Photosynthesis versus Exopolymer Degradation in the Formation of Microbialites on the Atoll of Kiritimati,Republic of Kiribati,Central Pacific

Aragonitic microbialites, characterized by a reticulate fabric, were discovered beneath lacustrine microbial mats on the atoll of Kiritimati, Republic of Kiribati, Central Pacific. The microbial mats, with cyanobacteria as major primary producers, grow in evaporated seawater modified by calcium carbonate and gypsum precipitation and calcium influx via surface and/or groundwaters. Despite the high aragonite supersaturation and a high photosynthetic activity, only minor aragonite precipitates are observed in the top parts of the microbial mats. Instead, major aragonite precipitation takes place in lower mat parts at the transition to the anoxic zone. The prokaryotic community shows a high number of phylotypes closely related to halotolerant taxa and/or taxa with preference to oligotrophic habitats. Soil- and plant- inhabiting bacteria underline a potential surface or subsurface influx from terrestrial areas, while chitinase-producing representatives coincide with the occurrence of insect remains in the mats. Strikingly, many of the clones have their closest relatives in microorganisms either involved in methane production or consumption of methane or methyl compounds. Methanogens, represented by the methylotrophic genus Methanohalophilus, appear to be one of the dominant organisms in anaerobic mat parts. All this points to a significant role of methane and methyl components in the carbon cycle of the mats. Nonetheless, thin sections and physicochemical gradients through the mats, as well as the ¹²C-depleted carbon isotope signatures of carbonates indicate that spherulitic components of the microbialites initiate in the photosynthesis-dominated orange mat top layer, and further grow in the green and purple layer below. Therefore, these spherulites are considered as product of an extraordinary high photosynthesis effect simultaneous to a high inhibition by pristine exopolymers. Then, successive heterotrophic bacterial activity leads to a condensation of the exopolymer framework, and finally to the formation of crevice-like zones of partly degraded exopolymers. Here initiation of horizontal aragonite layers and vertical aragonite sheets of the microbialite occurs, which are considered as a product of high photosynthesis at decreasing degree of inhibition. Finally, at low supersaturation and almost lack of inhibition, syntaxial growth of aragonite crystals at lamellae surfaces leads to thin fibrous aragonite veneers. While sulfate reduction, methylotrophy, methanogenesis and ammonification play an important role in element cycling of the mat, there is currently no evidence for a crucial role of them in CaCO₃ precipitation. Instead, photosynthesis and exopolymer degradation sufficiently explain the observed pattern and fabric of microbialite formation.     

Physical, chemical, and microbiological characteristics of microbial mats (kopara) in the South Pacific atolls of French Polynesia.

Microbial mats that develop in shallow brackish and hyposaline ponds in the rims of two French polynesian atolls (Rangiroa and Tetiaroa) were intensively investigated during the past three years. Comparative assessment of these mats (called kopara in polynesian language) showed remarkable similarities in their composition and structure. Due to the lack of iron, the color of the cyanobacterial pigments produced remained visible through the entire depth of the mats (20-40 cm depth), with alternate green, purple, and pink layers. Profiles of oxygen, sulfide, pH, and redox showed the anoxia of all mats from a depth of 2-3 mm. Analyses of bacterial pigments and bacterial lipids showed that all mats consisted of stratified layers of cyanobacteria (mainly Phormidium, Schizothrix, Scytonema) and purple and green phototrophic bacteria. The purple and green phototrophic bacteria cohabit with sulfate reducers (Desulfovibrio and Desulfobacter) and other heterotrophic bacteria. The microscopic bacterial determination emphasized the influence of salinity on the bacterial diversity, with higher diversity at low salinity, mainly for purple nonsulfur bacteria. Analyses of organic material and of exopolymers were also undertaken. Difference and similarities between mats from geomorphological, microbiological, and chemical points of view are discussed to provide multicriteria of classification of mats.     

