Methane production by microbial mats under low sulphate concentrations

Cyanobacterial mats collected in hypersaline salterns were incubated in a greenhouse under low sulphate concentrations ([]) and examined for their primary productivity and emissions of methane and other major carbon species. Atmospheric greenhouse warming by gases such as carbon dioxide and methane must have been greater during the Archean than today in order to account for a record of moderate to warm palaeoclimates, despite a less luminous early sun. It has been suggested that decreased levels of oxygen and sulphate in Archean oceans could have significantly stimulated microbial methanogenesis relative to present marine rates, with a resultant increase in the relative importance of methane in maintaining the early greenhouse. We maintained modern microbial mats, models of ancient coastal marine communities, in artificial brine mixtures containing both modern [] (c. 70 mm ) and ‘Archean’[] (<0.2 mm ). At low [], primary production in the mats was essentially unaffected, while rates of sulphate reduction decreased by a factor of three, and methane fluxes increased by up to 10‐fold. However, remineralization by methanogenesis still amounted to less than 0.4% of the total carbon released by the mats. The relatively low efficiency of conversion of photosynthate to methane is suggested to reflect the particular geometry and chemical microenvironment of hypersaline cyanobacterial mats. Therefore, such mats were probably relatively weak net sources of methane throughout their 3.5 Ga history, even during periods of low environmental levels oxygen and sulphate.     

Biodiversity of the microbial mat of the Garga hot spring

Background

Microbial mats are a good model system for ecological and evolutionary analysis of microbial communities. There are more than 20 alkaline hot springs on the banks of the Barguzin river inflows. Water temperature reaches 75 °C and pH is usually 8.0–9.0. The formation of microbial mats is observed in all hot springs. Microbial communities of hot springs of the Baikal rift zone are poorly studied. Garga is the biggest hot spring in this area.

Results

In this study, we investigated bacterial and archaeal diversity of the Garga hot spring (Baikal rift zone, Russia) using 16S rRNA metagenomic sequencing. We studied two types of microbial communities: (i) small white biofilms on rocks in the points with the highest temperature (75 °C) and (ii) continuous thick phototrophic microbial mats observed at temperatures below 70 °C. Archaea (mainly Crenarchaeota; 19.8% of the total sequences) were detected only in the small biofilms. The high abundance of Archaea in the sample from hot springs of the Baikal rift zone supplemented our knowledge of the distribution of Archaea. Most archaeal sequences had low similarity to known Archaea. In the microbial mats, primary products were formed by cyanobacteria of the genus Leptolyngbya. Heterotrophic microorganisms were mostly represented by Actinobacteria and Proteobacteria in all studied samples of the microbial mats. Planctomycetes, Chloroflexi, and Chlorobi were abundant in the middle layer of the microbial mats, while heterotrophic microorganisms represented mostly by Firmicutes (Clostridia, strict anaerobes) dominated in the bottom part. Besides prokaryotes, we detect some species of Algae with help of detection their chloroplasts 16 s rRNA.

Conclusions

High abundance of Archaea in samples from hot springs of the Baikal rift zone supplemented our knowledge of the distribution of Archaea. Most archaeal sequences had low similarity to known Archaea. Metagenomic analysis of microbial communities of the microbial mat of Garga hot spring showed that the three studied points sampled at 70 °C, 55 °C, and 45 °C had similar species composition. Cyanobacteria of the genus Leptolyngbya dominated in the upper layer of the microbial mat. Chloroflexi and Chlorobi were less abundant and were mostly observed in the middle part of the microbial mat. We detected domains of heterotrophic organisms in high abundance (Proteobacteria, Firmicutes, Verrucomicrobia, Planctomicetes, Bacteroidetes, Actinobacteria, Thermi), according to metabolic properties of known relatives, which can form complete cycles of carbon, sulphur, and nitrogen in the microbial mat. The studied microbial mats evolved in early stages of biosphere formation. They can live autonomously, providing full cycles of substances and preventing live activity products poisoning.

