Salinity Responses of Benthic Microbial Communities in a Solar Saltern (Eilat, Israel)

The salinity responses of cyanobacteria, anoxygenic phototrophs, sulfate reducers, and methanogens from the laminated endoevaporitic community in the solar salterns of Eilat, Israel, were studied in situ with oxygen microelectrodes and in the laboratory in slurries. The optimum salinity for the sulfate reduction rate in sediment slurries was between 100 and 120‰, and sulfate reduction was strongly inhibited at an in situ salinity of 215‰. Nevertheless, sulfate reduction was an important respiratory process in the crust, and reoxidation of formed sulfide accounted for a major part of the oxygen budget. Methanogens were well adapted to the in situ salinity but contributed little to the anaerobic mineralization in the crust. In slurries with a salinity of 180‰ or less, methanogens were inhibited by increased activity of sulfate-reducing bacteria. Unicellular and filamentous cyanobacteria metabolized at near-optimum rates at the in situ salinity, whereas the optimum salinity for anoxygenic phototrophs was between 100 and 120‰.     

Microbial communities and processes within a hypersaline gypsum crust in a saltern evaporation pond (Eilat,Israel)

Gypsum crusts containing multicolored, stratified microbial communities develop in the evaporation ponds of a commercial saltern in Eilat, Israel at salt concentrations between 190 and 240 g l⁻¹. The upper 0.5–2 cm of the crust is densely populated by orange-brown unicellular cyanobacteria. Below, a layer of green-colored filamentous cyanobacteria is found. Underneath, a bright purple layer of anoxygenic phototrophs is present, below which a reduced black layer is found. We have investigated the biological properties of this crust using a wide variety of techniques, and we here review the results of these interdisciplinary studies. The tests performed included microscopic examination of the biota, phylogenetic analyses based on 16S rRNA gene clone libraries and denaturing gradient gel electrophoresis, fatty acid analysis, light intensity and light quality measurements, microelectrode studies of oxygen profiles and oxygen evolution, determination of sulfate reduction using radioisotope methods, and measurement of methane evolution. The stable vertical stratification in the system enabled separate analyses of the different layers with a high spatial resolution. It was therefore possible to combine the different approaches and obtain information on the activities of the different types of oxygenic and anoxygenic phototrophs, dissimilatory sulfate reducers and methanogens in the different layers, as well as phylogenetic information on the nature of the microorganisms responsible for these processes. The gypsum crust thus becomes a paradigm for the study of a wide variety of microbial processes and their interrelationships in the presence of high salt concentrations. Guest Editors: J. John & B. Timms Salt Lake Research: Biodiversity and Conservation—Selected papers from the 9th Conference of the International Society for Salt Lake Research     

The structure and biogeochemical activity of the phototrophic communities from the Bol'sherechenskii alkaline hot spring

Microbial communities growing in the bed of the alkaline, sulfide hot spring Bol'sherechenskii (the Baikal rift area) were studied over many years (1986-2001). The effluent water temperature ranged from 72 to 74 degrees C, pH was from 9.25 to 9.8, and sulfide content was from 12 to 13.4 mg/ml. Simultaneous effects of several extreme factors restrict the spread of phototrophic microorganisms. Visible microbial fouling appears with a decrease in the temperature to 62 degrees C and in the sulfide content to 5.9 mg/l. Cyanobacteria predominated in all biological zones of the microbial mat. The filamentous cyanobacteria of the genus Phormidium are the major mat-forming organisms, whereas unicellular cyanobacteria and the filamentous green bacterium Chloroflexus aurantiacus are minor components of the phototrophic communities. No cyanobacteria of the species Mastigocladus laminosus, typical of neutral and subacid springs, were identified. Seventeen species of both anoxygenic phototrophic bacteria and cyanobacteria were isolated from the microbial mats, most of which exhibited optimum growth at 20 to 45 degrees C. The anoxygenic phototrophs were neutrophiles with pH optimum at about 7. The cyanobacteria were the most adapted to the alkaline conditions in the spring. Their optimum growth was observed at pH 8.5-9.0. As determined by the in situ radioisotope method, the optimal growth and decomposition rates were observed at 40-32 degrees C, which is 10 to 15 degrees C lower than the same parameter in the sulfide-deficient Octopus Spring (Yellowstone, United States). The maximum chlorophyll a concentration was 555 mg/m2 at 40 degrees C. Total rate of photosynthesis in the mats reached 1.3 g C/m2 per day. The maximum rate of dark fixation of carbon dioxide in the microbial mats was 0.806 g C/m2 per day. The maximum rate of sulfate reduction comprised 0.367 g S/m2 per day at 40 degrees C. The rate of methanogenesis did not exceed 1.188 micrograms C/m2 per day. The role of methanogenesis in the terminal decomposition of the organic matter was insignificant. Methane formation consumed 100 times less organic matter than sulfate reduction.     

