Solar Lake Research Articles

Deciphering the functional and structural complexity of the Solar Lake flat mat microbial benthic communities.

Fifty-five years ago, the extremophilic "Solar Lake" was discovered on the Red Sea shores, garnering microbiologists' interest worldwide from the 1970s to 1990s. Nevertheless, research on the lake paused at the turn of the millennium. In our study, we revisited the Solar Lake benthic community using a genome-centric approach and described the distinct microbial communities in the exposed versus shallow-water mat unveiling microbial zonation in the benthic communities surrounding the Solar Lake. Our findings highlighted the unique structural and functional adaptations employed by these microbial mat communities. Moreover, we report new methanogens and phototrophs, including an intriguing methanogen from the Archaeoglobales family. We describe how the Solar Lake's flat mat microbial community adapts to stressors like oxygen intrusion and drought due to summer water level changes, which provides insights into the genomic strategies of microbial communities to cope with altered and extreme environmental conditions.

Photosynthetic Versatility in the Genome of Geitlerinema sp. PCC 9228 (Formerly Oscillatoria limnetica 'Solar Lake'), a Model Anoxygenic Photosynthetic Cyanobacterium.

Anoxygenic cyanobacteria that use sulfide as the electron donor for photosynthesis are a potentially influential but poorly constrained force on Earth’s biogeochemistry. Their versatile metabolism may have boosted primary production and nitrogen cycling in euxinic coastal margins in the Proterozoic. In addition, they represent a biological mechanism for limiting the accumulation of atmospheric oxygen, especially before the Great Oxidation Event and in the low-oxygen conditions of the Proterozoic. In this study, we describe the draft genome sequence of Geitlerinema sp. PCC 9228, formerly Oscillatoria limnetica ‘Solar Lake’, a mat-forming diazotrophic cyanobacterium that can switch between oxygenic photosynthesis and sulfide-based anoxygenic photosynthesis (AP). Geitlerinema possesses three variants of psbA, which encodes protein D1, a core component of the photosystem II reaction center. Phylogenetic analyses indicate that one variant is closely affiliated with cyanobacterial psbA genes that code for a D1 protein used for oxygen-sensitive processes. Another version is phylogenetically similar to cyanobacterial psbA genes that encode D1 proteins used under microaerobic conditions, and the third variant may be cued to high light and/or elevated oxygen concentrations. Geitlerinema has the canonical gene for sulfide quinone reductase (SQR) used in cyanobacterial AP and a putative transcriptional regulatory gene in the same operon. Another operon with a second, distinct sqr and regulatory gene is present, and is phylogenetically related to sqr genes used for high sulfide concentrations. The genome has a comprehensive nif gene suite for nitrogen fixation, supporting previous observations of nitrogenase activity. Geitlerinema possesses a bidirectional hydrogenase rather than the uptake hydrogenase typically used by cyanobacteria in diazotrophy. Overall, the genome sequence of Geitlerinema sp. PCC 9228 highlights potential cyanobacterial strategies to cope with fluctuating redox gradients and nitrogen availability that occur in benthic mats over a diel cycle. Such dynamic geochemical conditions likely also challenged Proterozoic cyanobacteria, modulating oxygen production. The genetic repertoire that underpins flexible oxygenic/anoxygenic photosynthesis in cyanobacteria provides a foundation to explore the regulation, evolutionary context, and biogeochemical implications of these co-occurring metabolisms in Earth history.

Spirochaeta sinaica sp. nov., a halophilic spirochaete isolated from a cyanobacterial mat.