     

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

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

A Novel Approach for Bioremediation of a Coastal Marine Wastewater Effluent Based on Artificial Microbial Mats

A novel alternative for wastewater effluent bioremediation was developed using constructed microbial mats on low-density polyester. This biotechnology showed high removal efficiencies for nitrogen and phosphorous in a short retention time (48 h): 94% for orthophosphate (7.78 g m³ d⁻¹), 79% for ammonium (11.30 g m⁻³ d⁻¹), 78% for nitrite (7.46 g m⁻³ d⁻¹), and 83% for nitrate (8.55 g m⁻³ d⁻¹). The microbial mats were dominated by Cyanobacteria genera such as Chroococcus sp., Lyngbya sp., and bacteria of the subclass Proteobacteria representative of the Eubacteria Domain. Nitzschia sp. was the dominant Eukaryote Domain. Various N and P substrates in the wastewater permit the growth of self-forming and self-sustaining bacterial, microalgal, and cyanobacterial communities on a polyester support. The result is the continuous, self-sufficient growth of microbial mats. This is an innovative, economical, and environmentally safe alternative for the treatment of wastewater effluents in coastal marine environments.     

Use of specific PCR primers for the study of sulfate-reducing bacteria diversity in microbial mats of Ebro Delta,Spain

Microbial mats are prokaryotic communities that provide model systems to analyze microbial diversity and ecophysiological interactions. Sulfate-reducing bacteria (SRB) play a key role in sulfur and nutrient recycling in these ecosystems. In this work, specific primers for 16S rRNA encoding gene, previously described, were used to study the diversity of SRB in microbial mats of the Ebro Delta. We confirm that this method is reliable to identify the diversity of SRB in these ecosystems. However, some mismatches in obtained sequences had been observed in our system and must be taken under consideration. Various genera of SRB in Ebro Delta microbial mats were identified, such as Desulfonema, Desulfatitalea, Desulfosalsimonas, Desulfoccocus, and Desulfovibrio. The diversity observed in our samples is very similar to previously reported in other microbial mats communities.     

Microbiological Processes of the Carbon and Sulfur Cycles at Cold Methane Seeps of the North Atlantic

Functioning of microbial communities in surface sediments of the Haakon Mosby underwater mud volcano (lat. 72°N) and in gas seepage fields of the Vestnesa Ridge was investigated using Mir-1 and Mir-2 deep-sea submersibles during the 40th voyage of the research vessel Academician Mstislav Keldysh. Large areas of sedimentary deposits of the Haakon Mosby mud volcano (HMMV) and pockmarks of the Vestnesa Ridge (VR) are covered with bacterial mats 0.1 to 0.5 cm thick. The microbial community making up bacterial mats of the HMMV was dominated by large filamentous bacteria with filaments measuring up to 100 m in length and 2 to 8 m in width. The occurrence of rosettes allowed the observed filamentous bacteria to be referred to the morphologically similar genera Leucothrix or Thiothrix. Three morphological types of filamentous bacteria were identified in bacterial mats covering VR pockmarks. Filaments of type one are morphologically similar with representatives of the genera Thioploca or Desmanthos. Type two filaments had numerous inclusions of sulfur and resembled representatives of the genus Thiothrix. The third morphological type was constituted by single filaments made up of tightly connected disk-like cells and can be assigned to the genus Beggiatoa. The rates of methane oxidation (up to 1570 l C/(dm³ day)) and sulfate reduction (up to 17 mg S/(dm³day)) measured in the surface sediments of HMMV and VR were close to the maximum rates of these processes observed in heavily polluted regions of the northwestern shelf of the Black Sea. High rates of microbiological processes correlated with the high number of bacteria. The rate of methane production in sediments studied was notably lower and ranged from 0.1 to 3.5 CH₄/(dm³ day). Large areas of the HMMV caldera were populated by pogonophoras, represented by the two species Sclerolinum sp. and Oligobrachia sp. The mass development of Sclerolinum sp. in the HMMV caldera was by the activity of aerobic methane-oxidizing bacteria localized inside the cells of these animals. Bacterial cells were also found in the trophosome tissue of Oligobrachia sp., but in cells of these bacteria, we did not observe the membrane structures typical of methanotrophs. The localization pattern of pogonophoras on the surface of reduced sediments suggests that the predominant bacteria in Oligobrachia tissues are sulfur-oxidizing endosymbionts.     

