Microcoleus Chthonoplastes Research Articles

Comparative structure, primary production and biogenic stabilization of cohesive and non-cohesive marine sediments inhabited by microphytobenthos

The microstructure, rate of primary production and erodibility of noncohesive and cohesive marine sediments colonized by autotrophic microbial assemblages were investigated. Microbial development on non-cohesive sediments (Island of Texel, The Netherlands) formed millimetre-thick stratified mats. A surface sheath of extracellular polymeric substances (EPS) was visible on these mats; diatoms dominated the surface 500 μm with an underlying layer of cyanobacteria, mainly Microcoleus chthonoplastes. In comparison, biofilms on cohesive sediment (Bristol Channel, U.K.) were relatively thin (100 μm) and unstratified. The microspatial distribution of the algal biomass was confirmed by measurements of oxygen evolution and rates of primary production using oxygen micro-electrodes. While gross primary production was greater in the non-cohesive sediments, peak rates of photosynthetic activity and algal biomass were similar for the two sites. The erodibility of colonized areas was measured and the mechanisms of biogenic stabilization were examined by low-temperature scanning electron microscopy (LTSEM). Cohesive sediment stabilization was purely by the secretion of EPS while both EPS binding and network formation by cyanobacteria were visualized within the non-cohesive sediments. Areas where mat and biofilm systems were visible were more resistant to sediment erosion than adjacent areas without biofilm development. Direct comparison between biogenic effects at the two sites must be treated with caution because of the different nature of the sediments but the index of biogenic stabilization for both sites is reported. The stability of the non-cohesive sediments was greatly increased by the presence of a microbial mat. The hydration state of the EPS matrix was considered to have a potential effect on the critical erosion threshold of the sediments.

Biogeochemical cycles of carbon, sulfur, and free oxygen in a microbial mat

Complete budgets for carbon and oxygen have been constructed for cyanobacterial mats dominated by Microcoleus chthonoplastes from the evaporating ponds of a salt works located in Guerrero Negro, Baja California Sur, Mexico. Included in the budget are measured rates of O 2 production, sulfate reduction, and elemental exchange across the mat/brine interface, day and night, at various temperatures and times of the year. We infer from this data the various sinks for O 2, as well as the sources of carbon for primary production. To summarize, although seasonal variability exists, a major percentage of the O 2 produced during the day did not diffuse out of the mat but was used within the mat to oxidize both organic carbon and the sulfide produced by sulfate reduction. At night, most of the O 2 that diffused into the mat was used to oxidize sulfide, with O 2 respiration of minor importance. During the day, the internal mat processes of sulfate reduction and O 2 respiration generated as much or more inorganic carbon (DIC) for primary production as diffusion into the mat. Also, oxygenic photosynthesis was the most important process of carbon fixation, although anoxygenic photosynthesis may have been important at low light levels during some times of the year. At night, the DIC lost from the mat was mostly from sulfate reduction. Elemental fluxes across the mat/brine interface indicated that carbon with an oxidation state of greater than zero was taken up by the mat during the day and liberated from the mat at night. Overall, carbon with an average oxidation state of near zero accumulated in the mat. Both carbon fixation and carbon oxidation rates varied with temperature by a similar amount. These mats are thus closely coupled systems where rapid rates of photosynthesis both require and fuel rapid rates of heterotrophic carbon oxidation.

Lipid biogeochemistry of Phormidium and Microcoleus mats

A study on the predominant sources of organic matter and the main diagenetic processes in two different cyanobacterial mats from evaporite-controlled environments was performed. Fatty acids, hydrocarbons, alcohols, ketones and aldehydes were anlyzed in selected millimetre and submillimetre core sections. The changes in lipid composition were evaluated by comparison with the vertical distributions of the populations observed by optic microscopy and with the lipid patterns of enrichment cultures of species such as cyanobacteria, diatoms, purple bacteria, sulphate-reducers and methanogens obtained from the mats. The cyanobacteria Phormidium valderianum and Microcoleus chthonoplastes are the predominant primary producers, and occur almost as monocultures in the respective top layers. However, these mat-forming organisms only leave minor features in the solvent-extractable lipid sedimentary record. The predominant fatty acid distributions parallel the composition observed in the enrichment cultures of purple bacteria and appear mixed with acids characteristic of heterotrophic eubacteria such as sulphate-reducers. The concentrations of these lipids are, however, 5–10 times lower than the cyanobacterial acids from the top layer. De novo heterotrophic eubacterial synthesis is also observed in cases such as the highly branched isoprenoid eicosenes, the major hydrocarbon in the deep layers (>2 mm) of the Phormidium mat. Other major diagenetic changes involve dehydration and hydrogenation. These two processes take place concurrently under anoxic conditions and have been observed among the sterols and the isoprenoid alcohols. Significant amounts of 5ß(H)-stanols were observed in the more reducing sections where molecular indicators of methanogenic bacteria were also found.