Colorless Sulfur Bacteria, Beggiatoa spp. and Thiovulum spp., in O2 and H2S Microgradients

The interactions between colorless sulfur bacteria and the chemical microgradients at the oxygen-sulfide interface were studied in Beggiatoa mats from marine sediments and in Thiovulum veils developing above the sediments. The gradients of O₂, H₂S, and pH were measured by microelectrodes at depth increments of 50 μm. An unstirred boundary layer in the water surrounding the mats and veils prevented microturbulent or convective mixing of O₂ and H₂S. The two substrates reached the bacteria only by molecular diffusion through the boundary layer. The bacteria lived as microaerophiles or anaerobes even under stirred, oxic water. Oxygen and sulfide zones overlapped by 50 μm in the bacterial layers. Both compounds had concentrations in the range of 0 to 10 μmol liter⁻¹ and residence times of 0.1 to 0.6 s in the overlapping zone. The sulfide oxidation was purely biological. Diffusion calculations showed that formation of mats on solid substrates or of veils in the water represented optimal strategies for the bacteria to achieve a stable microenvironment, a high substrate supply, and an efficient competition with chemical sulfide oxidation. The continuous gliding movement of Beggiatoa cells in mats or the flickering motion of Thiovulum cells in veils were important for the availability of both O₂ and H₂S for the individual bacteria.     

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.     

Photosynthesis,Respiration and Exopolymer Calcium-Binding in Biofilm Calcification (Westerhöfer and Deinschwanger Creek,Germany)

The impact of microbial activity on biofilm calcification in aquatic environments is still a matter of debate, especially in settings where ambient water has high CaCO₃ mineral supersaturation. In this study, biofilms of two CO₂-degassing karst-water creeks in Germany, which attain high calcite supersaturation during their course downstream, were investigated with regard to water chemistry of the biofilm microenvironment. The biofilms mainly consisted of filamentous cyanobacteria (Phormidium morphotype) and heterotrophic bacteria (including sulfate-reducing bacteria), which affect the microenvironment and produce acidic exopolymers. In situ and ex situ microelectrode measurements showed that a strong pH increase, coupled with Ca^{2 +} consumption, occurred in light conditions at the biofilm surface, while the opposite occurred in the dark. Calcite supersaturation at the biofilm surface, calculated from ex situ Ca^{2 +} and CO₃ ^2? microelectrode measurements, showed that photosynthesis resulted in high omega values during illumination, while respiration slightly lowered supersaturation values in the dark, compared to values in the water column. Dissociation calculation demonstrated that the potential amount of Ca^{2 +} binding by exopolymers would be insufficient to explain the Ca^{2 +} loss observed, although Ca^{2 +} complexation to exopolymers might be crucial for calcite nucleation. No spontaneous precipitation occurred on biofilm-free limestone substrates under the same condition, regardless of high supersaturation. These facts indicate that photosynthesis is a crucial mechanism to overcome the kinetic barrier for CaCO₃ precipitation, even in highly supersaturated settings.     

Molecular and lipid biomarker analysis of a gypsum‐hosted endoevaporitic microbial community

Modern evaporitic microbial ecosystems are important analogs for understanding the record of earliest life on Earth. Although mineral‐depositing shallow‐marine environments were prevalent during the Precambrian, few such environments are now available today for study. We investigated the molecular and lipid biomarker composition of an endoevaporitic gypsarenite microbial mat community in Guerrero Negro, Mexico. The 16S ribosomal RNA gene‐based phylogenetic analyses of this mat corroborate prior observations indicating that characteristic layered microbial communities colonize gypsum deposits world‐wide despite considerable textural and morphological variability. Membrane fatty acid analysis of the surface tan/orange and lower green mat crust layers indicated cell densities of 1.6 × 10⁹ and 4.2 × 10⁹ cells cm^?3, respectively. Several biomarker fatty acids, ?7,10‐hexadecadienoic, iso‐heptadecenoic, 10‐methylhexadecanoic, and a ?12‐methyloctadecenoic, correlated well with distributions of Euhalothece, Stenotrophomonas, Desulfohalobium, and Rhodobacterales, respectively, revealed by the phylogenetic analyses. Chlorophyll (Chl) a and cyanobacterial phylotypes were present at all depths in the mat. Bacteriochlorophyl (Bchl) a and Bchl c were first detected in the oxic‐anoxic transition zone and increased with depth. A series of monomethylalkanes (MMA), 8‐methylhexadecane, 8‐methylheptadecane, and 9‐methyloctadecane were present in the surface crust but increased in abundance in the lower anoxic layers. The MMA structures are similar to those identified previously in cultures of the marine Chloroflexus‐like organism ‘Candidatus Chlorothrix halophila’ gen. nov., sp. nov., and may represent the Bchl c community. Novel 3‐methylhopanoids were identified in cultures of marine purple non‐sulfur bacteria and serve as a probable biomarker for this group in the lower anoxic purple and olive‐black layers. Together microbial culture and environmental analyses support novel sources for lipid biomarkers in gypsum crust mats.     