Leaching of silica and uranium and other quantitative aspects of the lithobiontic colonization in a radioactive thermal spring

The formation of microbial mats by thermophilic organisms on submerged rocks in radioactive thermal springs was followed quantitatively in situ as well as under experimental conditions, by determining the change in dry weight and organic matter as a function of time. Furthermore, the decay of the rock occurring in the springs could be shown to be directly related to the microbial colonization. Early in that process the formation of silicious gels, facilitating the settling of the organisms, could be observed. Simultaneously, this was accompanied by the leaching of silica from the underlying rock. This resulted in the destruction of the rock, which had been altered to a fine-grained dust underneath the colonizing mats; the microorganisms were found to move further downward within this layer. From the heavy metals present in the rock—iron (Fe), copper (Cu), manganese (Mn), uranium (U)— the leaching of uranium could be demonstrated, leading to the acquisition of this metal in the microbial mats in concentrations up to 15.34g/mg dry weight. Direct evidence for the leaching of Si (silicon) and U could be obtained by measurement of these elements after their release from ground rock chips in cultures with microorganisms from the hot springs at 50°C. X-ray analysis of the biomats strongly suggested that Cu, Mn, and Fe are also accumulated.     

Growth and metabolism of the purple sulfur bacterium Thiocapsa roseopersicina under combined light/dark and oxic/anoxic regimens

The dominant purple sulfur bacterium of laminated sediment ecosystems in temperate environments, Thiocapsa roseopersicina, was cultivated in sulfide-limited continuous cultures (D=0.03 h^-1) subjected to various combined diel regimen of aeration and illumination in order to simulate environmental conditions in microbial mats. For comparison, cultures were grown under similar illumination regimens but continuously anoxic conditions.Bacteriochlorophyll a (BChla) and carotenoid synthesis was restricted to anoxic-dark periods and did not occur during oxic-light periods. An increase in the length of the oxic-light periods resulted in decreased pigment contents. However, phototrophic growth remained possible even at 20 h oxic-light/4 h anoxic-dark regimens. When anoxic conditions were maintained throughtout, BChla synthesis occurred both during light and dark periods.Glycogen was synthesized in the light and degraded in the dark. Calculations showed that degradation of 1/4–1/5 of the glycogen is sufficient to account for the BChla and carotenoid synthesis in the dark.The data showed that T. roseopersicina is very well adapted to cope with the combined oxygen and light regimes as they occur in microbial mats, which may explain the dominance of this bacterium in the purple layer of these sediment ecosystems.Non-standard abbreviations BChl bacteriochlorophyll - specific growth rate - D dilution rate - S_R concentration of limiting substrate in reservoir bottle     

Phylogenetic Evidence for the Existence of Novel Thermophilic Bacteria in Hot Spring Sulfur-Turf Microbial Mats in Japan

