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

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

Biogeochemistry of microbial mats under Precambrian environmental conditions: a modelling study

Microbial mats have arguably been the most important ecosystem on Earth over its 3.5 Gyr inhabitation. Mats have persisted as consortia for billions of years and occupy some of Earth's most hostile environments. With rare exceptions (e.g. microbial mats developed on geothermal springs at Yellowstone National Park, USA), today's mats do not exist under conditions analogous to Precambrian habitats with substantially lower oxygen and sulphate concentrations. This study uses a numerical model of a microbial mat to investigate how mat composition in the past might have differed from modern mats. We present a numerical model of mat biogeochemistry that simulates the growth of cyanobacteria (CYA), colourless sulphur bacteria (CSB), and purple sulphur bacteria (PSB), with sulphate‐reducing bacteria (SRB) and heterotrophic bacteria represented by parameterized sulphate reduction rates and heterotrophic consumption rates, respectively. Variations in the availability of light, oxygen, sulphide, and sulphate at the upper boundary of the mat are the driving forces in the model. Mats with remarkably similar biomass and chemical profiles develop in models under oxygen boundary conditions ranging from 2.5 × 10^?13 to 0.25 mm and sulphate boundary concentrations ranging from 0.29 to 29 mm , designed to simulate various environments from Archean to modern. The modelled mats show little sensitivity to oxygen boundary conditions because, independent of the overlying oxygen concentrations, cyanobacterial photosynthesis creates similar O₂ concentrations of 0.45–0.65 mm in the upper reaches of the mat during the photoperiod. Varying sulphate boundary conditions have more effect on the biological composition of the mat. Sulphide generated from sulphate reduction controls the magnitude and distribution of the PSB population, and plays a part in the distribution of CSB. CSB are the most sensitive species to environmental change, varying with oxygen and sulphide.     

Intact Phospholipid and Quinone Biomarkers to Assess Microbial Diversity and Redox State in Microbial Mats

Microbial mats are stratified microbial communities composed by highly inter-related populations and therefore are frequently chosen as model systems to study diversity and ecophysiological strategies. The present study describes an integrated approach to analyze microbial quinones and intact polar lipids (IPLs) in microbial mats within layers as thin as 500 μm by liquid chromatography–tandem mass spectrometry. Quinone profiles revealed important depth-related differences in community composition in two mat systems. The higher abundance of ubiquinones, compared to menaquinones, reflected the clear predominance of microorganisms belonging to aerobic α-, β-, and γ-Proteobacteria in Ebro delta estuarine mats. Hypersaline photosynthetic Camargue mats (France) showed a predominance of menaquinone-9 at the top of the mat, which is consistent with an important contribution of facultative aerobic or anaerobic bacteria in its photic zone. Quinone indices also indicated a higher diversity of non-phototrophs and a more anaerobic character in the hypersaline mats. Besides, the dissimilarity index suggested that the samples were greatly influenced by a depth-related redox state gradient. In the analysis of IPLs, there was a predominance of phosphatidylglycerols and sulfoquinovosyldiacylglycerols, the latter being an abundant biomarker of Cyanobacteria. This combined approach based on quinone and IPL analysis has proven to be a useful method to establish differences in the microbial diversity and redox state of highly structure microbial mat systems at a fine-scale level.     

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.     

Diverse communities of Bacteria and Archaea flourished in Palaeoarchaean (3.5–3.3 Ga) microbial mats