Laminated cyanobacterial mats in sediments of solar salt works: some sedimentological implications

ABSTRACTFormation of microlaminated sediments in solar salt works along the Mediterranean coast in southern France only occurs within a restricted salinity range of 60–150 gl−1. These salinities are associated with development of a laminated cyanobacterial mat composed primarily of the filamentous cyanobacteria Microcoleus chthonoplastes interbedded with detrital laminae. Transplants of the cyanobacterial mat to a less saline zone (36–60 gl−1) indicated that the cyanobacterial mats failed to colonize the less saline waters due to herbivorous snails and competition for light from floating algal masses of Cladophora and Enteromorpha. Neither the snails nor the Cladophora and Enteromorpha masses are tolerant of salinities above 60 gl−1, and therefore the Microcoleus mats are restricted to those areas of the solar salt works with these higher salinities.Analyses of salinity, conductivity, dissolved oxygen and pH in shallow salt pans (with salinities of 60–150 gl−1) established a relationship between the daily development of oxygen supersaturation and cyanobacterial photosynthesis.Sediments are unlaminated in those portions of the solar salt works where there are no cyanobacterial mats. These mats are frequently drained of their overlying water, and thus desiccation cracks divide them into polygonal plates. The development and translocation of these plates is enhanced by gas bubbles which form under the surface of the mats. No correlation between the microlaminae in sections from two cores located approximately 1 m apart was observed. This was consistent with the hypothesis that the surface of the desiccation crack polygons can be removed by currents and redeposited on the top of other cyanobacterial mat polygons. This process results in a ‘patchwork quilt’of young and old cyanobacterial mat polygons with an irregular microlamination pattern. The presence of such an irregular pattern of laminae permits an important distinction to be made between sediments associated with stromatolite formation and those associated with the very fine and horizontal varved sediments of stratified (meromictic) water bodies. The sedimentological significance of these observations is reviewed in relation to the processes of stromatolite genesis.

Interactions between nitrogen fixation and oxegenic photosynthesis in a marine cyanobacterial mat

Abstract Cyanobacterial mats developed on fine sandy sediments of the upper littoral of the island of Mellum (North Sea). Freshly colonized sediment was dominated by the non-heterocystous, nitrogen-fixing cyanobacterium Oscillatoria limosa . Well established mats in which the cosmopolitan cyanobacterium Microcoleus chthonoplastes was the dominant organism also usually contained O. limosa as a minor component. This mat was about 1 mm thick and contained high biomass. Photosynthesis was maximal at about 150 μm depth and reached values of 280 μmol oxygen. 1 −1 · min −1 . On the other hand, in the dark, high respiratory activity turned the mat anaerobic within minutes. Freshly colonized sediment consisted of low cyanobacterial biomass loosely attached to the sand grains and present up to a depth of 2.5 mm. Respiratory activity was low and the sediment remained aerobic to a depth of 2 mm throughout the night. Nitrogen fixation (acetylene reduction) was measured during 24-h periods in both types of mats in order to elucidate interactions with oxygenic photosynthesis and oxygen concentration. Acetylene reduction in the mats showed very different diurnal patterns which depended on the type of mat investigated and the time of year. The results indicated that a temporary separation of oxygenic photosynthesis and nitrogen fixation occurred in the mat. Established mats fixed nitrogen predominantly during the transition from dark to light and vice versa, when oxygenic photosynthesis was reduced or absent. Freshly colonized sediment-fixed nitrogen throughout the night but often a stimulation was seen at dawn. The latter showed much higher specific activities than the established type. Also in spring, specific activities were much higher.