So-called sulfur-turf microbial mats, which are macroscopic white filaments or bundles consisting of large sausage-shaped bacteria and elemental sulfur particles, occur in sulfide-containing hot springs in Japan. However, no thermophiles from sulfur-turf mats have yet been isolated as cultivable strains. This study was undertaken to determine the phylogenetic positions of the sausage-shaped bacteria in sulfur-turf mats by direct cloning and sequencing of 16S rRNA genes amplified from the bulk DNAs of the mats. Common clones with 16S rDNA sequences with similarity levels of 94.8 to 99% were isolated from sulfur-turf mat samples from two geographically remote hot springs. Phylogenetic analysis showed that the phylotypes of the common clones formed a major cluster with members of the Aquifex-Hydrogenobacter complex, which represents the most deeply branching lineage of the domain bacteria. Furthermore, the bacteria of the sulfur-turf mat phylotypes formed a clade distinguishable from that of other members of the Aquifex-Hydrogenobacter complex at the order or subclass level. In situ hybridization with clone-specific probes for 16S rRNA revealed that the common phylotype of sulfur-turf mat bacteria is that of the predominant sausage-shaped bacteria.Microbial mats develop in a wide variety of aquatic environments, including geothermal hot springs and hydrothermal vents. There are several types of thermophilic microbial mats, e.g., those of cyanobacteria, anoxygenic phototrophic bacteria, and chemotrophic sulfur bacteria, which differ according to the physical and chemical conditions they favor and other environmental factors (10, 38). These microbial mats in thermal habitats have been studied extensively as a peculiar microbial community of the ecosystem, in relation to the phylogeny and evolution of thermophilic prokaryotes, or as a source of new functional enzymes.So-called sulfur-turf microbial mats are macroscopic bundles of white filaments consisting of colorless sulfur bacteria and elemental sulfur particles that form in shallow streams of sulfide-containing high-temperature hot springs. Since first reported by Miyoshi in 1897 (33), this kind of microbial mat has been recorded for several geographically remote hot springs in Japan, although there have been only scattered reports of sulfur-turf microbial mats or chemotrophic sulfur streamers in geothermal springs in other countries (9, 13, 14). The sulfur-turf mats generally develop within a temperature range of 45 to 73°C, within a pH range of 6 to 9, and at discrete sulfide-oxygen interfaces in geothermal springs. These characteristics suggest that the major constituents of the sulfur-turf prokaryotic community are (hyper)thermophilic, neutrophilic, microaerophilic, and chemolithotrophic bacteria. Early studies of these sulfur-turf mats distinguished microscopically three morphotypes of bacteria, two of which were tentatively named Thiovibrio miyoshi and Thiothrix miyoshi (15). Moreover, in situ ecophysiological and microscopic studies have shown that one of these bacteria, the large sausage-shaped “Thiovibrio miyoshi,” predominates in sulfur-turf mats and oxidizes environmental sulfide to elemental sulfur and then to sulfate via thiosulfate (27–31). So far, however, it has not been possible to isolate and cultivate any thermophilic prokaryotes from the sulfur-turf mats predominated by these sausage-shaped bacteria with artificial media, and no attempt has been made to clarify their taxonomic and phylogenetic positions.Determination of 16S rRNA genes is a useful research strategy for identifying uncultivated prokaryotes and is now commonly performed in ecological studies. This technique, involving PCR amplification of 16S rRNA genes or synthesis of cDNAs from bulk 16S rRNAs of natural mixed microbial populations, has been used successfully for the phylogenetic characterization of prokaryotes in hydrothermal environments (6, 7, 34, 40, 41, 47, 48). In the present study, this approach was applied to characterize the sausage-shaped bacteria in sulfur-turf mats without isolating and cultivating them. Here we report that sulfur-turf mats contain novel thermophilic bacteria belonging to the earliest-branching lineage of the domain bacteria.     

Benthic algal mats of some lakes of Inexpressible Island (northern Victoria Land,Antarctica)

 Benthic algal mats from four lakes located on Inexpressible Island (northern Victoria Land, Antarctica) were studied, analysing their structure and floristic composition. Thirty taxa of algae were identified: 11 Cyanophyta, 15 Bacillariophyta and 4 Chlorophyta. The lake mats contained from 15 (lake C) to 26 species (lake D). The number of diatom frustules in the mats varied from 771×10³ (lake C) to 9428×10³ frustules g^-1 (lake B). In terms of floristic composition and macromorphology, these mats were observed to be very similar to moat mats described from Lake Gondwana (northern Victoria Land) and the Dry Valleys lakes of southern Victoria Land. Received: 9 July 1995/Accepted: 12 February 1996     

Bacterial zonation,photosynthesis, and spectral light distribution in hot spring microbial mats of Iceland

The zonation and structure of phototrophic microbial mats were studied along two thermal gradients in sulfide-rich hot springs of southwest Iceland. The green, filamentous bacteriumChloroflexus and the unicellular, high-temperature form (HTF) ofMastigocladus formed mats growing up to a temperature limit of 62–66°C. The dominant phototrophs wereChloroflexus sp.,Mastigocladus laminosus, andPhormidium laminosum, respectively, at the three temperature intervals: >60°C, 60°C to 55–50°C, and <55–50°C. AChloroflexus mat growing at 60°C under 60M H₂S was anoxic in the light with the exception of a 0.5 mm thick band of HTFMastigocladus which produced oxygen. The oxygenic photosynthesis of these H₂S-sensitive cyanobacteria was probably dependent on a preceding sulfide depletion by the anoxygenicChloroflexus. Measurements of spectral radiance gradients with a fiberoptic microprobe showed maximum light attenuation by carotenoids and bacteriochlorophyllC. AM. laminosus mat growing at 52°C was oxic throughout and showed maximum light attenuation by carotenoids, chlorophyllA, and phycocyanin, but no detectable phycoerythrocyanin absorption.     