Limited taxonomic classification is possible for Archaean microbial mats and this is a fundamental limitation in constraining early ecosystems. Applying Fourier transform infrared spectroscopy (FTIR), a powerful tool for identifying vibrational motions attributable to specific functional groups, we characterized fossilized biopolymers in 3.5–3.3 Ga microbial mats from the Barberton greenstone belt (South Africa). Microbial mats from four Palaeoarchaean horizons exhibit significant differences in taxonomically informative aliphatic contents, despite high aromaticity. This reflects precursor biological heterogeneity since all horizons show equally exceptional preservation and underwent similar grades of metamorphism. Low methylene to end-methyl (CH₂/CH₃) absorbance ratios in mats from the 3.472 Ga Middle Marker horizon signify short, highly branched n-alkanes interpreted as isoprenoid chains forming archaeal membranes. Mats from the 3.45 Ga Hooggenoeg Chert H5c, 3.334 Ga Footbridge Chert, and 3.33 Ga Josefsdal Chert exhibit higher CH₂/CH₃ ratios suggesting mostly longer, unbranched fatty acids from bacterial lipid precursors. Absorbance ratios of end-methyl to methylene (CH₃/CH₂) in Hooggenoeg, Josefsdal and Footbridge mats yield a range of values (0.20–0.80) suggesting mixed bacterial and archaeal architect communities based on comparison with modern examples. Higher (0.78–1.25) CH₃/CH₂ ratios in the Middle Marker mats identify Archaea. This exceptional preservation reflects early, rapid silicification preventing the alteration of biogeochemical signals inherited from biomass. Since silicification commenced during the lifetime of the microbial mat, FTIR signals estimate the affinities of the architect community and may be used in the reconstruction of Archaean ecosystems. Together, these results show that Bacteria and Archaea flourished together in Earth's earliest ecosystems.     

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. Electronic Publication     

Structural and Geomicrobiological Characteristics of a Microbial Community from a Cold Sulfide Spring

The Ancaster sulfur spring is a cold (9°C) sulfur spring located near Ancaster, Ontario, Canada, which hosts an abundant and diverse microbial mat community. We conducted an extensive microscopical study of the microbial community of this spring using a number of techniques: phase light, confocal scanning laser microscopy, conventional scanning electron microscopy using both chemical/critical point drying and cryofixation preparative techniques, environmental scanning electron microscopy, and transmission electron microscopy. The latter two techniques were coupled with energy dispersive X-ray spectrometry for elemental analysis to complement wet geochemical data collected on bulk spring water and mat pore water. In the anoxic source of the spring, green and purple sulfur bacteria were found together with a sulfide-utilizing type of cyanobacteria that had the unusual characteristic of storing colloidal sulfur intracellularly. Deeper within the source, the mats were dominated by green sulfur bacteria and thick biofilms of cells that precipitated Fe and Zn sulfide minerals on their surfaces. Downstream from the source, thick, filamentous white mats lined the stream channel, formed by a diverse mass of nonphotosynthetic sulfur oxidizers, which were responsible for forming thick masses of spherical colloidal sulfur. These were distinguished by ESEM-EDS from cells by their simple elemental composition (only S was detected). Aqueous geochemistry analysis by ICP-MS showed that some elements (Fe, C, P, Zn, Mg, Ba) were present at higher levels in mat pore water than in bulk spring water. Our approach allowed us to gain an appreciation of the characteristics of this microbial community and allowed us to develop a good understanding of the types of microorganisms present and infer some of the relationships among the members of the community. In addition, we wish to convey the utility of a thorough microscopical approach in geomicrobiological and microbial ecology studies.     

Characterization of a sulfide-oxidizing biofilm developed in a packed-column reactor.

The potential of microbial mats to develop sulfide-oxidizing biofims was explored. A bioreactor specially designed for the treatment of sulfide-containing effluents was inoculated with a microbial-mat sample, and a complex microbial biofilm with sulfide-oxidation activity developed. The microbial composition of the biofilm was studied by pigment, microscopy, and 16S rRNA gene analyses. Purple sulfur bacteria and diatoms were observed by microscopy, chlorophyll a and bacteriochlorophyll a were detected in the pigment analysis, and high genetic diversity was found in the 16S rRNA gene library. Specialized anaerobic sulfur oxidizers (i.e., phototrophic purple and green sulfur bacteria) dominated the library. Aerobic phototrophs (diatoms) also developed and the oxygen produced allowed the growth of aerobic sulfide oxidizers, such as Thiomicrospira-like spp. Cyanobacteria, which are significant organisms in natural microbial mats, did not develop in the reactor but unexpected uncultured members from the Epsilonproteobacteria developed profusely. Moreover, a variety of more minor organisms, such as members of the Cytophaga-Flavobacterium-Bacteroides (CFB) and purple non-sulfur bacteria (Roseospirillum sp.), were also present. The results showed that a complex community with high genetic and metabolic diversity, including many uncultured organisms, can develop in a laboratory-scale reactor.     