Characterization of functional bacterial groups in a hypersaline microbial mat community (Salins-de-Giraud, Camargue, France)

A photosynthetic microbial mat was investigated in a large pond of a Mediterranean saltern (Salins-de-Giraud, Camargue, France) having water salinity from 70 per thousand to 150 per thousand (w/v). Analysis of characteristic biomarkers (e.g., major microbial fatty acids, hydrocarbons, alcohols and alkenones) revealed that cyanobacteria were the major component of the pond, in addition to diatoms and other algae. Functional bacterial groups involved in the sulfur cycle could be correlated to these biomarkers, i.e. sulfate-reducing, sulfur-oxidizing and anoxygenic phototrophic bacteria. In the first 0.5 mm of the mat, a high rate of photosynthesis showed the activity of oxygenic phototrophs in the surface layer. Ten different cyanobacterial populations were detected with confocal laser scanning microscopy: six filamentous species, with Microcoleus chthonoplastes and Halomicronema excentricum as dominant (73% of total counts); and four unicellular types affiliated to Microcystis, Chroococcus, Gloeocapsa, and Synechocystis (27% of total counts). Denaturing gradient gel electrophoresis of PCR-amplified 16S rRNA gene fragments confirmed the presence of Microcoleus, Oscillatoria, and Leptolyngbya strains (Halomicronema was not detected here) and revealed additional presence of Phormidium, Pleurocapsa and Calotrix types. Spectral scalar irradiance measurements did not reveal a particular zonation of cyanobacteria, purple or green bacteria in the first millimeter of the mat. Terminal-restriction fragment length polymorphism analysis of PCR-amplified 16S rRNA gene fragments of bacteria depicted the community composition and a fine-scale depth-distribution of at least five different populations of anoxygenic phototrophs and at least three types of sulfate-reducing bacteria along the microgradients of oxygen and light inside the microbial mat.     

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.     

The Structure and Biogeochemical Activity of the Phototrophic Communities from the Bol'sherechenskii Alkaline Hot Spring

Microbial communities growing in the bed of the alkaline, sulfide hot spring Bol'sherechenskii (the Baikal rift area) were studied over many years (1986–2001). The effluent water temperature ranged from 72 to 74°C, pH was from 9.25 to 9.8, and sulfide content was from 12 to 13.4 mg/ml. Simultaneous effects of several extreme factors restrict the spread of phototrophic microorganisms. Visible microbial mat appears with a decrease in the temperature to 62°C and in sulfide content to 5.9 mg/l. Cyanobacteria predominated in all biological zones of the microbial mat. The filamentous cyanobacteria of the genus Phormidium are the major mat-forming organisms, whereas unicellular cyanobacteria and the filamentous green bacterium Chloroflexus aurantiacus are minor components of the phototrophic communities. No cyanobacteria of the species Mastigocladus laminosus, typical of neutral and subacid springs, were identified. Seventeen species of both anoxygenic phototrophic bacteria and cyanobacteria were isolated from the microbial mats, most of which exhibited optimum growth at 20 to 45°C. The anoxygenic phototrophs were neutrophiles with pH optimum at about 7. The cyanobacteria were the most adapted to the alkaline conditions in the spring. Their optimum growth was observed at pH 8.5–9.0. As determined by the in situ radioisotope method, the optimal growth and decomposition rates were observed at 40–32°C, which is 10–15°C lower than the same parameter in the sulfide-deficient Octopus Spring (Yellowstone, United States). The maximum chlorophyll a concentration was 555 mg/m² at 40°C. The total rate of photosynthesis in the mats reached 1.3 g C/m² per day. The maximum rate of dark fixation of carbon dioxide in the microbial mats was 0.806 g C/m² per day. The maximum rate of sulfate reduction comprised 0.367 g S/m² per day at 40°C. The rate of methanogenesis did not exceed 1.188 g C/m² per day. The role of methanogenesis in the terminal decomposition of the organic matter was insignificant. Methane formation consumed 100 times less organic matter than sulfate reduction.     