A strain of free-living obligately anaerobic, halophilic spirochaete, SLT, was isolated from a sample of a cyanobacterial mat of the hypersaline Solar Lake, Sinai shore. The strain had motile helical cells, 0.35-0.40 × 6-10 μm. Strain SLT exhibited high resistance to NaCl among known halophilic spirochaetes growing at NaCl concentrations from 2 to 12% (optimum growth at 7%). The strain grew at temperatures from 10 to 32 °C (optimum at 28 °C) and pH from 6 to 8.5 (optimum at pH 7.0-7.5). Carbohydrates, but not alcohols, organic acids or nitrogenous compounds (peptone, yeast extract and amino acids), were used as energy substrates for growth. Ethanol, acetate, lactate, H2 and CO2 were the products of glucose fermentation. Sulfide was produced in the presence of S0 or thiosulfate in the medium. The DNA G+C content was 44.7 mol%. Based on 16S rRNA gene sequence analysis, strain SLT clustered within the genus Spirochaeta, exhibiting 94.2 and 93.7% similarity with its closest relatives, Spirochaeta bajacaliforniensis DSM 160554T and Spirochaeta smaragdinae DSM 11293T, respectively; similarity with other species did not exceed 86%. The phenotypic and chemotaxonomic characteristics of the strain, as well as the results of phylogenetic analysis support the classification of strain SLT as representing a novel species of the genus Spirochaeta, for which the name Spirochaeta sinaica sp. nov. is proposed. The type strain is SLT ( = DSM 14994 = NIQEM U 783).

Egypt's Red Sea coast: phylogenetic analysis of cultured microbial consortia in industrialized sites.

The Red Sea possesses a unique geography, and its shores are rich in mangrove, macro-algal and coral reef ecosystems. Various sources of pollution affect Red Sea biota, including microbial life. We assessed the effects of industrialization on microbes along the Egyptian Red Sea coast at eight coastal sites and two lakes. The bacterial communities of sediment samples were analyzed using bacterial 16S rDNA pyrosequencing of V6-V4 hypervariable regions. The taxonomic assignment of 131,402 significant reads to major bacterial taxa revealed five main bacterial phyla dominating the sampled sites: Proteobacteria (68%), Firmicutes (13%), Fusobacteria (12%), Bacteriodetes (6%), and Spirochetes (0.03%). Further analysis revealed distinct bacterial consortia that primarily included (1) marine Vibrio spp.—suggesting a “marine Vibrio phenomenon”; (2) potential human pathogens; and (3) oil-degrading bacteria. We discuss two divergent microbial consortia that were sampled from Solar Lake West near Taba/Eilat and Saline Lake in Ras Muhammad; these consortia contained the highest abundance of human pathogens and no pathogens, respectively. Our results draw attention to the effects of industrialization on the Red Sea and suggest the need for further analysis to overcome the hazardous effects observed at the impacted sites.

Desulfovibrio brasiliensis sp. nov., a moderate halophilic sulfate-reducing bacterium from Lagoa Vermelha (Brazil) mediating dolomite formation

A novel halotolerant sulfate-reducing bacterium, Desulfovibrio brasiliensis strain LVform1, was isolated from sediments of a dolomite-forming hypersaline coastal lagoon, Lagoa Vermelha, in the state of Rio de Janeiro, Brazil. The cells are vibrio-shaped and 0.30 to 0.45 microm by 1.0 to 3.5 microm in size. These bacteria mediate the precipitation of dolomite [CaMg(CO3)2] in culture experiments. The strain was identified as a member of the genus Desulfovibrio in the delta-subclass of the Proteobacteria on the basis of its 16S rRNA gene sequence, its physiological and morphological properties. Strain LVform1 is obligate sodium-dependent and grows at NaCl concentrations of up to 15%. The 16S rRNA sequence revealed that this strain is closely related to Desulfovibrio halophilus (96.2% similarity) and to Desulfovibrio oxyclinae (96.8% similarity), which were both isolated from Solar Lake, a hypersaline coastal lake in the Sinai, Egypt. Strain LVform1 is barotolerant, growing under pressures of up to 370 bar (37 MPa). We propose strain LVform1 to be the type strain of a novel species of the genus Desulfovibrio, Desulfovibrio brasiliensis (type strain LVform1 = DSMZ No. 15816 and JCM No. 12178). The GenBank/EMBL accession number for the 16S rDNA sequence of strain LVform1 is AJ544687.

