominately aquatic bacteria that grow photosynthetically under anoxic conditions. Similar to Chromatiaceae (purple sulfur bacteria) and purple nonsulfur bacteria (Proteobacteria), the photosynthetic metabolism of green sulfur bacteria differs from that of Cyanobacteria, algae, and green plants in that water cannot serve as an electron-donating substrate and molecular oxygen is not generated. The external electron donors for photosynthetic CO2 assimilation have low redox potentials and comprise reduced sulfur compounds and molecular hydrogen. Green sulfur bacteria can be distinguished from the Chromatiaceae by their distinct bacteriochlorophylls (BChls), the presence of special light-harvesting structures, and their exclusively photolithotrophic metabolism. Chlorobi / “Chlorobia” / Subclass I “Chlorobiales” / Green Sulfur Bacteria / Green Sulfur Bacteria The mol% G + C of the DNA is: 47.8–58.1 (Bd). Type genus: Chlorobium Nadson 1906, 190. Keywords: Green Sulfur Bacteria; Chlorobium; “Chlorobiaceae”

Okenone is a carotenoid pigment unique to certain members of Chromatiaceae, the dominant family of purple sulfur bacteria (PSB) found in euxinic photic zones. Diagenetic alteration of okenone produces okenane, the only recognized molecular fossil unique to PSB. The in vivo concentrations of okenone and bacteriochlorophyll a (Bchl a) on a per cell basis were monitored and quantified as a function of light intensity in continuous cultures of the purple sulfur bacterium Marichromatium purpuratum (Mpurp1591). We show that okenone-producing PSB have constant bacteriochlorophyll to carotenoid ratios in light-harvesting antenna complexes. The in vivo concentrations of Bchl a, 0.151 ± 0.012 fmol cell^(−1), and okenone, 0.103 ± 0.012 fmol cell^(−1), were not dependent on average light intensity (10–225 Lux) at both steady and non-steady states. This observation revealed that in autotrophic continuous cultures of Mpurp1591, there was a constant ratio for okenone to Bchl a of 1:1.5. Okenone was therefore constitutively produced in planktonic cultures of PSB, regardless of light intensity. This confirms the legitimacy of okenone as a signature for autotrophic planktonic PSB and by extrapolation water column euxinia. We measured the δ^(13)C, δ^(15)N, and δ^(34)S bulk biomass values from cells collected daily and determined the isotopic fractionations of Mpurp1591. There was no statistical relationship in the bulk isotope measurements or stable isotope fractionations to light intensity or cell density under steady and non-steady-state conditions. The carbon isotope fractionation between okenone and Bchl a with respect to overall bulk biomass (^(13)ε_pigment – biomass) was 2.2 ± 0.4‰ and −4.1 ± 0.9‰, respectively. The carbon isotopic fractionation (^(13)ε_pigment-CO_2) for the production of pigments in PSB is more variable than previously thought with our reported values for okenone at −15.5 ± 1.2‰ and −21.8 ± 1.7‰ for Bchl a.

We demonstrate that Blue-diode-based pulse amplitude modulation (PAM) technology can be used to measure the photosynthetic electron transport rate (ETR) of purple sulfur bacteria (Thermochromatium tepidum, Chromatiaceae). Previous studies showed that PAM technology could be used to estimate photosynthesis in purple nonsulfur bacteria and so PAM technology can be used to estimate photosynthesis of both kinds of purple photosynthetic bacteria. The absorptance of Thermochromatium films on glass fiber disks was measured and used to calculate actual ETR. ETR vs Irradiance (P vs E) curves fitted the waiting-in-line model (ETR = (ETRmax × E/Eopt) × exp (1−E/Eopt)). Yield (Y) was only ≈ 0.3–0.4. Thermochromatium saturates at 325 ± 13.8 μmol photons m(−2) s(−1) or ≈15% sunlight and shows photoinhibition at high irradiances. A pond of Thermochromatium would exhibit classic surface inhibition. Photosynthesis is extremely low in the absence of an electron source: ETR increases in the presence of acetate (5 mol m(−3)) provided as an organic carbon source and also increases in the presence of sulfite (3 mol m(−3)) but not sulfide and is only marginally increased by the presence of Fe(2+). Nonphotochemical quenching does occur in Thermochromatium but at very low levels compared to oxygenic photo-organisms or Rhodopseudomonads.