Biomarkers from individual carbonate phases of an Oligocene cold‐seep deposit,Washington State,USA

An Oligocene cold‐seep limestone (Lincoln Creek Formation, Washington State, USA) was studied for its lipid biomarker inventory. Biomarker analysis on minute amounts of sample (tens of mg) and complementary ¹³C_carbonate analyses allowed us to link biogeochemical processes with individual, closely intertwined carbonate phases. The ancient seep deposit exhibits four major carbonate phases, according to the paragenetic sequence of (I) micrite, (II) yellow aragonite, (III) clear aragonite and (IV) equant calcite spar. For the micrite, varying but significant amounts of archaea‐derived isoprenoids clearly indicate that the precipitation of this phase was induced by the microbial anaerobic oxidation of methane (AOM). However, water column‐derived lipids present in this carbonate phase reflect the incorporation of organic matter from background sediment cemented by authigenic micrite. Yellow aragonite made up only a minor rock component (<10% vol.), but contained a major portion of lipid biomarkers indicative of AOM. Along with low δ¹³C_carbonate values (less than ?30‰ Pee Dee Belemnite), this points to an intimate spatial association of AOM consortia with the precipitation of yellow aragonite. Clear aragonite showed similar δ¹³C_carbonate values but much lower, if any, contents of AOM biomarkers. This suggests that AOM‐derived carbonate ions diffused over a greater distance to the site of precipitation compared with yellow aragonite. The latest phase, equant calcite spar, did not yield appreciable biomarkers, but showed a notable ¹³C_carbonate‐enrichment that is most likely caused by methanogenesis that prevailed in the sediments after AOM activity had ceased. A comparison of the ancient seep carbonates with modern counterparts from Hydrate Ridge (offshore Oregon, USA) revealed a remarkable coincidence of the respective mineral phases and their biomarker patterns. This suggests that the mechanisms of carbonate formation and the associated biogeochemical processes remained unchanged over geological times. □Biomarkers, carbonates, cold seeps, Hydrate Ridge, isotopes, Oligocene, Washington.     

Exopolymeric substances of sulfate-reducing bacteria: Interactions with calcium at alkaline pH and implication for formation of carbonate minerals

Sulfate‐reducing bacteria (SRB) have been recognized as key players in the precipitation of calcium carbonate in lithifying microbial communities. These bacteria increase the alkalinity by reducing sulfate ions, and consuming organic acids. SRB also produce copious amounts of exopolymeric substances (EPS). All of these processes influence the morphology and mineralogy of the carbonate minerals. Interactions of EPS with metals, calcium in particular, are believed to be the main processes through which the extracellular matrix controls the precipitation of the carbonate minerals. SRB exopolymers were purified from lithifying mat and type cultures, and their potential role in CaCO₃ precipitation was determined from acid‐base titrations and calcium‐binding experiments. Major EPS characteristics were established using infrared spectroscopy and gas chromatography to characterize the chemical functional groups and the sugar monomers composition. Our results demonstrate that all of the three SRB strains tested were able to produce large amounts of EPS. This EPS exhibited three main buffering capacities, which correspond to carboxylic acids (pK_a = 3.0), sulfur‐containing groups (thiols, sulfonic and sulfinic acids – pK_a = 7.0–7.1) and amino groups (pK_a = 8.4–9.2). The calcium‐binding capacity of these exopolymers in solution at pH 9.0 ranged from 0.12g_Ca g_EPS^?1–0.15 g_Ca g_EPS^?1. These results suggest that SRB could play a critical role in the formation of CaCO₃ in lithifying microbial mats. The unusually high sulfur content, which has not been reported for EPS before, indicates a possible strong interaction with iron. In addition to changing the saturation index through metabolic activity, our results imply that SRB affect the rock record through EPS production and its effect on the CaCO₃ precipitation. Furthermore, EPS produced by SRB may account for the incorporation of metals (e.g. Sr, Fe, Mg) associated with carbonate minerals in the rock record.     