Distribution of chloropigments in suspended particulate matter and benthic microbial mat of a meromictic lake, Lake Kaiike, Japan

We investigated the distribution of chloropigments in a small meromictic lake, Lake Kaiike, south-west Japan. In the water-column, concentrations of Chl a related to cyanobacteria, BChl a related to purple sulphur bacteria, and three types of BChl e homologues (BChls e1, e2 and e3) related to brown-coloured green sulphur bacteria, were maximal at the redox boundary. Below the redox boundary, absolute concentrations of Chl a and BChl a gradually decreased with depth, whereas BChls e remained rather constant. Suspended particulate matter (SPM) at the deeper region of the anoxic water-column was enriched in highly alkylated BChl e homologues compared with SPM at the redox boundary. The shift in the relative content of highly alkylated BChl e homologues beneath the boundary was associated with community related adaptation of brown-coloured green sulphur bacteria to changes in light quality/quantity, resulting from the optical absorption and reflectance of SPMs in the overlying water-column. Benthic microbial mats were characterized by high abundances of BChls e, in which highly alkylated homologues were substantially abundant. This suggests that the BChls e in the microbial mat may be derived from the low-light adapted brown-coloured green sulphur bacteria forming the bacterial mat.     

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.     

Phylogenetic and functional prokaryotic diversity in the Hoito-Gol mesothermal mineral spring (Eastern Sayan Mountains,Buryat Republic)

High-throughput sequencing was used for comparative analysis of microbial communities of the water and mat from the Hoito-Gol mesothermal mineral sulfide spring (Eastern Sayan Mountains, Buryat Republic). Activity of microbial communities was determined. While both spring biotopes were dominated by members of three bacterial phyla—Proteobacteria, Bacteroidetes, and Firmicutes—they differed drastically in the composition of predominant phylotypes (at the genus level). In the water, the organisms widespread in aquatic environments were predominant, mostly aerobic chemoorganotrophs of the genera Acinetobacter, Pedobacter, and Flavobacterium. In the microbial mat, the organisms actively involved in the sulfur cycle predominated, including sulfur-reducing bacteria Sulfurospirillum, sulfate-reducing deltaproteobacteria, sulfuroxidizing chemoautotrophic bacteria, anoxygenic phototrophic bacteria of the phyla Chloroflexi and Chlorobi, as well as purple bacteria belonging to the α-, ß-, and γ-Proteobacteria. Microbial mats of the spring exhibited higher phylogenetic diversity compared to high-temperature mats containing photosynthetic microorganisms.     

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.     

Mathematical simulation of the diel O, S, and C biogeochemistry of a hypersaline microbial mat

The creation of a mathematical simulation model of photosynthetic microbial mats is important to our understanding of key biogeochemical cycles that may have altered the atmospheres and lithospheres of early Earth. A model is presented here as a tool to integrate empirical results from research on hypersaline mats from Baja California Sur (BCS), Mexico into a computational system that can be used to simulate biospheric inputs of trace gases to the atmosphere. The first version of our model, presented here, calculates fluxes and cycling of O(2), sulfide, and dissolved inorganic carbon (DIC) via abiotic components and via four major microbial guilds: cyanobacteria (CYA), sulfate reducing bacteria (SRB), purple sulfur bacteria (PSB) and colorless sulfur bacteria (CSB). We used generalized Monod-type equations that incorporate substrate and energy limits upon maximum rates of metabolic processes such as photosynthesis and sulfate reduction. We ran a simulation using temperature and irradiance inputs from data collected from a microbial mat in Guerrero Negro in BCS (Mexico). Model O(2), sulfide, and DIC concentration profiles and fluxes compared well with data collected in the field mats. There were some model-predicted features of biogeochemical cycling not observed in our actual measurements. For instance, large influxes and effluxes of DIC across the MBGC mat boundary may reveal previously unrecognized, but real, in situ limits on rates of biogeochemical processes. Some of the short-term variation in field-collected mat O(2) was not predicted by MBGC. This suggests a need both for more model sensitivity to small environmental fluctuations for the incorporation of a photorespiration function into the model.     