A comprehensive marine biomarker record of green and purple sulfur bacteria (GSB and PSB, respectively) is required to test whether anoxygenic photosynthesis represented a greater fraction of marine primary productivity during the Precambrian than the Phanerozoic, as current models of ocean redox evolution suggest. For this purpose, we analyzed marine rock extracts and oils from the Proterozoic to the Paleogene for C40 diagenetic products of carotenoid pigments using new analytical methods. Gas chromatography coupled with tandem mass spectrometry provides a new perspective on the temporal distributions of carotenoid biomarkers for phototrophic sulfur bacteria, specifically okenane, chlorobactane, and paleorenieratane. According to conventional paleoredox interpretations, this revised stratigraphic distribution of the GSB and PSB biomarkers implies that the shallow sunlit surface ocean (

A compact handheld Raman spectrometer equipped with a 532 nm excitation laser was used to detect pigments as possible biomarkers in autotrophic (cyanobacteria and purple sulfur bacteria) and heterotrophic halophilic microorganisms (Archaea of the family Halobacteriaceae, Salinibacter). Common as well as less common carotenoids, including α-bacterioruberin, salinixanthin, and spirilloxanthin derivatives were detected in cell pellets of model organisms belonging to the genera Haloferax, Haloarcula, Halobacterium (Archaea), Salinibacter (Bacteroidetes), and Ectothiorhodospira (Gammaproteobacteria). Direct measurements on such cultures provide fast and reliable identification of pigments. Bacterioruberin was detected as the dominant carotenoid in pellets of cells collected from the saltern crystallizer ponds in Eilat, Israel. Raman analysis of the colored layered microbial communities in a benthic gypsum crust in the saltern evaporation ponds showed signals consistent with the presence of myxoxanthophyll and echinenone carotenoids in the upper orange and dark-green cyanobacterial layers. Chlorophyll a and phycocyanin expected in the green layer were not detected using the green excitation. Spirilloxanthin dominates the red layer below, inhabited mainly by purple sulfur bacteria. To our best knowledge, this is the first attempt to detect and identify pigments in natural microbial communities consisting of different types of halophilic microorganisms by direct Raman spectrometric measurements using light compact handheld devices. Copyright © 2013 John Wiley & Sons, Ltd.

Mahoney Lake, British Columbia, Canada, is a stratified, 15-m deep saline lake with a euxinic (anoxic, sulfidic) hypolimnion. A dense plate of phototrophic purple sulfur bacteria is found at the chemocline, but to date the rest of the Mahoney Lake microbial ecosystem has been underexamined. In particular, the microbial community that resides in the aphotic hypolimnion and/or in the lake sediments is unknown, and it is unclear whether the sulfate reducers that supply sulfide for phototrophy live only within, or also below, the plate. Here we profiled distributions of 16S rRNA genes using gene clone libraries and PhyloChip microarrays. Both approaches suggest that microbial diversity is greatest in the hypolimnion (8 m) and sediments. Diversity is lowest in the photosynthetic plate (7 m). Shallower depths (5 m, 7 m) are rich in Actinobacteria, Alphaproteobacteria, and Gammaproteobacteria, while deeper depths (8 m, sediments) are rich in Crenarchaeota, Natronoanaerobium, and Verrucomicrobia. The heterogeneous distribution of Deltaproteobacteria and Epsilonproteobacteria between 7 and 8 m is consistent with metabolisms involving sulfur intermediates in the chemocline, but complete sulfate reduction in the hypolimnion. Overall, the results are consistent with the presence of distinct microbial niches and suggest zonation of sulfur cycle processes in this stratified system.