Vesiculation of thylakoids in Myxobaktron salinum Walsby strain PCC 8305

Myxobaktron salinum WALSBY strain PCC 8305 (also known as Dactylococcopsis salina) is a unicellular cyanobacterium isolated from hypersaline waters of Solar Lake (Sinai). This cyanobacterium is characterised by fusiform cells showing a netlike cytoplasm, i.e. keritomy. Transmission electron microscopy studies showed that the keritomy is due to the enlargement of thylakoid lumen in electron-transparent areas, i.e. intrathylakoidal vesicles. In young cells grown photo-autotrophically under standard conditions the content of intrathylakoidal vesicles appears mostly electron-transparent. With the culture ageing the content becomes more electron-dense, the intrathylakoidal vesicles are sometimes filled with small osmiophilic granules, and the vesiculation of thylakoids increases; under dark condition there is a progressive decrease of the electron-dense content up to complete disappearance of the osmiophilic granules. There is also a reduction of the dimension of intrathylakoidal vesicles. During growth under nitrogen depleted conditions there is a marked accumulation of low electron-dense granules inside the intrathylakoidal vesicles. These granules tend to disappear when the culture is transferred to dark conditions, and the intrathylakoidal vesicle content becomes completely electron-transparent. This behaviour is typical of carbohydrates, which are usually consumed under dark conditions. Using a specific staining (THIERY 1967) we confirmed the polysaccharidic nature of the content of the intrathylakoidal vesicles. The dynamic of the formation and role of these vesicles are discussed together with their occurrence among other unicellular cyanobacteria.

Bioremediation of oil by marine microbial mats.

Cyanobacterial mats developing in oil-contaminated sabkhas along the African coasts of the Gulf of Suez and in the pristine Solar Lake, Sinai, were collected for laboratory studies. Samples of both mats showed efficient degradation of crude oil in the light, followed by development of an intense bloom of Phormidium spp. and Oscillatoria spp. Isolated cyanobacterial strains, however, did not degrade crude oil in axenic cultures. Strains of sulfate-reducing bacteria and aerobic heterotrophs were capable of degrading model compounds of aliphatic and aromatic hydrocarbons. Results indicate that degradation of oil was done primarily by aerobic heterotrophic bacteria. The oxygenic photosynthesis of oil-insensitive cyanobacteria supplied the molecular oxygen for the efficient aerobic metabolism of organisms, such as Marinobacter sp. The diurnal shifts in environmental conditions at the mat surface, from highly oxic conditions in the light to anaerobic sulfide-rich habitat in the dark, may allow the combined aerobic and anaerobic degradation of crude oil at the mat surface. Hence, coastal cyanobacterial mats may be used for the degradation of coastline oil spills. Oxygen microelectrodes detected a significant inhibition of photosynthetic activity subsequent to oil addition. This prevailed for a few hours and then rapidly recovered. In addition, shifts in bacterial community structure following exposure to oil were determined by denaturing gradient gel electrophoresis of PCR-amplified fractions of 16S rRNA from eubacteria, cyanobacteria and sulfate-reducing bacteria. Since the mats used for the present study were obtained from oil-contaminated environments, they were believed to be preequilibrated for petroleum remediation. The mesocosm system at Eilat provided a unique opportunity to study petroleum degradation by mats formed under different salinities (up to 21%). These mats, dominated by cyanobacteria, can serve as close analogues to the sabkhas contaminated during the Gulf War in Kuwait and Saudi Arabia.

Oligonucleotide microarray for 16S rRNA gene-based detection of all recognized lineages of sulfate-reducing prokaryotes in the environment.

For cultivation-independent detection of sulfate-reducing prokaryotes (SRPs) an oligonucleotide microarray consisting of 132 16S rRNA gene-targeted oligonucleotide probes (18-mers) having hierarchical and parallel (identical) specificity for the detection of all known lineages of sulfate-reducing prokaryotes (SRP-PhyloChip) was designed and subsequently evaluated with 41 suitable pure cultures of SRPs. The applicability of SRP-PhyloChip for diversity screening of SRPs in environmental and clinical samples was tested by using samples from periodontal tooth pockets and from the chemocline of a hypersaline cyanobacterial mat from Solar Lake (Sinai, Egypt). Consistent with previous studies, SRP-PhyloChip indicated the occurrence of Desulfomicrobium spp. in the tooth pockets and the presence of Desulfonema- and Desulfomonile-like SRPs (together with other SRPs) in the chemocline of the mat. The SRP-PhyloChip results were confirmed by several DNA microarray-independent techniques, including specific PCR amplification, cloning, and sequencing of SRP 16S rRNA genes and the genes encoding the dissimilatory (bi)sulfite reductase (dsrAB).