Interactions between nitrogen fixation and oxegenic photosynthesis in a marine cyanobacterial mat

Cyanobacterial mats developed on fine sandy sediments of the upper littoral of the island of Mellum (North Sea). Freshly colonized sediment was dominated by the non-heterocystous, nitrogen-fixing cyanobacterium Oscillatoria limosa. Well established mats in which the cosmopolitan cyanobacterium Microcoleus chthonoplastes was the dominant organism also usually contained O. limosa as a minor component. This mat was about 1 mm thick and contained high biomass. Photosynthesis was maximal at about 150 μm depth and reached values of 280 μmol oxygen. 1 −1 · min −1. On the other hand, in the dark, high respiratory activity turned the mat anaerobic within minutes. Freshly colonized sediment consisted of low cyanobacterial biomass loosely attached to the sand grains and present up to a depth of 2.5 mm. Respiratory activity was low and the sediment remained aerobic to a depth of 2 mm throughout the night. Nitrogen fixation (acetylene reduction) was measured during 24-h periods in both types of mats in order to elucidate interactions with oxygenic photosynthesis and oxygen concentration. Acetylene reduction in the mats showed very different diurnal patterns which depended on the type of mat investigated and the time of year. The results indicated that a temporary separation of oxygenic photosynthesis and nitrogen fixation occurred in the mat. Established mats fixed nitrogen predominantly during the transition from dark to light and vice versa, when oxygenic photosynthesis was reduced or absent. Freshly colonized sediment-fixed nitrogen throughout the night but often a stimulation was seen at dawn. The latter showed much higher specific activities than the established type. Also in spring, specific activities were much higher.

In situ fluctuations of oxygen and sulphide in marine microbial sediment ecosystems

Laminated microbial ecosystems (microbial mats) on the island of Schiermonnikoog (The Netherlands) were studied with respect to variation in oxygen and sulphide profiles, depth distributions of photopigments and viable number and cell volume of purple sulphur bacteria. Cyanobacteria occurred in the top 2 mm, the dominant species being Microcoleus chthonoplastes. The blooming of purple sulphur bacteria below the cyanobacterial layer was observed in autumn, the dominant species being the immotile Thiocapsa roseopersicina. Cell volume of this species is indicative of its growth rate. In situ measurements showed strong diel fluctuations in oxygen and sulphide profiles. Frequently, cyanobacteria and purple sulphur bacteria were exposed to oxygen during the day, and to anoxic conditions at night. Sulphide sometimes reached the layer of the cyanobacteria. The cyanobacteria and the purple sulphur bacteria both are very well adapted to these diel fluctuations. In addition, strong seasonal variations were observed, whereas short-term fluctuations of oxygen occurred due to changing light-climate and rainfall. Attention was paid to the unusual occurrence of microbial mats on the North Sea beach during the autumn of 1987.

Nitrogen fixation (acetylene reduction) in nonheterocystous cyanobacterial mats from the Dampier Archipelago, Western Australia

Discs punched from non-heterocystous cyanobacterial mats, one containing Microcoleus chthonoplastes, Oscillatoria sp. and Phormidium sp., the other Phorrnidium sp. and Aphanocapsa sp., were incubated for 23 days in artificial sea-water of salinity 0 to 140 g L-1. The chlorophyll a content of both mats increased over this salinity range, with lower increases above 100 g L-1. There was little change in the species composition of mats at salinities 240 g L-1; ≥ 40 g L-1, mats produced essentially monospecific thalli containing small quantities of the other species. Acetylene reduction ranged from zero at the highest salinity to a maximum of 1100-1500 pmol C2H2 reduced m-2 h-1 at 20-60 g L-1. Maximum fixation rates were two orders of magnitude higher than in situ measurements (8-60, mean 16 pmol C2H2 reduced m-2 h-1). The salinity range observed in the field was 40-60 g L-1, but maximum fixation rates in the field (60 Fmol C2H2 reduced m-2 h-1) were much lower than those observed in the laboratory.