Characterization of bacterial diversity associated with microbial mats,gypsum evaporites and carbonate microbialites in thalassic wetlands: Tebenquiche and La Brava,Salar de Atacama,Chile

In this paper, we report the presence of sedimentary microbial ecosystems in wetlands of the Salar de Atacama. These laminated systems, which bind, trap and precipitate mineral include: microbial mats at Laguna Tebenquiche and Laguna La Brava, gypsum domes at Tebenquiche and carbonate microbialites at La Brava. Microbial diversity and key biogeochemical characteristics of both lakes (La Brava and Tebenquiche) and their various microbial ecosystems (non-lithifying mats, flat and domal microbialites) were determined. The composition and abundance of minerals ranged from trapped and bound halite in organic-rich non-lithifying mats to aragonite-dominated lithified flat microbialites and gypsum in lithified domal structures. Pyrosequencing of the V4 region of the 16s rDNA gene showed that Proteobacteria comprised a major phylum in all of the microbial ecosystems studied, with a marked lower abundance in the non-lithifying mats. A higher proportion of Bacteroidetes was present in Tebenquiche sediments compared to La Brava samples. The concentration of pigments, particularly that of Chlorophyll a, was higher in the Tebenquiche than in La Brava. Pigments typically associated with anoxygenic phototrophic bacteria were present in lower amounts. Organic-rich, non-lithifying microbial mats frequently formed snake-like, bulbous structures due to gas accumulation underneath the mat. We hypothesize that the lithified microbialites might have developed from these snake-like microbial mats following mineral precipitation in the surface layer, producing domes with endoevaporitic communities in Tebenquiche and carbonate platforms in La Brava. Whereas the potential role of microbes in carbonate platforms is well established, the contribution of endoevaporitic microbes to formation of gypsum domes needs further investigation.     

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.     

Microbial mats in tidal channels at San Carlos,Baja California Sur,Mexico

Microbial communities of stratified phototrophic bacteria in laminated intertidal sediments north of Estuary El Puente, near San Carlos, Baja California Sur, Mexico,‐were studied. This study describes the macroscopic and microscopic characteristics of the mats, including their annual growth. The mats were located in and along meandering mangrove‐lined tidal channels. Their thickness ranged from 0.5 to 25 cm. Square‐meter areas of polygonal mats were detected in several ponds infiltrated by sea water. The principal microbial community of the upper surface of various morphotypes of microbial mats was identified as cyanobacteria belonging to the genera Microcoleus, Lyngbya, Phormidium, and Oscillatoria. Other cyanobacte‐rial genera such as Pseudanabaena, Spirulina, Synechococcus, and Gloeocapsa, as well as many unidentified diatoms, were also present but at lower population densities. The second inward reddish layers of the microbial mats contained similar cyano‐bacterial genera plus anoxygenic phototrophic bacteria belonging to the genera Chloroflexus, Thiocapsa, Chromatium, Prosthecochloris, Rhodopseudomonas, and Chlorobium, as well as several unidentified bacteria. In situ measurements on the growth of the mats, from intermittent tide sites, showed an annual buildup of two layers: green and reddish. These layers corresponded to a vertical growth of 1.4 ± 0.27 mm/year. Permanently submerged mats did not show vertical growth during the same period of time.     

Anoxygenic phototrophic bacteria from microbial communities of Goryachinsk thermal spring (Baikal Area,Russia)

Species composition of anoxygenic phototrophic bacteria in microbial mats of the Goryachinsk thermal spring was investigated along the temperature gradient. The spring belonging to nitrogenous alkaline hydrotherms is located at the shore of Lake Baikal 188 km north-east from Ulan-Ude. The water is of the sulfate-sodium type, contains trace amounts of sulfide, and salinity does not exceed 0.64 g/L, pH 9.5. The temperature at the outlet of the spring may reach 54°C. The cultures of filamentous anoxygenic phototrophic bacteria, nonsulfur and sulfur purple bacteria, and aerobic anoxygenic phototrophic bacteria were identified using the pufLM molecular marker. The fmoA marker was used for identification of green sulfur bacteria. Filamentous cyanobacteria predominated in the mats, with anoxygenic phototrophs comprising a minor component of the phototrophic communities. Thermophilic bacteria Chloroflexus aurantiacus were detected in the samples from both the thermophilic and mesophilic mats. Cultures of nonsulfur purple bacteria similar to Blastochloris sulfoviridis and Rhodomicrobium vannielii were isolated from the mats developed at high (50.6–49.4°C) and low temperatures (45–20°C). Purple sulfur bacteria Allochromatium sp. and Thiocapsa sp., as well as green sulfur bacteria Chlorobium sp., were revealed in low-temperature mats. Truly thermophilic purple and green sulfur bacteria were not found in the spring. Anoxygenic phototrophic bacteria found in the spring were typical of the sulfur communities, for which the sulfur cycle is mandatory. The presence of aerobic bacteriochlorophyll a-containing bacteria identified as Agrobacterium (Rhizobium) tumifaciens in the mesophilic (20°C) mat is of interest.     