Fine‐scale measurement of diffusivity in a microbial mat with nuclear magnetic resonance imaging

Noninvasive 1H‐nuclear magnetic resonance (NMR) imaging was used to investigate the diffusive properties of microbial mats in two dimensions. Pulsed field gradient NMR was used to acquire images of the H2O diffusion coefficient, Ds, and multiecho imaging NMR was used to obtain images of the water density in two structurally different microbial mats sampled from Solar Lake (Egypt). We found a pronounced lateral and vertical variability of both water density and water diffusion coefficient, correlated with the laminated and heterogeneous distribution of microbial cells and exopolymers within the mats. The average water density varied from 0.5 to 0.9, whereas the average water diffusion coefficient ranged from 0.4 to 0.9 relative to the values obtained in the stagnant water above the mat samples. The apparent water diffusivities estimated from NMR imaging compared well to apparent O2 diffusivities measured with a diffusivity microsensor. Analysis of measured O2 concentration profiles with a diffusion‐reaction model showed that both the magnitude of calculated rates and the depth distribution of calculated O2 consumption/production zones changed when the observed variations of diffusivity were taken into account. With NMR imaging, diffusivity can be determined at high spatial resolution, which can resolve inherent lateral and vertical heterogeneities found in most natural benthic systems.

Long-term compositional changes after transplant in a microbial mat cyanobacterial community revealed using a polyphasic approach.

Using a polyphasic approach that included microscopy, cultivation and 16S rRNA-based cultivation-independent molecular fingerprinting, we compared the cyanobacterial composition of Solar Lake microbial mats and samples thereof transplanted and maintained in new settings for extended periods of time. Significant changes in community composition, with clear replacement of the dominant cyanobacterium, Microcoleus chthonoplastes, were detected in all cases. The most dramatic shifts occurred in a sample kept in the laboratory for 3 years, which resulted in dominance by an Oscillatoria-like cyanobacterium whose 16S rRNA closely matched that of a morphologically similar isolate from mats in Mexico. Transfer of Solar Lake mat to an artificial experimental pond with incubation under seminatural conditions resulted in an increase in cyanobacterial diversity. Judging from the molecular signatures, two novel, previously unrecognized and phylogenetically well-delimited cyanobacterial populations became dominant. Through cultivation, one population was shown to correspond to a filamentous, non-heterocystous group of Cyanobacteria with very narrow trichomes (approximately equals 0.75-1.5 microm). The most dominant novel molecular signature, however, could not be identified by cultivation efforts or correlation with microscopy and, upon phylogenetic analyses, its 16S rRNA genes showed no particular close association to known cyanobacterial groups.

RELATIONSHIP BETWEEN ALGOSPHERE AND FREE WATER BACTERIA IN SOLAR LAKE (SINAI)

Cyanobacterial mats of solar lake were followed up during the four seasons of a year 1997 through daylight and night. Also, the free water microorganisms groups related to soil fertility were followed included: Total colony counts, asymbiotic-N-fixers, nitrifying bacteria, sulphur-oxidizing and reducing bacteria. The ratio between total phytoplanktons and bacteria in water was calculated as indicator for biological activity in the Solar Lake The obtained results showed that there are a positive correlation between the counts of free water bacteria and phytoplanktonic mass in the lake. The highest bacterial counts were recorded during day time of spring season. The ratio of algosphere to free water bacteria was generally more than one, reached highest value beginning of stratification period (autumn season). On the other hand at the highest figure of total aerobic bacterial counts and the lowest figure was recorded by sulphur reducing bacteria.