Aerobic Nitrogen Fixation in Aggregate-forming Cultures of the Nonheterocystous Cyanobacterium Microcoleus chthonoplastes

Microcoleus chthonoplastes is a filamentous, nonheterocystous cyanobacterium which fixes gaseous nitrogen in axenic culture under aerobic conditions. Growth on agar plates reflects the benthic habit observed in the natural environment, and in liquid culture clumps or aggregates are produced. Scanning electron microscopy suggests that these aggregates are primarily the result of random movement of trichomes against one another. The aim of this study was to ascertain whether aggregation or any intracellular differentiation might be important in protection of the oxygen-labile nitrogenase in this species. Observations using light and transmission electron microscopy revealed no cellular or intracellular differentiation, and tetrazolium labelling experiments provided no evidence that a proportion of cells might act in a heterocyst-like fashion. The results presented suggest that nitrogen fixation in M. chthonoplastes is unlikely to be enhanced in oxygen-deficient microsites formed as a result of aggregation.

Acetylene reduction and hydrogen metabolism by a cyanobacterial/sulfate‐reducing bacterial mat ecosystem

Abstract We report a study of nitrogenase activity (acetylene reduction) and hydrogen gas metabolism in intact smooth cyanobacterial mats from Hamelin Pool, Shark Bay, Western Australia. The predominant cyanobacterial population in these mats is Microcoleus chthonoplastes. The mats had a significant capacity for nitrogen fixation, predominantly attributable to the photosyn‐thetic component. By physical and chemical perturbation we revealed an active hydrogen metabolism within the mats. Most of the H2 formation was attributed to fermentative processes, whereas hydrogen was consumed in light‐dependent, together with oxygen‐ and sulfate‐dependent respiratory processes. It was concluded that H2 formed by fermentative bacteria in the dark drives a significant proportion of sulfate reduction in the mats, but there was little H2 transfer from the cyanobacteria to the sulfate‐reducing bacteria. Thus photosynthetically produced H2 gas is unlikely to significantly alter the previously measured carbon: sulfur ratio ...

SYSTEMATICS AND ECOLOGY OF LYNGBYA SPP. AND ASSOCIATED SPECIES (CYANOPHYTA) IN A NEW ENGLAND SALT MARSH1,2

ABSTRACTField populations of the most abundant filamentous blue‐green algae (cyanobacteria) from a northern New England salt marsh were examined for the presence of natural morphological clusters. A continuum in the distribution of average cell sizes and shapes for individual trichomes was evident. However, there was a high frequency of certain cell sizes (six cluster, which were also separated by several qualitatives characters (end cell shape, granule location, and cell color). In clonal cultures the trichomes of these culsters maintained the morphology of the original isolated trichome and thus morphology apperated to be stable. The clusters were either named according to Gietler's (1932.) terminology or assigned numbers if no adequate descriptions were availble: (1) Lyngyya and Phormidium spp., (2) Microcoleus chthonoplastes Thur. (3) Lyngbya sp.1, (4)Lyngbya sp. 2 (5) Lyngbya aestruarii (Mert.) Liebmann, and, (6) Lyngbya aestuarii (mert. Liebmann, and, (6) Lyngbya sp. 3. All goups had maximum asbundance in late summer. Microcoleus chthonoplastes occurred throughtout the; year, while Lyngbya aestuarri and Lynobya sp. 3 occurred throughout the summer and fall. It is concluded that both field and culture information are useful in enhancing our knowledge of blue‐green algal systematics.

The evolution of glutathione metabolism in phototrophic microorganisms.