Detection and Capturing of <Superscript>14</Superscript>C Radioactively-Labeled Small Subunit rRNA from Mixed Microbial Communities of a Microbial Mat Using Magnetic Beads

Carbon cycling in the hypersaline microbial mats from Chiprana Lake, Spain is primarily dependent on phototrophic microorganisms with the ability to fix CO₂ into organics that can be further utilized by aerobic as well as anaerobic heterotrophic bacteria. Here, mat pieces were incubated in seawater amended with ¹⁴C sodium bicarbonate and the incorporation of the radiocarbon in the small subunit ribosomal RNA (SSU rRNA) of mat organisms was followed using scintillation counter and autoradiography. Different domains of SSU rRNA were separated from the total RNA by means of streptavidin-coated magnetic beads and biotin-labeled oligonucleotide probes. The ¹⁴C label was detected in isolated RNA by both scintillation counter and autoradiography, however the latter technique was less sensitive. Using scintillation counter, the radiolabel incorporation increased with time with a maximum rate of 0.18 Bq ng⁻¹ detected after 25 days. The bacterial SSU rRNA could be captured using the magnetic beads, however the hybridization efficiency was around 20%. The captured RNA was radioactively labeled, which could be mainly due to the fixation of radiocarbon by phototrophic organisms. In conclusion, the incubation of microbial mats in the presence of radiolabeled bicarbonate leads to the incorporation of the ¹⁴C label into RNA molecules through photosynthesis and this label can be detected using scintillation counter. The used approach could be useful in studying the fate of fixed carbon and its uptake by other microorganisms in complex microbial mats, particularly when species-specific probes are used and the hybridization efficiency and RNA yield are further optimized.     

Phylogenetic Analysis of a Microbialite-Forming Microbial Mat from a Hypersaline Lake of the Kiritimati Atoll,Central Pacific

On the Kiritimati atoll, several lakes exhibit microbial mat-formation under different hydrochemical conditions. Some of these lakes trigger microbialite formation such as Lake 21, which is an evaporitic, hypersaline lake (salinity of approximately 170‰). Lake 21 is completely covered with a thick multilayered microbial mat. This mat is associated with the formation of decimeter-thick highly porous microbialites, which are composed of aragonite and gypsum crystals. We assessed the bacterial and archaeal community composition and its alteration along the vertical stratification by large-scale analysis of 16S rRNA gene sequences of the nine different mat layers. The surface layers are dominated by aerobic, phototrophic, and halotolerant microbes. The bacterial community of these layers harbored Cyanobacteria (Halothece cluster), which were accompanied with known phototrophic members of the Bacteroidetes and Alphaproteobacteria. In deeper anaerobic layers more diverse communities than in the upper layers were present. The deeper layers were dominated by Spirochaetes, sulfate-reducing bacteria (Deltaproteobacteria), Chloroflexi (Anaerolineae and Caldilineae), purple non-sulfur bacteria (Alphaproteobacteria), purple sulfur bacteria (Chromatiales), anaerobic Bacteroidetes (Marinilabiacae), Nitrospirae (OPB95), Planctomycetes and several candidate divisions. The archaeal community, including numerous uncultured taxonomic lineages, generally changed from Euryarchaeota (mainly Halobacteria and Thermoplasmata) to uncultured members of the Thaumarchaeota (mainly Marine Benthic Group B) with increasing depth.     

Formation and calcification of modern gypsum-dominated stromatolites,EMISAL, Fayium,Egypt

Recent gypsum stromatolites are forming along the margin as well as on the pond floor of the EMISAL saltworks, Fayium, Egypt. Gypsum precipitates are classified according to their morphology, fabrics, and crystal size into (1) subaqueous bottom gypsum crusts, (2) selenitic gypsum facies, (3) stromatolitic gypsum dome facies, and (4) gypsolite facies. Two types of microbial mats, lithifying and non-lithifying, can be identified. The lithifying mat is shallow and composed of an alternation of gypsum and microbial layers that are seasonally controlled. The non-lithifying mat, formed in the deeper part of the pond, is a greenish-brown slime-rich layer that exhibits a frothy macro texture and produces a firm gelatinous film covering the sediment surface. Calcium carbonate (mostly aragonite) particles, identified by light and scanning electron microscopy, and X-ray diffraction occur within the deeper part of the lithified mat and are associated with living and degrading biofilm. Precipitation of aragonite is associated with the dissolution of gypsum, which may have resulted from bacterial sulphate reduction. The latter process increases alkalinity and ultimately results in the replacement of gypsum by aragonite.     