Anoxygenic Photosynthesis and Nitrogen Fixation by a Microbial Mat Community in a Bahamian Hypersaline Lagoon

Simultaneous measurements of photosynthesis (both oxygenic and anoxygenic) and N(inf2) fixation were conducted to discern the relationships between photosynthesis, N(inf2) fixation, and environmental factors potentially regulating these processes in microbial mats in a tropical hypersaline lagoon (Salt Pond, San Salvador Island, Bahamas). Major photoautotrophs included cyanobacteria, purple phototrophic bacteria, and diatoms. Chemosystematic photopigments were used as indicators of the relative abundance of mat phototrophs. Experimental manipulations consisted of light and dark incubations of intact mat samples exposed to the photosystem II inhibitor DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea], a dissolved organic carbon source (D-glucose), and normal seawater (37(permil)). Photosynthetic rates were measured by both O(inf2) and (sup14)C methods, and nitrogenase activity (NA) was estimated by the acetylene reduction assay. Moderate reductions in salinity (from 74 to 37(permil)) had no measurable effect on photosynthesis, O(inf2) consumption, or NA. CO(inf2) fixation in DCMU-amended samples was (symbl)25% of that in the control (nonamended) samples and demonstrated photosynthetic activity by anoxygenic phototrophs. NA in DCMU-amended samples, which was consistently higher (by a factor of 2 to 3) than the other (light and dark) treatments, was also attributed to purple phototrophic bacteria. The ecological implication is that N(inf2) fixation by anoxygenic phototrophs (purple phototrophic bacteria and possibly cyanobacteria) may be regulated by the activity of oxygenic phototrophs (cyanobacteria and diatoms). Consortial interactions that enhance the physiological plasticity of the mat community may be a key for optimizing production, N(inf2) fixation, and persistence in these extreme environments.     

Biogeochemistry of a gypsum-encrusted microbial ecosystem

Gypsum crusts containing multicolored stratified microbial populations grow in the evaporation ponds of a commercial saltern in Eilat, Israel. These crusts contain two prominent cyanobacterial layers, a bright purple layer of anoxygenic phototrophs, and a lower black layer with active sulphate reduction. We explored the diel dynamics of oxygen and sulphide within the crust using specially constructed microelectrodes, and further explored the crust biogeochemistry by measuring rates of sulphate reduction, stable sulphur isotope composition, and oxygen exchange rates across the crust–brine interface. We explored crusts from ponds with two different salinities, and found that the crust in the highest salinity was the less active. Overall, these crusts exhibited much lower rates of oxygen production than typical organic‐rich microbial mats. However, this was mainly due to much lower cell densities within the crusts. Surprisingly, on a per cell‐volume basis, rates of photosynthesis were similar to organic‐rich microbial mats. Due to relatively low rates of oxygen production and deep photic zones extending from 1.5 to 3 cm depth, a large percentage of the oxygen produced during the day accumulated into the crusts. Indeed, only between 16% to 34% of the O₂ produced in the crust escaped, and the remainder was internally recycled, used mainly in O₂ respiration. We view these crusts as potential homologs to ancient salt‐encrusted microbial ecosystems, and we compared them to the 3.45 billion‐year‐old quartz barite deposits from North Pole, Australia, which originally precipitated gypsum.     

Effects of imposed salinity gradients on dissimilatory arsenate reduction, sulfate reduction, and other microbial processes in sediments from two California soda lakes