Of the many roles ascribed to glutathione (GSH) the one most clearly established is its role in the protection of higher eucaryotes against oxygen toxicity through destruction of thiol-reactive oxygen byproducts. If this is the primary function of GSH then GSH metabolism should have evolved during or after the evolution of oxygenic photosynthesis. That many bacteria do not produce GSH is consistent with this view. In the present study we have examined the low-molecular-weight thiol composition of a variety of phototrophic microorganisms to ascertain how evolution of GSH production is related to evolution of oxygenic photosynthesis. Cells were extracted in the presence of monobromobimane (mBBr) to convert thiols to fluorescent derivatives, which were analyzed by high-pressure liquid chromatography. Significant levels of GSH were not found in the green bacteria (Chlorobium thiosulfatophilum and Chloroflexus aurantiacus). Substantial levels of GSH were present in the purple bacteria (Chromatium vinosum, Rhodospirillum rubrum, Rhodobacter sphaeroides, and Rhodocyclus gelatinosa), the cyanobacteria [Anacystis nidulans, Microcoleus chthonoplastes S.G., Nostoc muscorum, Oscillatoria amphigranulata, Oscillatoria limnetica, Oscillatoria sp. (Stinky Spring, Utah), Oscillatoria terebriformis, Plectonema boryanum, and Synechococcus lividus], and eucaryotic algae (Chlorella pyrenoidsa, Chlorella vulgaris, Euglena gracilis, Scenedesmus obliquus, and Chlamydomonas reinhardtii). Other thiols measured included cysteine, gamma-glutamylcysteine, thiosulfate, coenzyme A, and sulfide; several unidentified thiols were also detected. Many of the organisms examined also exhibited a marked ability to reduce mBBr to syn-(methyl,methyl)bimane, an ability that was quenched by treatment with 2-pyridyl disulfide or 5,5'-bisdithio-(2-nitrobenzoic acid) prior to reaction with mBBr. These observations indicate the presence of a reducing system capable of electron transfer to mBBr and reduction of reactive disulfides. The distribution of GSH in phototrophic eubacteria indicates that GSH synthesis evolved at or around the time that oxygenic photosynthesis evolved.

Association of a new type of gliding, filamentous, purple phototrophic bacterium inside bundles of Microcoleus chthonoplastes in hypersaline cyanobacterial mats.

An unidentified filamentous purple bacterium, probably belonging to a new genus or even a new family, is found in close association with the filamentous, mat-forming cyanobacterium Microcoleus chthonoplastes in a hypersaline pond at Guerrero Negro, Baja California Sur, Mexico, and in Solar Lake, Sinai, Egypt. This organism is a gliding, segmented trichome, 0.8-0.9 micrometer wide. It contains intracytoplasmic stacked lamellae which are perpendicular and obliquely oriented to the cell wall, similar to those described for the purple sulfur bacteria Ectothiorhodospira. These bacteria are found inside the cyanobacterial bundle, enclosed by the cyanobacterial sheath. Detailed transmission electron microscopical analyses carried out in horizontal sections of the upper 1.5 mm of the cyanobacterial mat show this cyanobacterial-purple bacterial association at depths of 300-1200 micrometers, corresponding to the zone below that of maximal oxygenic photosynthesis. Sharp gradients of oxygen and sulfide are established during the day at this microzone in the two cyanobacterial mats studied. The close association, the distribution pattern of this association and preliminary physiological experiments suggest a co-metabolism of sulfur by the two-membered community. This probable new genus of purple bacteria may also grow photoheterotrophically using organic carbon excreted by the cyanobacterium. Since the chemical gradients in the entire photic zone fluctuate widely in a diurnal cycle, both types of metabolism probably take place. During the morning and afternoon, sulfide migrates up to the photic zone allowing photoautotrophic metabolism with sulfide as the electron donor. During the day the photic zone is highly oxygenated and the purple bacteria may either use oxidized species of sulfur such as elemental sulfur and thiosulfate in the photoautotrophic mode or grow photoheterotrophically using organic carbon excreted by M. chthonoplastes. The new type of filamentous purple sulfur bacteria is not available yet in pure culture, and its taxonomical position cannot be fully established. This organism is suggested to be a new type of gliding, filamentous, purple phototroph.