Purple sulfur bacteria (PSB) mainly occur in anoxic aquatic and benthic environments, where they play important roles in cycling carbon and sulfur. Many PSB characteristically produce the unique keto-carotenoid, okenone, which is important not only for its light absorption and photoprotection properties but also because of its diagenesis product, okenane, which is a biomarker for ancient sediments derived from anoxic environments. The specific methylation pattern of the χ-ring of okenane is unlikely to be formed by diagenetic processes and should therefore reflect an enzymatic activity from okenone biosynthesis. This study describes two enzymes that produce the χ-ring of okenone, the only structural element of okenone preserved in okenane. Genes encoding enzymes of carotenogenesis were identified in the draft genome sequence of an okenone-producing PSB, Thiodictyon sp. strain CAD16. Two divergently transcribed genes encoded a CrtY-type lycopene cyclase and a CrtU/CruE-type γ-carotene desaturase/methyltransferase. Expression of crtY in Escherichia coli showed that this gene encoded a lycopene cyclase that produced γ-carotene as the only product. Although the sequence of the γ-carotene desaturase/methyltransferase was more similar to CrtU sequences of green sulfur bacteria than to CruE sequences of cyanobacteria, expression of the crtU gene in Chlorobaculum tepidum showed that the enzyme produced carotenoids with χ-rings rather than φ-rings. Phylogenetic analysis of the carotene desaturase/methyltransferases revealed that enzymes capable of converting β-rings to χ-rings have independently evolved at least two times. These results indicate that it probably will not be possible to deduce the activity of carotene desaturase/methyltransferases solely from sequence data.

Organic biomarkers in marine sedimentary rocks hold important clues about the early history of Earth’s surface environment. The chemical relicts of carotenoids from anoxygenic sulfur bacteria are of particular interest to geoscientists because of their potential to signal episodes of marine photic-zone euxinia such as those proposed for extended periods in the Proterozoic as well as brief intervals during the Phanerozoic. It is therefore critical to constrain the environmental and physiological factors that influence carotenoid production and preservation in modern environments. Here, we present the results of coupled pigment and nucleic acid clone library analyses from planktonic and benthic samples collected from a microbially dominated meromictic lake, Fayetteville Green Lake (New York). Purple sulfur bacteria (PSB) are abundant and diverse both in the water column at the chemocline and in benthic mats below oxygenated shallow waters, with different PSB species inhabiting the two environments. Okenone (from PSB) is an abundant carotenoid in both the chemocline waters and in benthic mats. Green sulfur bacteria and their primary pigment Bchl e are also represented in and below the chemocline. However, the water column and sediments are devoid of the green sulfur bacteria carotenoid isorenieratene. The unexpected absence of isorenieratene and apparent benthic production of okenone provide strong rationale for continued exploration of the microbial ecology of biomarker production in modern euxinic environments.

Summary Culture-based and culture-independent methods were used to explore the diversity of phototrophic purple bacteria in Soap Lake, a small meromictic soda lake in the western USA. Among soda lakes, Soap Lake is unusual because it consists of distinct upper and lower water bodies of vastly different salinities, and its deep waters contain up to 175 mM sulfide. From Soap Lake water new alkaliphilic purple sulfur bacteria of the families Chromatiaceae and Ectothiorhodospiraceae were cultured, and one purple non-sulfur bacterium was isolated. Comparative sequence analysis of pufM, a gene that encodes a key photosynthetic reaction centre protein universally found in purple bacteria, was used to measure the diversity of purple bacteria in Soap Lake. Denaturing gradient gel electrophoresis and subsequent phylogenetic analyses of pufMs amplified from Soap Lake water revealed that a significant diversity of purple bacteria inhabit this soda lake. Although close relatives of several of the pufM phylotypes obtained from cultured species could also be detected in Soap Lake water, several other more divergent pufM phylotypes were also detected. It is possible that Soap Lake purple bacteria are major contributors of organic matter into the ecosystem of this lake, especially in its extensive anoxic and sulfidic deep waters.