Salinity effects on microbial community structure and on potential rates of arsenate reduction, arsenite oxidation, sulfate reduction, denitrification, and methanogenesis were examined in sediment slurries from two California soda lakes. We conducted experiments with Mono Lake and Searles Lake sediments over a wide range of salt concentrations (25 to 346 g liter(-1)). With the exception of sulfate reduction, rates of all processes demonstrated an inverse relationship to total salinity. However, each of these processes persisted at low but detectable rates at salt saturation. Denaturing gradient gel electrophoresis analysis of partial 16S rRNA genes amplified from As(V) reduction slurries revealed that distinct microbial populations grew at low (25 to 50 g liter(-1)), intermediate (100 to 200 g liter(-1)), and high (>300 g liter(-1)) salinity. At intermediate and high salinities, a close relative of a cultivated As-respiring halophile was present. These results suggest that organisms adapted to more dilute conditions can remain viable at high salinity and rapidly repopulate the lake during periods of rising lake level. In contrast to As reduction, sulfate reduction in Mono Lake slurries was undetectable at salt saturation. Furthermore, sulfate reduction was excluded from Searles Lake sediments at any salinity despite the presence of abundant sulfate. Sulfate reduction occurred in Searles Lake sediment slurries only following inoculation with Mono Lake sediment, indicating the absence of sulfate-reducing flora. Experiments with borate-amended Mono Lake slurries suggest that the notably high (0.46 molal) concentration of borate in the Searles Lake brine was responsible for the exclusion of sulfate reducers from that ecosystem.     

Effects of Imposed Salinity Gradients on Dissimilatory Arsenate Reduction, Sulfate Reduction, and Other Microbial Processes in Sediments from Two California Soda Lakes

Salinity effects on microbial community structure and on potential rates of arsenate reduction, arsenite oxidation, sulfate reduction, denitrification, and methanogenesis were examined in sediment slurries from two California soda lakes. We conducted experiments with Mono Lake and Searles Lake sediments over a wide range of salt concentrations (25 to 346 g liter⁻¹). With the exception of sulfate reduction, rates of all processes demonstrated an inverse relationship to total salinity. However, each of these processes persisted at low but detectable rates at salt saturation. Denaturing gradient gel electrophoresis analysis of partial 16S rRNA genes amplified from As(V) reduction slurries revealed that distinct microbial populations grew at low (25 to 50 g liter⁻¹), intermediate (100 to 200 g liter⁻¹), and high (>300 g liter⁻¹) salinity. At intermediate and high salinities, a close relative of a cultivated As-respiring halophile was present. These results suggest that organisms adapted to more dilute conditions can remain viable at high salinity and rapidly repopulate the lake during periods of rising lake level. In contrast to As reduction, sulfate reduction in Mono Lake slurries was undetectable at salt saturation. Furthermore, sulfate reduction was excluded from Searles Lake sediments at any salinity despite the presence of abundant sulfate. Sulfate reduction occurred in Searles Lake sediment slurries only following inoculation with Mono Lake sediment, indicating the absence of sulfate-reducing flora. Experiments with borate-amended Mono Lake slurries suggest that the notably high (0.46 molal) concentration of borate in the Searles Lake brine was responsible for the exclusion of sulfate reducers from that ecosystem.     

Anoxygenic phototrophic bacteria from extreme environments

A diverse group of anoxygenic phototrophic bacteria thrive in habitats characterized by extremes of temperature, pH, or salinity. These `extremophilic' anoxygenic phototrophs are optimally adapted to the conditions of their habitats and are ideal model systems for defining the physiochemical limits of photosynthesis. Extremophilic phototrophs have provided new insight into the evolution of photosynthesis and play ecological roles as primary producers in their unusual habitats. This revised version was published online in August 2006 with corrections to the Cover Date.     

Diurnal Cycles of Sulfate Reduction under Oxic Conditions in Cyanobacterial Mats

Diurnal cycles of sulfate reduction were examined in a well-developed cyanobacterial mat which grew in an outdoor experimental hypersaline pond system at a constant salinity of 75 ± 5% for 3 years. Vertical profiles of sulfate reduction were determined for the upper 12 mm of the microbial mat. Sulfate reduction activities were compared with diurnal variations of oxygen and sulfide concentrations measured by microelectrodes. Significant activity of sulfate-reducing bacteria was detected under aerobic conditions during the daytime, with maximal activity at 2 p.m. When comparing sulfate reduction activities in sediment cores taken at 6 a.m. and 12 a.m. and incubated at a constant temperature in the light and in the dark, a distinct stimulation of the activity in the vertical profile of sulfate reduction by light was evident. It is therefore concluded that the maximal in situ activities, measured at 2 p.m. in the chemocline of the cyanobacterial mat, cannot be attributed to diurnal changes of temperature alone. The response of sulfate-reducing bacteria to the addition of specific carbon sources was significantly different in the cyanobacterial layer, the anoxygenic phototrophic bacterial layer, and the permanently reduced layer of the microbial mat. Sulfate reduction in the mat layer exposed to high oxygen concentrations as a result of cyanobacterial oxygenic photosynthesis was enhanced only by glycolate; in the microzone where the chemocline is found during the daytime, ethanol was the only carbon source to enhance sulfate reduction, while both ethanol and lactate enhanced this activity in the permanently reduced zone.     