Structure and function of natural sulphide-oxidizing microbial mats under dynamic input of light and chemical energy

We studied the interaction between phototrophic and chemolithoautotrophic sulphide-oxidizing microorganisms in natural microbial mats forming in sulphidic streams. The structure of these mats varied between two end-members: one characterized by a layer dominated by large sulphur-oxidizing bacteria (SOB; mostly Beggiatoa-like) on top of a cyanobacterial layer (B/C mats) and the other with an inverted structure (C/B mats). C/B mats formed where the availability of oxygen from the water column was limited (<5 μm). Aerobic chemolithotrophic activity of the SOB depended entirely on oxygen produced locally by cyanobacteria during high light conditions. In contrast, B/C mats formed at locations where oxygen in the water column was comparatively abundant (>45 μM) and continuously present. Here SOB were independent of the photosynthetic activity of cyanobacteria and outcompeted the cyanobacteria in the uppermost layer of the mat where energy sources for both functional groups were concentrated. Outcompetition of photosynthetic microbes in the presence of light was facilitated by the decoupling of aerobic chemolithotrophy and oxygenic phototrophy. Remarkably, the B/C mats conserved much less energy than the C/B mats, although similar amounts of light and chemical energy were available. Thus ecosystems do not necessarily develop towards optimal energy usage. Our data suggest that, when two independent sources of energy are available, the structure and activity of microbial communities is primarily determined by the continuous rather than the intermittent energy source, even if the time-integrated energy flux of the intermittent energy source is greater.     

Salinity, depth and the structure and composition of microbial mats in continental Antarctic lakes

1. Lakes and ponds in the Larsemann Hills and Bølingen Islands (East‐Antarctica) were characterised by cyanobacteria‐dominated, benthic microbial mats. A 56‐lake dataset representing the limnological diversity among the more than 150 lakes and ponds in the region was developed to identify and quantify the abiotic conditions associated with cyanobacterial and diatom communities. 2. Limnological diversity in the lakes of the Larsemann Hills and Bølingen Islands was associated primarily with conductivity and conductivity‐related variables (concentrations of major ions and alkalinity), and variation in lake morphometry (depth, catchment and lake area). Low concentrations of pigments, phosphate, nitrogen, DOC and TOC in the water column of most lakes suggest extremely low water column productivity and hence high water clarity, and may thus contribute to the ecological success of benthic microbial mats in this region. 3. Benthic communities consisted of prostrate and sometimes finely laminated mats, flake mats, epilithic and interstitial microbial mats. Mat physiognomy and carotenoid/chlorophyll ratios were strongly related to lake depth, but not to conductivity. 4. Morphological‐taxonomic analyses revealed the presence of 26 diatom morphospecies and 33 cyanobacterial morphotypes. Mats of shallow lakes (interstitial and flake mats) and those of deeper lakes (prostrate mats) were characterised by different dominant cyanobacterial morphotypes. No relationship was found between the distribution of these morphotypes and conductivity. In contrast, variation in diatom species composition was strongly related to both lake depth and conductivity. Shallow ponds were mainly characterised by aerial diatoms (e.g. Diadesmis cf. perpusilla and Hantzschia spp.). In deep lakes, communities were dominated by Psammothidium abundans and Stauroforma inermis. Lakes with conductivities higher than ±1.5 mS cm^?1 became susceptible to freezing out of salts and hence pronounced conductivity fluctuations. In these lakes P. abundans and S. inermis were replaced by Amphora veneta. Stomatocysts were important only in shallow freshwater lakes. 5. Ice cover influenced microbial mat structure and composition both directly by physical disturbance in shallow lakes and by influencing light availability in deeper lakes, as well as indirectly by generating conductivity increases and promoting the development of seasonal anoxia. 6. The relationships between diatom species composition and conductivity, and diatom species composition and depth, were statistically significant. Transfer functions based on these data can therefore be used in paleolimnological reconstruction to infer changes in the precipitation–evaporation balance in continental Antarctic lakes.     

Food items of the grey mullet Mugil cephalus in the Banc d'Arguin area (Mauritania)

The stomach and gut content of 22 Mugil cephalus from the Banc d'Arguin shallows (Mauritanian coast, West Africa) is examined. Quartz grains are the predominant material found in the stomach together with small portions of benthic diatoms and flakes, aggregates of fine-grained inorganic and organic particles. As the composition of this material is modified by a sorting procedure during feeding it is difficult to conclude, whether seagrass stands (muddy and mixed sediments with sand fraction) or sandy flats are preferred as feeding habitats. The animals studied did not or not relevantly utilize the non-diatom microphytes, though the environment of the Banc d'Arguin is rich in microbial mats (cyanobacteria), Vaucheria beds and epiphytic vegetation of seagrass leaves.     