SYSTEMATICS AND ECOLOGY OF LYNGBYA SPP. AND ASSOCIATED SPECIES (CYANOPHYTA) IN A NEW ENGLAND SALT MARSH1,2

ABSTRACTField populations of the most abundant filamentous blue‐green algae (cyanobacteria) from a northern New England salt marsh were examined for the presence of natural morphological clusters. A continuum in the distribution of average cell sizes and shapes for individual trichomes was evident. However, there was a high frequency of certain cell sizes (six clusters), which were also separated by several qualitative characters (end cell shape, granule location, and cell color). In clonal cultures the trichomes of these clusters maintained the morphology of the original isolated trichome and thus morphology appeared to he stable. The clusters were either named according to Geitler's (1932) terminology or assigned numbers if no adequate descriptions were available: (1) Lyngbya and Phormidium spp., (2) Microcoleus chthonoplastes Thur. (3) Lyngbya sp. 1, (4) Lyngbya sp. 2, (5) Lyngbya aestuarii (Mert.) Liebmann. and, (6) Lyngbya sp. 3. All groups had maximum abundance in late summer. Microcoleus chthonoplastes occurred throughout the year, while Lyngbya aestuarii and Lyngbya sp. 3 occurred throughout the summer and fall. It is concluded that both field and culture information are useful in enhancing our knowledge of blue‐green algal systematics.

Transition from Anoxygenic to Oxygenic Photosynthesis in a Microcoleus chthonoplastes Cyanobacterial Mat

Benthic cyanobacterial mats with the filamentous Microcoleus chthonoplastes as the dominant phototroph grow in oxic hypersaline environments such as Solar Lake, Sinai. The cyanobacteria are in situ exposed to chemical variations between 200 mumol of sulfide liter at night and 1 atm pO(2) during the day. During experimental H(2)S to O(2) transitions the microbial community was shown to shift from anoxygenic photosynthesis, with H(2)S as the electron donor, to oxygenic photosynthesis. Microcoleus filaments could carry out both types of photosynthesis concurrently. Anoxygenic photosynthesis dominated at high sulfide levels, 500 mumol liter, while the oxygenic reaction became dominant when the sulfide level was reduced below 100 to 300 mumol liter (25 to 75 mumol of H(2)S liter). An increasing inhibition of the oxygenic photosynthesis was observed upon transition to oxic conditions from increasing sulfide concentrations. Oxygen built up within the Microcoleus layer of the mat even under 5 mmol of sulfide liter (500 mumol of H(2)S liter) in the overlying water. The implications of such a localized O(2) production in a highly reducing environment are discussed in relation to the evolution of oxygenic photosynthesis during the Proterozoic era.

The microbial community at Laguna Figueroa, Baja California Mexico: from miles to microns.

Laguna Figueroa is a lagoonal complex on the Pacific coast of the Baja California penisula 200 km south of the Mexican-United States border. The hypersaline lagoon is 16 km long and 2-3 km wide with a salt marsh and evaporite flat and is separated from the ocean by a barrier dune and beach. At the salt marsh-evaporite flat interface a stratified microbial community dominated by Microcoleus chthonoplastes is depositing laminated sediments. Similar stratiform deposits with associated microbial mat communities have been found in cherts of the Fig Tree Group, South Africa which are 3.4 GE in age. Heavy rains in the winters of 1978-1979 and 1979-1980 flooded the evaporite flat with 1-3 meters of meteoric water and buried the laminated sediment under 5-10 cm of siliciclastic and clay sediment. These flooding events had a dramatic effect on the composition of the mat community. The Microcoleus dominated community, with species of Chloroflexus sp. and an Ectothiorhodospira-like filamentous purple phototroph, disappeared leaving a community dominated by the purple phototrophs Chromatium sp. and Thiocapsa sp. Recolonization of the surface by species of the cyanobacteria Oscillatoria sp. and Spirulina sp. preceded the return of the Microcoleus community. Field conditions were monitored by ground based observations and supplemented with LandSat and Skylab imagery. The microbial community was studied with light microscopy and transmission electron microscopy. The change in dominating microbial species was correlated with the episodes of flooding.

Diel N 2 fixation in an intertidal marine cyanobacterial mat community

Diurnal and diel rates of nitrogen fixation (acetylene reduction) were measured in an intertidal marine mat community consisting of Microcoleus chthonoplastes, Lyngbya aestuarii, and an underlying layer of anaerobic bacteria (probably photosynthetic, fermentative, and sulfate reducing bacteria). The community is located on Shackleford Banks, a barrier island off the coast of North Carolina. Rates of nitrogenase activity during the diurnal period were generally highest in the morning. Rates measured during the day, but incubated in the dark, at times equalled those of samples incubated continuously in the light. Fixation also occurred at night. Mat samples deprived of light prior to the acetylene reduction assay continued to reduce acetylene during the night and into the following day, although at lower rates than samples exposed to light. The persistence of dark nitrogenase activity serves as evidence that reducing power initially generated through photosynthesis can be stored and subsequently utilized through heterotrophic metabolism. Although cyanobacteria are responsible for light-mediated nitrogenase activity, non-photosynthetic bacterial contributions to dark-mediated nitrogenase activity cannot be ruled out.