Effect of salinity changes on the bacterial diversity, photosynthesis and oxygen consumption of cyanobacterial mats from an intertidal flat of the Arabian Gulf

The effects of salinity fluctuation on bacterial diversity, rates of gross photosynthesis (GP) and oxygen consumption in the light (OCL) and in the dark (OCD) were investigated in three submerged cyanobacterial mats from a transect on an intertidal flat. The transect ran 1 km inland from the low water mark along an increasingly extreme habitat with respect to salinity. The response of GP, OCL and OCD in each sample to various salinities (65 per thousand, 100 per thousand, 150 per thousand and 200 per thousand) were compared. The obtained sequences and the number of unique operational taxonomic units showed clear differences in the mats' bacterial composition. While cyanobacteria decreased from the lower to the upper tidal mat, other bacterial groups such as Chloroflexus and Cytophaga/Flavobacteria/Bacteriodetes showed an opposite pattern with the highest dominance in the middle and upper tidal mats respectively. Gross photosynthesis and OCL at the ambient salinities of the mats decreased from the lower to the upper tidal zone. All mats, regardless of their tidal location, exhibited a decrease in areal GP, OCL and OCD rates at salinities > 100 per thousand. The extent of inhibition of these processes at higher salinities suggests an increase in salt adaptation of the mats microorganisms with distance from the low water line. We conclude that the resilience of microbial mats towards different salinity regimes on intertidal flats is accompanied by adjustment of the diversity and function of their microbial communities.     

Characterization of the endoevaporitic microbial communities in a hypersaline gypsum crust by fatty acid analysis

We have used fatty acid analyses to study the community structure of a layered endoevaporitic microbial community within a gypsum crust that covers the bottom of a saltern evaporation pond in Eilat, Israel. This community, living at a salinity of 218–238 g l⁻¹ total dissolved salts, consists of an upper brown layer dominated by unicellular cyanobacteria, a green layer with filamentous cyanobacteria, a red-purple layer with both Chromatium and Ectothiorhodospira/Halorhodospira type of purple sulfur bacteria, and a black layer in which dissimilatory sulfate reduction occurs. An olive-green layer is sometimes present below the red-purple layer. Analysis by gas chromatography/mass spectrometry of the fatty acid methyl esters prepared from the different fractions showed characteristic patterns in each layer, and these could be related to fatty acid composition data from the literature and to fatty acid analyses of representative halophilic microorganisms isolated from the site. The nature of the fatty acids in the green layer suggests that the cyanobacteria present there use the oxygen-independent pathway for production of unsaturated fatty acids, a pathway only occasionally encountered in filamentous cyanobacteria. The facultative anaerobic nature of the cyanobacteria in the green layer was confirmed by their ability to perform anoxygenic photosynthesis with sulfide as electron donor. Specific signature fatty acids identified for each layer corresponded well with the microscopic and functional analysis of the biota present. Guest Editor: John M. Melack Saline Waters and their Biota     

Effect of salinity on diazotrophic activity and microbial composition of phototrophic communities from Bitter-1 soda lake (Kulunda Steppe,Russia)

Bitter-1 is a shallow hypersaline soda lake in Kulunda Steppe (Altai region, Russia). During a study period between 2005 and 2016, the salinity in the littoral area of the lake fluctuated within the range from 85 to 400 g/L (in July of each year). Light-dependent nitrogen fixation occurred in this lake up to the salt-saturating conditions. The rates increased with a decrease in salinity, both under environmental conditions and in laboratory simulations. The salinities below 100 g/L were favorable for light-dependent nitrogen fixation, while the process was dramatically inhibited above 200 g/L salts. The analysis of nifH genes in environmental samples and in enrichment cultures of diazotrophic phototrophs suggested that anaerobic fermenting and sulfate-reducing bacteria could participate in the dark nitrogen fixation process up to soda-saturating conditions. However, we cannot exclude the possibility that haloalkaliphilic nonheterocystous cyanobacteria (Euhalothece sp. and Geitlerinema sp.) and anoxygenic purple sulfur bacteria (Ectothiorhodospira sp.) might also play a role in the process at light conditions. The heterocystous cyanobacterium Nodularia sp. develops at low salinity (below 80 g/L) that is not characteristic for Bitter-1 Lake and thus does not make a significant contribution to the nitrogen fixation in this lake.