Irradiance and temperature regulation of oxygenic photosynthesis and O 2 consumption in a hypersaline cyanobacterial mat (Solar Lake, Egypt)

Short-term effects of temperature and irradiance on oxygenic photosynthesis and O2 consumption in a hypersaline cyanobacterial mat were investigated with O2 microsensors in a laboratory. The effect of temperature on O2 fluxes across the mat–water interface was studied in the dark and at a saturating high surface irradiance (2162 μmol photons m−2 s−1) in the temperature range from 15 to 45 °C. Areal rates of dark O2 consumption increased almost linearly with temperature. The apparent activation energy of 18 kJ mol−1 and the corresponding Q 10 value (25 to 35 °C) of 1.3 indicated a relative low temperature dependence of dark O2 consumption due to mass transfer limitations imposed by the diffusive boundary layer at all temperatures. Areal rates of net photosynthesis increased with temperature up to 40 °C and exhibited a Q 10 value (20 to 30 °C) of 2.8. Both O2 dynamics and rates of gross photosynthesis at the mat surface increased with temperature up to 40 °C, with the most pronounced increase of gross photosynthesis at the mat surface between 25 and 35 °C (Q 10 of 3.1). In another mat sample, measurements at increasing surface irradiances (0 to 2319 μmol photons m−2 s−1) were performed at 25, 33 (the in situ temperature) and 40 °C. At all temperatures, areal rates of gross photosynthesis saturated with no significant reduction due to photoinhibition at high irradiances. The initial slope and the onset of saturation (E k = 148 to 185 μmol photons m−2 s−1) estimated from P versus E d curves showed no clear trend with temperature, while maximal photosynthesis increased with temperature. Gross photosynthesis was stimulated by temperature at each irradiance except at the lowest irradiance of 54 μmol photons m−2 s−1, where oxygenic gross photosynthesis and also the thickness of the photic zone was significantly reduced at 40 °C. The compensation irradiance increased with temperature, from 32 μmol photons m−2 s−1 at 25 °C to 77 μmol photons m−2 s−1 at 40 °C, due to increased rates of O2 consumption relative to gross photosynthesis. Areal rates of O2 consumption in the illuminated mat were higher than dark O2 consumption at corresponding temperatures, due to an increasing O2 consumption in the photic zone with increasing irradiance. Both light and temperature enhanced the internal O2 cycling within hypersaline cyanobacterial mats.

Distribution and diversity of archaea corresponding to the limnological cycle of a hypersaline stratified lake (Solar lake, Sinai, Egypt).

The vertical and seasonal distribution and diversity of archaeal sequences was investigated in a hypersaline, stratified, monomictic lake, Solar Lake, Sinai, Egypt, during the limnological development of stratification and mixing. Archaeal sequences were studied via phylogenetic analysis of 16S rDNA sequences as well as denaturing gradient gel electrophoresis analysis. The 165 clones studied were grouped into four phylogenetically different clusters. Most of the clones isolated from both the aerobic epilimnion and the sulfide-rich hypolimnion were defined as cluster I, belonging to the Halobacteriaceae family. The three additional clusters were all isolated from the anaerobic hypolimnion. Cluster II is phylogenetically located between the genera Methanobacterium and Methanococcus. Clusters III and IV relate to two previously documented groups of uncultured euryarchaeota, remotely related to the genus Thermoplasma. No crenarchaeota were found in the water column of the Solar Lake. The archaeal community in the Solar Lake under both stratified and mixed conditions was dominated by halobacteria in salinities higher than 10%. During stratification, additional clusters, some of which may possibly relate to uncultured halophilic methanogens, were found in the sulfide- and methane-rich hypolimnion.

Short-term temperature effects on oxygen and sulfide cycling in a hypersaline cyanobacterial mat (Solar Lake, Egypt)