Zodletone spring in Oklahoma is a unique environment with high concentrations of dissolved-sulfide (10 mm) and short-chain gaseous alkanes, exhibiting characteristics that are reminiscent of conditions that are thought to have existed in Earth's history, in particular the late Archean and early-to-mid Proterozoic. Here, we present a process-oriented investigation of the microbial community in two distinct mat formations at the spring source, (1) the top of the sediment in the source pool and (2) the purple streamers attached to the side walls. We applied a combination of pigment and lipid biomarker analyses, while functional activities were investigated in terms of oxygen production (microsensor analysis) and carbon utilization ((13)C incorporation experiments). Pigment analysis showed cyanobacterial pigments, in addition to pigments from purple sulfur bacteria (PSB), green sulfur bacteria (GSB) and Chloroflexus-like bacteria (CLB). Analysis of intact polar lipids (IPLs) in the source sediment confirmed the presence of phototrophic organisms via diacylglycerol phospholipids and betaine lipids, whereas glyceroldialkylglyceroltetraether additionally indicated the presence of archaea. No archaeal IPLs were found in the purple streamers, which were strongly dominated by betaine lipids. (13)C-bicarbonate- and -acetate-labeling experiments indicated cyanobacteria as predominant phototrophs in the source sediment, carbon was actively fixed by PSB/CLB/GSB in purple streamers by using near infrared light. Despite the presence of cyanobacteria, no oxygen could be detected in the presence of light, suggesting anoxygenic photosynthesis as the major metabolic process at this site. Our investigations furthermore indicated photoheterotrophy as an important process in both habitats. We obtained insights into a syntrophically operating phototrophic community in an ecosystem that bears resemblance to early Earth conditions, where cyanobacteria constitute an important contributor to carbon fixation despite the presence of high sulfide concentrations.

We investigated the genotypic diversity of oxygenic and anoxygenic phototrophic microorganisms in microbial mat samples collected from three hot spring localities on the east coast of Greenland. These hot springs harbour unique Arctic microbial ecosystems that have never been studied in detail before. Specific oligonucleotide primers for cyanobacteria, purple sulfur bacteria, green sulfur bacteria and Choroflexus/Roseiflexus-like green non-sulfur bacteria were used for the selective amplification of 16S rRNA gene fragments. Amplification products were separated by denaturing gradient gel electrophoresis (DGGE) and sequenced. In addition, several cyanobacteria were isolated from the mat samples, and classified morphologically and by 16S rRNA-based methods. The cyanobacterial 16S rRNA sequences obtained from DGGE represented a diverse, polyphyletic collection of cyanobacteria. The microbial mat communities were dominated by heterocystous and non-heterocystous filamentous cyanobacteria. Our results indicate that the cyanobacterial community composition in the samples were different for each sampling site. Different layers of the same heterogeneous mat often contained distinct and different communities of cyanobacteria. We observed a relationship between the cyanobacterial community composition and the in situ temperatures of different mat parts. The Greenland mats exhibited a low diversity of anoxygenic phototrophs as compared with other hot spring mats which is possibly related to the photochemical conditions within the mats resulting from the Arctic light regime. © 2006 The Authors.

Stable carbon and nitrogen isotopic compositions were determined for individual photosynthetic pigments isolated and purified from the saline meromictic Lake Kaiike, Japan, to investigate species-independent biogeochemical processes of photoautotrophs in the natural environment. In the anoxic monimolimnion and benthic microbial mats, the carbon isotopic compositions of BChls e and isorenieratene related to brown-coloured strains of green sulfur bacteria are substantially ( approximately 10 per thousand) depleted in (13)C relative to those found in the chemocline. In conjunction with 16S rDNA evidence reported previously, it strongly suggests that Pelodyctyon luteolum inhabited and photosynthesized in the anoxic monimolimnion and benthic microbial mats by using (13)C-depleted regenerated CO(2). By contrast, both Chl a and BChl a in the monimolimnion and microbial mats have similar isotopic compositions as they do in the chemocline, implying that the source organisms live only in the chemocline. In the chemocline, the nitrogen isotopic compositions of BChl e homologues ranges from -7.7 to-6.5 per thousand, whereas that of BChl a is -2.1 per thousand. These isotopic compositions suggest that green sulfur bacteria Chlorobium phaeovibrioides would conduct nitrogen fixation in the chemocline, whereas purple sulfur bacteria Halochromatium sp. and cyanobacteria Synechococcus sp. may assimilate nitrite.