Structure and development of a benthic marine microbial mat

Vertically stratified microbial communities of phototrophic bacteria in the upper intertidal zones of the North Sea island of Mellum were investigated. Growth and population dynamics of the cyanobacterial mat were followed over three successive years. It was concluded that the initial colonization of the sandy sediments was by the cyanobacterium Oscillatoria. In well-established mats, however, the dominant organism was Microcoleus chthonoplastes. The observed succession of cyanobacteria during mat development is correlated with nitrogen fixation. Nitrogen fixation is necessary in this low-nutrient environment to ensure colonization by mat-constructing cyanobacteria. Under certain conditions, a red layer of purple sulfur bacteria developed underneath the cyanobacterial mat in which Chromatium and Thiocapsa spp. dominated, but Thiopedia and Ectothiorhodospira spp. have also been observed. Measurements of light penetrating the cyanobacterial mat indicated that sufficient light is available for the photosynthetic growth of purple sulfur bacteria. Profiles of oxygen, sulfide and redox potential within the microbial mat were measured using microelectrodes. Maximum oxygen concentrations, measured at a depth of 0.7 mm, reached levels more than twice the normal air saturation. Dissolved sulfide was not detected by the microelectrodes. Determination of acid-distilled sulfide, however, revealed appreciable amounts of bound sulfide in the mat. Redox profiles measured in the mat led to the conclusion that the upper 10 mm of the sedimentary sequence is in a relatively oxidized state.

Structure and development of a benthic marine microbial mat

Vertically stratified microbial communities of phototrophic bacteria in the upper intertidal zones of the North Sea island of Mellum were investigated. Growth and population dynamics of the cyanobacterial mat were followed over three successive years. It was concluded that the initial colonization of the sandy sediments was by the cyanobacterium Oscillatoria. In well-established mats, however, the dominant organism was Microcoleus chthonoplastes. The observed succession of cyanobacteria during mat development is correlated with nitrogen fixation. Nitrogen fixation is necessary in this low-nutrient environment to ensure colonization by mat-constructing cyanobacteria. Under certain conditions, a red layer of purple sulfur bacteria developed underneath the cyanobacterial mat in which Chromatium and Thiocapsa spp. dominated, but Thiopedia and Ectothiorhodospira spp. have also been observed. Measurements of light penetrating the cyanobacterial mat indicated that sufficient light is available for the photosynthetic growth of purple sulfur bacteria. Profiles of oxygen, sulfide and redox potential within the microbial mat were measured using microelectrodes. Maximum oxygen concentrations, measured at a depth of 0.7 mm, reached levels more than twice the normal air saturation. Dissolved sulfide was not detected by the microelectrodes. Determination of acid-distilled sulfide, however, revealed appreciable amounts of bound sulfide in the mat. Redox profiles measured in the mat led to the conclusion that the upper 10 mm of the sedimentary sequence is in a relatively oxidized state.

Motility of the cyanobacteriumMicrocoleus chthonoplastesin mud

The cyanobacterium Microcoleus chthonoplastes Thuret was abundant in the upper littoral mudflats along the Anglesey shores of the Menai Straits. The majority of the Microcoleus population was found in the top 2 mm of freshly cut cores but the filaments migrated by gliding movement to the surface if this was kept moist. Quantitative estimations of the populations were made by using lens tissue traps placed in contact with the core surface. Although the filaments are phototactic they were found to glide to the original surface of the core in the dark; their preference for the original surface seemed to indicate an aerotactic response. Geotaxis and magnetotaxis were tested and found not to occur. The filaments migrated up through mud slurry overlays of up to 5·5 mm deep in 18 h. The ability to do this would be of benefit in a habitat subject to silting.