We investigated short-term temperature effects on oxygen and sulfide cycling with O 2 , pH, and H 2 S microsensors in a hypersaline cyanobacterial mat, incubated in darkness and at a down-welling irradiance, E d (PAR), of 425 μmol photons m -2 s -1 in a laboratory. The incubation temperature was increased from 25 to 40°C in 5°C intervals. Areal rates of gross and net photosynthesis, of O 2 consumption in the aphotic zone and of dark O 2 consumption were maximal at 30°C, i.e. close to the in situ temperature of the natural habitat. Areal rates of dark oxygen consumption showed only a minor temperature dependence as O 2 consumption was diffusion limited at all temperatures. Sulfide production increased strongly with temperature in both the dark and light incubated mat (Q 10 = 1.8 to 3.2), and this led to saturation of sulfide oxidation and an increased sulfide efflux out of the dark incubated mat, which was maximal at 35°C. In the uppermost layer of the dark incubated mat, pH decreased due to aerobic respiration, sulfide oxidation and fermentation, and this decrease was enhanced with temperature. In the light incubated mat, the thickness of the photic zone decreased with temperature from 0.9 to 0.5 mm. Oxygen penetration and peak oxygen concentration decreased with temperature, whereas the upper sulfide boundary and thus the zone of sulfide oxidation rose closer to the mat surface in the light incubated mat. Areal rates of sulfide oxidation increased more than 2-fold from 25 to 40°C in the light incubated mat. The relative contribution of sulfide oxidation to oxygen consumption in the aphotic zone increased significantly with temperature, indicating that at elevated temperatures incomplete sulfide oxidation occurred in the light incubated mat. Both the photosynthetically induced pH maximum and the overall pH of the mat decreased with increasing temperature due to enhanced heterotrophic activity, sulfide oxidation, and a changed depth distribution of these processes. Our data demonstrate a close coupling of oxygen and sulfur cycling in hypersaline microbial mats, that is strongly regulated by temperature.

Phytoplankton Activities in Hypersaline, Anoxic Conditions. I—Dynamics of Phytoplankton Succession in the Solar Lake (Taba, Egypt)

The seasonal and vertical distribution of planktonic algae in the Solar Lake (Taba–Egypt) and their relation to some aquatic environmental factors were discussed through the period (Sep. 1996–Aug. 1997). Surface water analysis showed that no remarkable changes were observed in nitrate, silicate and iron, while turbidity, salinity and chloride were changed according to the season. In water column profile, H2S, temperature and salinity showed clear variations between stratification and holomixis. Maximum salinity (199‰) was recorded during stratification at 3-4 m depth. Stratification (Oct-June) appears clearly with changes in temperature (surface 16°C, thermocline up to 53.3°C and the bottom temperature around 47.7°C). Cyanobacterial mats of the Solar Lake are classified into two layers, the upper layer was dominated by coccoide forms while the lower was dominated by filamentous forms. During the period of study, 39 algal species were identified among which were 17 species of Cyanobacteria, 18 species of Bacillariophyta and 2 species of either Chlorophyta or Dinoflagellates.

Phytoplankton Activities in Hypersaline, Anoxic Conditions. II—Photosynthetic Activity of some Sulphide Adapted Cyanobacterial Strains Isolated from Solar Lake, Taba, Egypt

Five cyanobacterial strains namely Oscillatoria limnetica, Aphanotheca stagnina, Synechococcus aeruginosus, Spirulina tenissima and Dactylococcopsis salina were isolated from an anaerobic sulphide rich hypersaline Solar Lake, Taba- Egypt. At oxygenic conditions Spirulina was the most tolerant strain to sulphide concentrations, while Aphanotheca and Synechococcus were the most sensitive. Aphanotheca have higher anoxygenic photoassimilation activity under a broad range of sulphide concentrations. The response of the five cyanobacterial strains to sulphide under oxygenic and anoxygenic conditions depends on the organism, time of exposure, presence or absence of DCMU and also on sulphide concentration.