Coral transplantation as an aid to reef rehabilitation: evaluation of a case study in the Maldive Islands

As part of a study of reef rehabilitation, whole coral colonies (primarily Acropora, Pocillopora, Porites, Eavia and Favites) were transplanted and cemented in place onto three approximately 20 m² areas of Armorflex concrete mats on a 0.8–1.5 m deep reef-flat in the Maldives which had been severely degraded by coral mining. Growth, in situ mortality, and losses from mats due to wave action of a total of 530 transplants were monitored over 28 months. Natural recruitment of corals to both the transplanted Armorflex areas and concrete mats without transplants was also studied. Overall survivorship of corals 28 moths after transplantation was 51%. Most losses of transplants due to wave action occurred during the first 7 months when 25% were lost, with only a further 5% of colonies being lost subsequently. Within 16 months most colonies had accreted naturally to the concretemats. Thirty-two percent of transplants which remained attached died with Acropora hyacinthus and Pocillopora perrucosa having the highest mortality rates (approx. 50% nortality over two years) and Porites lobata and P. lutea the lowest (2.8 and 8.1% mortality respectively over two years). Growth rates were very variable with a quarter to a third of transplants showing negative growth during each inter-survey period. Acropora hyacinthus, A. cytherea and A. divaricata transplants had the highest growth rates (colony mean linear radial extension 4.15–5.81 cm y^-1), followed by Pocillopora verrucosa (mean 2.51 cm y^-1). Faviids and poritids had lowest growth rates. Favia and Favites showed the poorest response to transplantation whilst Acropora divaricata, which combined a high growth rate with relatively low mortality, appeared particularly amenable to transplantation. Natural recruitment did not differ significantly between concrete mats with and without transplanted corals. Visible recruits wer first recorded 10 months after emplacement of the mats and were predominantly Acropora and Pocillopora. On near vertical surfaces their density was almost 18 m^-2. Recruits grew fast producing many 20–30 cm diameter colonies on the mats within 3.5 years. Growth and survival of transplants are compared with results of transplantation studies in other locations. We conclude: (1) species transplanted should be selected with care as certain species are significantly more amenable than others to transplantation, (2) the choice of whether fragments or whole colonies are transplanted may profoundly influence survival, (3) considerable loss of transplants is likely from higher energy sites whatever method of attachment, (4) transplantation should, in general, be undertaken only if recovery following natural recruitment is unlikely.     

Use of the crossflow-microscreen technique for SCP production from dilute substrates

Summary A crossflow-microscreen cultivation technique was successfully used to select and maintain an easily harvestable microbial culture with a limited number of species under non-aseptic conditions in diluted cheese whey. The microbial selective pressure exerted by the system could be manipulated by varying the hydraulic () and mean cell () residence times. The optimum system parameters were =1 h and =10 h, resulting in a selected microbial population comprising three species only, namely Geotrichum candidum, Streptococcus cremoris and Leuconostoc lactophilum. The amino acid profile of the SCP produced compared favourably with other types of protein. The crossflow-microscreen technique makes SCP production possible from dilute, waste organic effluents.     

Dynamics of Dimethyl Sulfide in a Marine Microbial Mat

Abstract The microbial mat was chosen as a model ecosystem to study dynamics of dimethyl sulfide (DMS) in marine sediments in order to gain insight into key processes and factors which determine emission rates. A practical advantage, compared to open ocean ecosystems, is that microbial mats contain high biomasses of different functional groups of bacteria involved in DMS dynamics, and that DMS concentrations are generally high enough to allow direct measurement of emission rates. Field data showed that, during the seasonal development of microbial mats, concentrations of chlorophyll a corresponded to dimethylsulfoniopropionate (DMSP). DMSP is an important precursor of DMS. It was demonstrated, with laboratory cultures, that various species of benthic diatoms produce substantial amounts of DMSP. The abundances of aerobic and anaerobic DMS- or DMSO-utilizing bacteria were estimated using the most-probable-number technique. Laboratory experiments with relatively undisturbed sediment cores showed that microbial mats act as a sink for DMS under oxic/light (day) conditions, and as a source of DMS under anoxic/dark (night) conditions. Axenic culture studies with Chromatium vinosum M2 and Thiocapsa pfennigii M8 (isolated from a microbial mat) showed that, under anoxic/light conditions, DMS was quantitatively converted to dimethylsulfoxide (DMSO). T. roseopersicina M11 converted DMSP to DMS and acrylate, apparently without use of either substrate. Received: 5 May 1997; Accepted: 21 August 1997     