     

Sulfate Reducers Can Outcompete Methanogens at Freshwater Sulfate Concentrations

Acetate and hydrogen metabolism by sulfate reducers and methanogens in the profundal sediments of an oligotrophic lake were examined. Inhibition of sulfate reduction with molybdate stimulated methane production from both hydrogen and acetate. Molybdate did not stimulate methane production in sediments that were preincubated to deplete the sulfate pool. Sulfate reduction accounted for 30 to 81% of the total of terminal metabolism proceeding through sulfate reduction and methane production in Eckman grab samples of surface sediments. The ability of sulfate reducers to effectively compete with methanogens for acetate was related to the sulfate reducers' lower half-saturation constant for acetate metabolism at in situ sulfate concentrations. Processes other than sulfate reduction and methanogenesis consumed hydrogen at elevated hydrogen partial pressures and prevented a kinetic analysis of hydrogen uptake by sulfate reducers and methanogens. The demonstration that sulfate reducers can successfully compete with methanogens for hydrogen and acetate in sediments at in situ sulfate concentrations of 60 to 105 μM extends the known range of sediment habitats in which sulfate reduction can be a dominant terminal process.     

High abundances of aerobic anoxygenic phototrophs in saline steppe lakes

We studied the distribution of anoxygenic phototrophs in 23 steppe lakes in the Transbaikal region (Russia), in Uzbekistan (Central Asia) and in the Crimean peninsula (Ukraine). The lakes varied in their mineral content and composition (salinities from 0.2 to 300 g L(-1) ). The Transbaikal lakes were alkaline (pH>9), with high amounts of soda. The Uzbek and Crimean lakes were more pH neutral, frequently with high amounts of sulfates. The presence of anoxygenic phototrophs was registered by infrared epifluorescence microscopy, infrared fluorometry and pigment analyses. In mostly shallow, fully oxic lakes, the anoxygenic phototrophs represented 7-65% of the total prokaryotes, with the maxima observed in Transbaikal soda lakes Gorbunka (32%), Khilganta (65%), Zanday (58%) and Zun-Kholvo (46%). Some of the lakes contained over 1 μg bacteriochlorophyll L(-1) . In contrast, only small amounts of anoxygenic phototrophs were present in highly mineralized lakes (>100 g total salts L(-1) ); Borzinskoe, Tsagan-Nur (Transbaikal), Staroe (Crimea) and in the residual part of the south-west Aral Sea (Uzbekistan). The oxic environment and the specific diurnal changes of bacteriochlorophyll concentration observed suggest that the phototrophic community was mostly composed of aerobic anoxygenic phototrophs. The high abundances and bacteriochlorophyll concentrations point to an important role of aerobic anoxygenic phototrophs in the habitats studied.     

Diversity of phototrophic bacteria in microbial mats from Arctic hot springs (Greenland)

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.     

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.     

Pigments, light penetration, and photosynthetic activity in the multi-layered microbial mats of Great Sippewissett Salt Marsh, Massachusetts

The multi-layered microbial mats in the sand flats of Great Sippewissett Salt Marsh were found to have five distinct layers of phototrophic organisms. The top 1–3 mm contained oxygenic phototrophs. The lower 3–4 mm contained anoxygenic phototrophic bacteria. The uppermost gold layer contained diatoms and cyanobacteria, and chlorophyll a was the major chlorophyll. The next layer down was green and was composed of primarily filamentous cyanobacteria containing chlorophyll a. This was followed by a bright pink layer of bacteriochlorophyll b-containing purple sulfur bacteria. The lowest layer was a thin dull green layer of green sulfur bacteria containing bacteriochlorophyll c. The distribution of the chlorophylls with depth revealed that two-thirds of the total chlorophyll in the mat was composed of bacteriochlorophylls present in the anoxygenic phototrophys. The cyanobacterial layers and both purple sulfur bacterial layers had photoautotrophic activity. Light was attenuated in the uppermost layers so that less than 5% of the total radiation at the surface penetrated to the layers of anoxygenic phototrophys.