Methylmercury enters an aquatic food web through acidophilic microbial mats in Yellowstone National Park, Wyoming

Microbial mats are a visible and abundant life form inhabiting the extreme environments in Yellowstone National Park (YNP), WY, USA. Little is known of their role in food webs that exist in the Park's geothermal habitats. Eukaryotic green algae associated with a phototrophic green/purple Zygogonium microbial mat community that inhabits low-temperature regions of acidic (pH ∼ 3.0) thermal springs were found to serve as a food source for stratiomyid (Diptera: Stratiomyidae) larvae. Mercury in spring source water was taken up and concentrated by the mat biomass. Monomethylmercury compounds (MeHg⁺), while undetectable or near the detection limit (0.025 ng l⁻¹) in the source water of the springs, was present at concentrations of 4–7 ng g⁻¹ dry weight of mat biomass. Detection of MeHg⁺ in tracheal tissue of larvae grazing the mat suggests that MeHg⁺ enters this geothermal food web through the phototrophic microbial mat community. The concentration of MeHg⁺ was two to five times higher in larval tissue than mat biomass indicating MeHg⁺ biomagnification occurred between primary producer and primary consumer trophic levels. The Zygogonium mat community and stratiomyid larvae may also play a role in the transfer of MeHg⁺ to species in the food web whose range extends beyond a particular geothermal feature of YNP.     

Calcium dynamics in microbialite‐forming exopolymer‐rich mats on the atoll of Kiritimati,Republic of Kiribati,Central Pacific

Microbialite‐forming microbial mats in a hypersaline lake on the atoll of Kiritimati were investigated with respect to microgradients, bulk water chemistry, and microbial community composition. O₂, H₂S, and pH microgradients show patterns as commonly observed for phototrophic mats with cyanobacteria‐dominated primary production in upper layers, an intermediate purple layer with sulfide oxidation, and anaerobic bottom layers with sulfate reduction. Ca²⁺ profiles, however, measured in daylight showed an increase of Ca²⁺ with depth in the oxic zone, followed by a sharp decline and low concentrations in anaerobic mat layers. In contrast, dark measurements show a constant Ca²⁺ concentration throughout the entire measured depth. This is explained by an oxygen‐dependent heterotrophic decomposition of Ca²⁺‐binding exopolymers. Strikingly, the daylight maximum in Ca²⁺ and subsequent drop coincides with a major zone of aragonite and gypsum precipitation at the transition from the cyanobacterial layer to the purple sulfur bacterial layer. Therefore, we suggest that Ca²⁺ binding exopolymers function as Ca²⁺ shuttle by their passive downward transport through compression, triggering aragonite precipitation in the mats upon their aerobic microbial decomposition and secondary Ca²⁺ release. This precipitation is mediated by phototrophic sulfide oxidizers whose action additionally leads to the precipitation of part of the available Ca²⁺ as gypsum.     

Chemolithotrophic sulfur bacteria in sediments,mats, and stromatolites of western Australian saline lakes

Samples of stromatolites, microbial mats, and sediments from four saline lakes (approximate seasonal salinity ranges 20–220%o) in Western Australia were used to establish enrichments for elective cultures of aerobic and anaerobic denitrifying chemolithoautotrophs that could grow with thiosulfate as sole energy source. Organisms of these types were obtained from all sources tested. Twenty‐four pure cultures were isolated, all of which were gram‐negative, rod‐shaped bacteria exhibiting a considerable diversity of metabolic capability. Isolation of these obligate and facultative sulfur‐oxidizing chemolithotrophs from the stromatolite and mat habitats indicates the possibility that these rod‐shaped bacteria contribute to the oxidative phase of the sulfur cycle in these habitats, in addition to oxidation by phototrophs or Beggiatoa. Only four of the pure cultures could grow without salt, but all 24 showed significant halophily, some tolerating 3 M NaCl. Three novel isolates of NaCl‐dependent, thiosulfate‐oxidizing, aerobic and denitrifying obligate chemolithotrophs are described. In addition, a facultatively heterotrophic halophilic strain growing either methylotrophically on methylamine or chemolithotrophically on thiosulfate aerobically or with anaerobic denitrification was found.     

Structure and development of a benthic marine microbial mat

Abstract 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.     