Excretion products of algae and their occurrence in Solar Lake. Taba, Egypt

The present study aimed to investigate the nature of organic compounds liable to be released by blue-green algae (cyanobacteria) and organic compounds detected in the Solar Lake in Taba, Egypt. The liquid-liquid extraction method and liquid chromatography technique were applied for extraction, clean-up and separation of the organics. Gas chromatography/mass spectrometry (GC/MS) was used to carry out qualitative analysis. The most dominant species in the algal samples collected from Solar lake in Taba are Aphanotheca stagnina, Synechococcus aeruginosus, Oscillatoria limnetica and Microcoleus chthonoplastes. n-Alkane hydrocarbons that ranged from C12 to C25 were identified in lake water samples, most of them were derived from algal species. Several fatty acids were isolated from water samples with predominant tridecanoic acid (C13), tetradecanoic acid (C14) and hexadecanoic acid (C16). Meanwhile, the identification of fatty acids from algal suspension samples indicated the predominance of hexadecanoic acid (C16) and heptadecanoic acid (C17), with carbon number that ranged from C10 to C17. The dominant compounds were C15, C17, C17:1, C19 n-alkanes. C13, C15, C16 fatty acids, alkyl esters of propyl decanoate, esters of benzenedicarboxlate, diphenyl amine, geosmin and 2-methyl isoborneol (2-MIB) in water and in algal suspension samples were probably enough to distinguish the cyanobacterial mat community of a saline lake shore.

Phytoplankton activities in hypersaline, anoxic conditions. I—Dynamics of phytoplankton succession in the solar lake (Taba, Egypt)

The seasonal and vertical distribution of planktonic algae in the Solar Lake (Taba-Egypt) and their relation to some aquatic environmental factors were discussed through the period (Sep. 1996-Aug. 1997). Surface water analysis showed that no remarkable changes were observed in nitrate, silicate and iron, while turbidity, salinity and chloride were changed according to the season. In water column profile, H2S, temperature and salinity showed clear variations between stratification and holomixis. Maximum salinity (199%.) was recorded during stratification at 3–4 m depth. Stratification (Oct-June) appears clearly with changes in temperature (surface 16°C, thermocline up to 53.3°C and the bottom temperature around 47.7°C). Cyanobacterial mats of the Solar Lake are classified into two layers, the upper layer was dominated by coccoide forms while the lower was dominated by filamentous forms. During the period of study, 39 algal species were identified among which were 17 species of Cyanobacteria, 18 species of Bacillariophyta and 2 species of either Chlorophyta or Dinoflagellates.

Phytoplankton activities in hypersaline, anoxic conditions. II-Photosynthetic activity of some sulphide adapted cyanobacterial strains isolated from Solar Lake, Taba, Egypt

Five cyanobacterial strains namely Oscillatoria limnetica, Aphanotheca stagnina, Synechococcus aeruginosus, Spirulina tenissima and Dactylococcopsis salina were isolated from an anaerobic sulphide rich hypersaline Solar Lake, Taba- Egypt. At oxygenic conditions Spirulina was the most tolerant strain to sulphide concentrations, while Aphanotheca and Synechococcus were the most sensitive. Aplianotheca have higher anoxygenic photoassimilation activity under a broad range of sulphide concentrations. The response of the five cyanobacterial strains to sulphide under oxygenic and anoxygenic conditions depends on the organism, time of exposure, presence or absence of DCMU and also on sulphide concentration.

Sulfate-Reducing Bacteria and Their Activities in Cyanobacterial Mats of Solar Lake (Sinai, Egypt)

The sulfate-reducing bacteria within the surface layer of the hypersaline cyanobacterial mat of Solar Lake (Sinai, Egypt) were investigated with combined microbiological, molecular, and biogeochemical approaches. The diurnally oxic surface layer contained between 10(6) and 10(7) cultivable sulfate-reducing bacteria ml-1 and showed sulfate reduction rates between 1,000 and 2, 200 nmol ml-1 day-1, both in the same range as and sometimes higher than those in anaerobic deeper mat layers. In the oxic surface layer and in the mat layers below, filamentous sulfate-reducing Desulfonema bacteria were found in variable densities of 10(4) to 10(6) cells ml-1. A Desulfonema-related, diurnally migrating bacterium was detected with PCR and denaturing gradient gel electrophoresis within and below the oxic surface layer. Facultative aerobic respiration, filamentous morphology, motility, diurnal migration, and aggregate formation were the most conspicuous adaptations of Solar Lake sulfate-reducing bacteria to the mat matrix and to diurnal oxygen stress. A comparison of sulfate reduction rates within the mat and previously published photosynthesis rates showed that CO2 from sulfate reduction in the upper 5 mm accounted for 7 to 8% of the total photosynthetic CO2 demand of the mat.