The photosynthetic reaction center (RC) is a transmembrane protein complex that catalyzes light-driven electron transport across the photosynthetic membrane. The complete amino-acid sequence of the H subunit of the RC from a thermophilic purple sulfur bacterium, Thermochromatium tepidum, has been determined for the first time among purple sulfur bacteria. The H subunit consists of 259 amino acids and has a molecular mass of 28 187. The deduced amino-acid sequences of this H subunit showed a significant (40%) degree of identity with those from mesophilic purple nonsulfur bacteria. The determined primary structure of the H subunit was compared with the structures of mesophilic B. viridis and R. sphaeroides based on the three-dimensional structure of the H subunit from T. tepidum, which has been recently determined by X-ray crystallography. One lipid molecule was found in the crystal structure of the T. tepidum RC, and the head group of the lipid appears to be stabilized by the electrostatic interactions with the conserved basic residues in the H subunit. The above comparison has suggested the existence of a lipid-binding site on the molecular surface at which a lipid molecule can interact with the RC in a specific manner.

Phototrophic purple bacteria are Proteobacteria which synthesize a photosynthetic apparatus, located in internal membrane systems and the cytoplasmic membrane. They thrive in all kinds of aquatic habitats and sediments that receive sufficient light and provide oxygen deficient and anoxic conditions. They occur widely distributed in nature. Metabolically specialized species live in specific ecological niches, metabolically versatile species have adapted to a wide range of habitats. Phototrophic purple sulfur bacteria, the Chromatiaceae and the Ectothiorhodospiraceae families, are Gammaproteobacteria. Genera of the purple nonsulfur bacteria are Alpha- and Betaproteobacteria phylogenetically closely related to purely chemotrophic bacteria. Keywords: phototrophic bacteria; Chromatiaceae; Ectothiorhodospiraceae; purple nonsulfur bacteria

In meromictic Mahoney Lake (British Columbia), purple sulfur bacteria formed a dense, IO-cm-thick layer at the primary chemocline. The dominant species (Amoebobucterpurpureus) reached a maximal cell concentration of 4 x lOa cells ml-l, 274/o of which were viable. A maximal concentration of 20,900 pg bacteriochlorophyll n liter’ was determinedthe highest concentration ever found in a natural body of water. Steep vertical gradients of conductivity, light, and sulfide concentration were detected across the bacterial layer, while the concentration of oxygen increased above it. Integrated primary production of the layer was 15 mg C m-2 h-l. Bacteriochlorophyllspecific assimilation rates reached maxima at the top of the layer. Anoxygenic photosynthesis was limited by reduced sulfur compounds at the top of the layer, whereas strong attenuation of irradiance resulted in light limitation for most of the cell population below. The sulfide flux from below the layer accounted for 63% of the sulfide oxidized during anoxygenic photosynthesis must have been produced within the layer. This calculation agrees well with sulfide production rates measured in situ (12.4 pm01 liter-’ h--l). In contrast to other lakes, considerable uptake of [14C]acetate by purple sulfur bacteria and comparatively high numbers of purple nonsulfur bacteria indicate an important role of dissolved organic matter and a short cycle of carbon and redox compounds in the dense purple microbial layer.