The terrestrial biota prior to the origin of land plants (embryophytes): a review of the evidence

It is often assumed that life originated and diversified in the oceans prior to colonizing the land. However, environmental constraints in chemical evolution models point towards critical steps leading to the origin of life as having occurred in subaerial settings. The earliest fossil record does not include finds from terrestrial deposits, so much of our understanding about the presence of a terrestrial microbial cover prior to the Proterozoic is based on inference and geochemical proxies that indicate biospheric carbon cycling during the Archaean. Our assessment is that by 2.7 Ga, microbial ecosystems in terrestrial settings were driven by oxygen‐generating, photosynthetic cyanobacteria. Studies of modern organisms indicate that both the origin and primary diversification of the eukaryotes could have occurred in terrestrial settings, shortly after 2.0 Ga, but there is no direct fossil evidence of terrestrial eukaryotes until about 1.1 Ga. At this time, it appears that the diversity of life in non‐marine habitats exceeded that found in marine settings where sulphidic seas may have impaired eukaryotic physiology and retarded evolution. Geochemical proxies indicate the establishment of an extensive soil‐forming microbial cover by 850 Ma, and it is possible that a rise in atmospheric oxygen at this time was due to the evolutionary expansion of green algae into terrestrial habitats. Direct fossil evidence of the earliest terrestrial biotas in the Phanerozoic consists of problematical palynomorphs from the Cambro‐Ordovician of Laurentia. These indicate that the evolution of the first land plants (embryophytes) during the Middle Ordovician took place within a landscape that included aeroterrestrial algae which were actively adapting to selection in subaerial settings.     

The gas vacuole: An early organelle of prokaryote motility?

Several lines of evidence suggest that the gas vesicle may have been an early organelle of prokaryote motility. First, it is found in bacteria that are thought to be representatives of primitive groups. Second, it is a simple structure, and the structure alone imparts the function of motility. Thirdly, it is widely distributed amongst prokaryotes, having been found in the purple and green sulfur photosynthetic bacteria, cyanobacteria, methanogenic bacteria, obligate and facultative anaerobic heterotrophic bacteria, as well as aerobic heterotrophic bacteria that divide by budding and binary transverse fission. Recent evidence suggests that in some bacteria the genes for gas vesicle synthesis occur on plasmids. Thus, the wide distribution of this characteristic could be due to recent evolution and rapid dispersal, though early evolution is not precluded. Though the gas vesicle structure itself appears to be highly conserved among the various groups of bacteria, it seems doubtful that the regulatory mechanism to control its synthesis could be the same for the diverse gas vacuolate bacterial groups.Proceedings of the Fourth College Park Colloquium on Chemical Evolution:Limits of Life, University of Maryland, College Park, 18–20 October 1978.     

Spectral Irradiance and Distribution of Pigments in a Highly Layered Marine Microbial Mat

The spectral irradiance from 400 to 1,100 nm was measured with depth in the intertidal sand mats at Great Sippewissett Salt Marsh, Mass. These mats contained at least four distinct layers, composed of cyanobacteria, purple sulfur bacteria containing bacteriochlorophyll a (Bchl a), purple sulfur bacteria containing Bchl b, and green sulfur bacteria. Spectral irradiance was measured directly by layering sections of mat on a cosine receptor. Irradiance was also approximated by using a calibrated fiber-optic tip. With the tip, irradiance measurements could be obtained at depth intervals less than 250 μm. The irradiance spectra were correlated qualitatively and quantitatively with the distribution of the diverse chlorophyll pigments in this mat and were compared with spectra recorded in plain sand lacking pigmented phototrophs. We found that the shorter wavelengths (400 to 550 nm) were strongly attenuated in the top 2 mm of the mat. The longer wavelengths (red and near infrared) penetrated to much greater depths, where they were attenuated by Bchl a, b, and c-containing anoxygenic phototrophic bacteria. The specific attenuation bands in the irradiance spectra correlated with the specific in vivo absorption bands of the Bchl-protein complexes in the bacteria. We concluded that the pigments in the phototrophs had a profound affect on the light environment within the mat. It seems likely that the diverse Bchl-protein complexes found in the anoxygenic phototrophs evolved in dense mat environments as a result of competition for light.