Age attenuates the T‐type CaV3.2‐RyR axis in vascular smooth muscle

Caveolae position Ca_V3.2 (T‐type Ca²⁺ channel encoded by the α‐3.2 subunit) sufficiently close to RyR (ryanodine receptors) for extracellular Ca²⁺ influx to trigger Ca²⁺ sparks and large‐conductance Ca²⁺‐activated K⁺ channel feedback in vascular smooth muscle. We hypothesize that this mechanism of Ca²⁺ spark generation is affected by age. Using smooth muscle cells (VSMCs) from mouse mesenteric arteries, we found that both Ca_v3.2 channel inhibition by Ni²⁺ (50 µM) and caveolae disruption by methyl‐ß‐cyclodextrin or genetic abolition of Eps15 homology domain‐containing protein (EHD2) inhibited Ca²⁺ sparks in cells from young (4 months) but not old (12 months) mice. In accordance, expression of Ca_v3.2 channel was higher in mesenteric arteries from young than old mice. Similar effects were observed for caveolae density. Using SMAKO Ca_v1.2^?/? mice, caffeine (RyR activator) and thapsigargin (Ca²⁺ transport ATPase inhibitor), we found that sufficient SR Ca²⁺ load is a prerequisite for the Ca_V3.2‐RyR axis to generate Ca²⁺ sparks. We identified a fraction of Ca²⁺ sparks in aged VSMCs, which is sensitive to the TRP channel blocker Gd³⁺ (100 µM), but insensitive to Ca_V1.2 and Ca_V3.2 channel blockade. Our data demonstrate that the VSMC Ca_V3.2‐RyR axis is down‐regulated by aging. This defective Ca_V3.2‐RyR coupling is counterbalanced by a Gd³⁺ sensitive Ca²⁺ pathway providing compensatory Ca²⁺ influx for triggering Ca²⁺ sparks in aged VSMCs.     

Anoxygenic phototrophic bacteria from gypsum karst lakes of Lithuania

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

Biogeochemical evolution of a sulfur-iron rich aquatic system in a reflooded wetland environment (Lake Agmon,northern Israel)

Major biogeochemical processes in the newly created, shallow Lake Agmon (Hula Valley, northern Israel) were investigated from 1994 to 1996. Sediment cores, lake water and porewater were analyzed for chemical composition and spatial distribution. Sediment analyses revealed that Lake Agmon has two different sediment types: peat sediments in the northern and central parts, and marls in the southern part. The basic composition of the lake's water was controlled mainly by the mixing of two distinct water types (Jordan River and water drainage), and by evaporation. About 3/4 of the lake water originated from the Jordan Inlet, a quarter through the Z Canal Inlet (peat drainage) and a minor amount from groundwater seepage. Lake Agmon is unique among freshwater wetlands owing to its high SO ₄ ^2– content, which is ca. 1/3 that of sea water. This characteristic is ascribed to the dissolution of secondary gypsum, formed in the peat soils since the drainage of the historic Hula Marsh. Leaching gypsum from the shallow sediments during the first few months after flooding was followed by a later stage of constant diffusion and advection of SO ₄ ^2– from gypsum dissolution in deeper sediments. Gypsum dissolution in lake sediments contributed ca. half of the SO₄ ^2– and Ca²⁺ inputs to the lake. The concomitant increase of Ca²⁺ combined with alkalinity release due to organic matter decomposition in the sediments led to the precipitation of CaCO₃. This precipitation was enhanced by photosynthesis, particularly during summers, and consumed about a tenth of the Ca²⁺ and third of the alkalinity outputs from the lake. Iron-hydroxide was the main agent for microbial oxidation of organic matter, surpassing by far the role of sulfate, nitrate and manganese as electron acceptors. The produced Fe²⁺ was transported upward by diffusion and advection and oxidized to ferric iron at the sediment-water interface. There is evidence, however, that some sulfate was reduced, but most of the produced sulfide reacted with ferrous iron and accumulated in the sediments as FeS minerals. Therefore, despite high sulfate concentrations, the high iron availability restricted release of toxic sulfides into the water and thereby served to maintain reasonable water quality.     