During a bloom of purple sulfur bacteria in Lake Ciso (Spain) data were collected on diel changes in the concentrations of sulfide, sulfur, glycogen, and poly-β-hydroxybutyrate (PHB), the rates of CO2 fixation and H2S oxidation, and the viability of cells along the vertical profile. During the day, sulfide oxidation resulted in the intracellular accumulation of sulfur and glycogen. At night, the concentrations of glycogen and sulfur decreased concomitant with the production of sulfide and PHB. Throughout the day, CO2 fixation and H2S oxidation were maximal in the zone of maximal population density (peak of the layer); however, in the top of the layer the specific rates were 2–6 times higher. The ratio of CO2 fixation to H2S oxidation, the sulfur content of the cells, and their specific density all indicated that sulfide was predominantly oxidized to sulfur. In the top and the peak of the layer cell viability was high but decreased rapidly with increasing depth. It was concluded that the existence of the bloom is the combined result of slow growth at the peak of the layer and a much faster rate of reproduction at the top of the layer. The cells at the top stored sulfur extensively and then sank to deeper layers where they could not oxidize the sulfur because of insufficient light.

The anaerobic zones of Lakes Cis6 and Vilar (Banyoles karstic area, NE Spain) had mass developments of purple sulfur bacteria during summer 1982. In Lake Vilar, Chromatium spp. was dominant (up to 92% of the microbial biovolume). In Lake Ciso, the predominant microorganisms were Chromatium spp. (up to 7 1%) and another purple sulfur bacterium forming aggregates (20%). The bacterial layer could be divided according to the physiological state of the cells into a top part of maximal specific activity, a peak of maximal abundance, and a bottom part of inactive cells. The bacteria in the peak were predominantly limited by light; sulfide, phosphate, and acetate were not limiting in the middle of the day. The light limitation started at the depth having the maximal concentration of cells; the top of the layer appeared to be sulfide-limited. Specific contents of photopigments, elemental sulfur, and reserve polymers decreased from the top to the bottom of the bacterial layer. These phenomena point to the crucial role of light in the development of layers of phototrophic bacteria in stratified lakes.

Daily rates of oxygenic photosynthesis (OP) by phytoplankton, anoxygenic photosynthesis (AP) by purple sulfur bacteria, and chemoautotrophic productivity (CP = dark CO, assimilation) were measured once each season in saline, meromictic Big Soda Lake. Total daily productivity and the relative importance of each autotrophic process varied with seasonal changes in vertical mixing, light availability, and the biomass of phototrophs. Daily productivity was highest (2,830 mg C.rnmz) and was dominated by OP in winter when the mixolimnion was isothermal, the biomass of phytoplankton was high, and the biomass of purple sulfur bacteria was low. During the summer-fall period of thermal stratification, phytoplankton biomass decreased, a plate of purple sulfur bacteria formed below the oxycline, and daily rates of dark CO, assimilation (CP = 390-680 mg C.rnmz) exceeded phototrophic productivity (OP + AP = ZOO-370 mg C.rnmz). Total annual productivity was about 500 g C.me2, of which 60% was produced by phytoplankton (mostly in winter), 30% by chemoautotrophs (nitrifying and sulfuroxidizing bacteria), and only

The photosynthesis-light curve for purple sulfur bacteria had a steeper inclination than that for green sulfur bacteria at low light intensities. Light saturation occurred at intensities of S-7 klux in the former, in the latter at 10-30 klux. Light inhibition was observed in purple sulfur bacteria but was negligible in green sulfur bacteria. The optimal temperature for photosynthesis of these bacteria is considerably higher than that of most phytoplankton or green plants. Photosynthetic sulfur bacteria appear ordinarily in the contact layer between oxidative and reductive zones of meromictic or stagnant holomictic lakes; the light intensity in this contact layer is usually less than 10% of that at the surface. On the assmuption that the photosynthetic rate of these bacteria is limited mainly by the interaction of hydrogen sulfide concentration with light intensity, their growth was analyzed with a mathematical model. The properties of the growth phase observed in lakes were similar to those calculated. The main factors determining the growth of photosynthetic sulfur bacteria in lakes are the II23 concentration in the upper layer and the light conditions in the deeper layer.