Periphyton flora of some lotic and lentic environments of Hope Bay (Antarctic Peninsula)

A taxonomic study was made of the periphytic algal flora of some freshwater environments at Hope Bay, Antarctic Peninsula (63°27S; 56°59W), during summer 1992–1993. Seven lakes and two pools that are characterized by different trophic levels were studied. The largest, Boeckella Lake, has an important outflow (Prasiola Stream) that was also analyzed. Ninety-seven algal taxa, including specific and infraspecific entities, are recorded. Thirty-one new records for the Antarctic continent are described and illustrated. The flora comprises 44.4% Cyanophyceae, 21.6% Bacillariophyceae, 18.5% Chlorophyceae, 9.3% Tribophyceae, 4.2% Chrysophyceae and 2% Zygophyceae. The epilithic flora of Hope Bay shows a high species richness. Phormidiumfragile and Lyngbya lagerheimii were the most abundant species among the Cyanophyceae. In the algal mats we also found Chrysophyceae (mainly Hydrurus foetidus and Chrysococcus rufescens) and the macroscopic chlorophycean Prasiola crispa. Tribonema australis sp. nov. (Tribonematales, Tribophyceae) is put forward as a new species. Hormidium fluitans (Gay) Heering is renamed Klebshormidium fluitans.     

Characterization of sulfide-oxidizing microbial mats developed inside a full-scale anaerobic digester employing biological desulfurization

The microbial mats responsible for biological desulfurization from biogas in a full-scale anaerobic digester were characterized in terms of their structure, as well as their chemical and microbial properties. Filament-shaped elemental sulfur 100–500 μm in length was shown to cover the mats, which cover the entire headspace of the digester. This is the first report on filamentous sulfur production in a non-marine environment. The results of the analysis of the mats suggest that the key players in the sulfide oxidation and sulfur production in the bio-desulfurization in the headspace of the digester were likely to be two sulfide-oxidizing bacteria (SOB) species related to Halothiobacillus neapolitanus and Sulfurimonas denitrificans, and that the microbial community, cell density, activity for sulfide oxidation varied according to the environmental conditions at the various locations of the mats. Since the water and nutrients necessary for the SOB were provided by the digested sludge droplets deposited on the mats, and our results show that a higher rate of sulfide oxidation occurred with more frequent digested sludge deposition, the habitat of the SOB needs to be made in the lower part of the headspace near the liquid level of the digested sludge to maintain optimal conditions.     

Canopy height and competition explain species segregation in wet heathlands

Questions

What is the general pattern of species co‐occurrence in managed heathlands? Is the pattern consistent among functional groups? Is it ruled by species competition, or by contrasting environments at a fine scale? Does grazing pressure and herbivore species condition species interactions?

Location

Erica mackayana wet heaths, Galicia, NW Iberian Peninsula.

Methods

A null model approach was used to compare species co‐occurrence with generated random matrices from 54 10‐m transects. The C‐score was obtained from the multispecies presence/absence matrix for each transect of shrubs and graminoids recorded at 25‐cm intervals. Differences in canopy height were recorded to assess the importance of the environment compared to inter‐specific competition. Results were linked to different levels of grazing pressure and herbivore species.

Results

Species segregation was the main pattern for all species, but mainly among graminoid species compared to shrubs. Graminoids showed an even proportion of segregated pairs explained by different canopy heights and competition. These differences were mainly species environmental requirements of canopy height. Levels of grazing pressure enhanced species segregation in graminoids but had no effect on shrubs or the total species set.

Conclusions

Competition and canopy height affect the E. mackayana heathland composition, but differently for functional groups. A heterogeneous vegetation profile with shrub mats and open gaps created by light grazing promotes species co‐existence within mats and competition in gaps. I suggest this is an optimum structure for the habitat to be targeted through management.