Dendritic calcium accumulation regulates wind sensitivity via short‐term depression at cercal sensory‐to‐giant interneuron synapses in the cricket

An in vivo Ca²⁺ imaging technique was applied to examine the cellular mechanisms for attenuation of wind sensitivity in the identified primary sensory interneurons in the cricket cercal system. Simultaneous measurement of the cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and membrane potential of a wind‐sensitive giant interneuron (GI) revealed that successive air puffs caused the Ca²⁺ accumulation in dendrites and diminished the wind‐evoked bursting response in the GI. After tetanic stimulation of the presynaptic cercal sensory nerves induced a larger Ca²⁺ accumulation in the GI, the wind‐evoked bursting response was reversibly decreased in its spike number. When hyperpolarizing current injection suppressed the [Ca²⁺]_i elevation during tetanic stimulation, the wind‐evoked EPSPs were not changed. Moreover, after suprathreshold tetanic stimulation to one side of the cercal nerve resulted in Ca²⁺ accumulation in the GI's dendrites, the slope of EPSP evoked by presynaptic stimulation of the other side of the cercal nerve was also attenuated for a few minutes after the [Ca²⁺]_i had returned to the prestimulation level. This short‐term depression at synapses between the cercal sensory neurons and the GI (cercal‐to‐giant synapses) was also induced by a depolarizing current injection, which increased the [Ca²⁺]_i, and buffering of the Ca²⁺ rise with a high concentration of a Ca²⁺ chelator blocked the induction of short‐term depression. These results indicate that the postsynaptic Ca²⁺ accumulation causes short‐term synaptic depression at the cercal‐to‐giant synapses. The dendritic excitability of the GI may contribute to postsynaptic regulation of the wind‐sensitivity via Ca²⁺‐dependent depression. © 2001 John Wiley & Sons, Inc. J Neurobiol 46: 301–313, 2001     

The Arabidopsis heterotrimeric G‐protein β subunit,AGB1, is required for guard cell calcium sensing and calcium‐induced calcium release

Cytosolic calcium concentration ([Ca²⁺]_cyt) and heterotrimeric G‐proteins are universal eukaryotic signaling elements. In plant guard cells, extracellular calcium (Ca_o) is as strong a stimulus for stomatal closure as the phytohormone abscisic acid (ABA), but underlying mechanisms remain elusive. Here, we report that the sole Arabidopsis heterotrimeric Gβ subunit, AGB1, is required for four guard cell Ca_o responses: induction of stomatal closure; inhibition of stomatal opening; [Ca²⁺]_cyt oscillation; and inositol 1,4,5‐trisphosphate (InsP3) production. Stomata in wild‐type Arabidopsis (Col) and in mutants of the canonical Gα subunit, GPA1, showed inhibition of stomatal opening and promotion of stomatal closure by Ca_o. By contrast, stomatal movements of agb1 mutants and agb1/gpa1 double‐mutants, as well as those of the agg1agg2 Gγ double‐mutant, were insensitive to Ca_o. These behaviors contrast with ABA‐regulated stomatal movements, which involve GPA1 and AGB1/AGG3 dimers, illustrating differential partitioning of G‐protein subunits among stimuli with similar ultimate impacts, which may facilitate stimulus‐specific encoding. AGB1 knockouts retained reactive oxygen species and NO production, but lost YC3.6‐detected [Ca²⁺]_cyt oscillations in response to Ca_o, initiating only a single [Ca²⁺]_cyt spike. Experimentally imposed [Ca²⁺]_cyt oscillations restored stomatal closure in agb1. Yeast two‐hybrid and bimolecular complementation fluorescence experiments revealed that AGB1 interacts with phospholipase Cs (PLCs), and Ca_o induced InsP3 production in Col but not in agb1. In sum, G‐protein signaling via AGB1/AGG1/AGG2 is essential for Ca_o‐regulation of stomatal apertures, and stomatal movements in response to Ca_o apparently require Ca²⁺‐induced Ca²⁺ release that is likely dependent on Gβγ interaction with PLCs leading to InsP3 production.