Dynamics of anoxygenic photosynthesis in an experimental green sulphur bacteria biofilm

The dynamics of sulphide oxidation in an experimental biofilm of the green sulphur bacterium, Prosthecochloris aestuarii , were studied using a newly developed light–dark cycling procedure. The biofilm was grown for 6 weeks in a benthic gradient chamber, in which gradients of light, sulphide and oxygen were imposed experimentally. The H₂S concentrations and pH were measured with microsensors as a function of depth in the biofilm and of time after a change in illumination status. The sulphide oxidation rates were calculated as a function of time and depth in the biofilm using a numerical procedure to solve the non-stationary general diffusion equation. A close agreement was found between the areal rates of anoxygenic photosynthesis during the cycling procedure and the steady state before the cycling experiment. For the different layers of the biofilm, the maximum activity was observed after 10–12 min of light exposure. After this maximum, sulphide oxidation decreased concomitantly with sulphide concentration, indicating sulphide limitation of anoxygenic photosynthesis. This lag time limits the application of the standard dark–light shift method with a brief light exposure of a few seconds and, therefore, the numerical procedure described in this study enables the depth distribution of anoxygenic photosynthesis rates in microbial mats to be determined more accurately.     

ANTARCTIC CYANOBACTERIA: LIGHT,NUTRIENTS, AND PHOTOSYNTHESIS IN THE MICROBIAL MAT ENVIRONMENT1

The microenvironmental and photosynthetic characteristics of Antarctic microbial mats were measured in a series of ponds near McMurdo Sound. As elsewhere in Antarctica, these cold-water benthic communities were dominated by oscillatoriacean cyanobacteria. Despite large variations in mat thickness, surface morphology, and color, all of the communities had a similar pigment organization, with a surface carotenoid-rich layer that overlaid a deep chlorophyll maximum (DCM) enriched in phycocyanin as well as chlorophyll a. Spectroradiometric analyses showed that the DCM population inhabited an orange-red shade environment. In several of the mats, the deep-living trichomes migrated up to the surface of the mat within 2 h in response to a 10-fold decrease in surface irradiance. The euphotic layer of the mats was supersaturated in oxygen and contained ammonium and dissolved reactive phosphorus concentrations in excess of 100 mg N·m^?3 or P·m^?3. Integral photosynthesis by core samples was saturated at low irradiances and varied two- to threefold throughout the continuous 24-h radiation cycle. Oxygen microelectrode analyses showed that the photosynthetic rates were slow to negligible near the surface and maximal in the DCM. These compressed, nutrient-rich euphotic zones have some properties analogous to planktonic systems, but the integrated photosynthetic responses of the community reflect the strong self-shading within the mat and physiological dominance by the motile, DCM populations.     

Cyanobacterial life at low O(2): community genomics and function reveal metabolic versatility and extremely low diversity in a Great Lakes sinkhole mat

Cyanobacteria are renowned as the mediators of Earth's oxygenation. However, little is known about the cyanobacterial communities that flourished under the low-O(2) conditions that characterized most of their evolutionary history. Microbial mats in the submerged Middle Island Sinkhole of Lake Huron provide opportunities to investigate cyanobacteria under such persistent low-O(2) conditions. Here, venting groundwater rich in sulfate and low in O(2) supports a unique benthic ecosystem of purple-colored cyanobacterial mats. Beneath the mat is a layer of carbonate that is enriched in calcite and to a lesser extent dolomite. In situ benthic metabolism chambers revealed that the mats are net sinks for O(2), suggesting primary production mechanisms other than oxygenic photosynthesis. Indeed, (14)C-bicarbonate uptake studies of autotrophic production show variable contributions from oxygenic and anoxygenic photosynthesis and chemosynthesis, presumably because of supply of sulfide. These results suggest the presence of either facultatively anoxygenic cyanobacteria or a mix of oxygenic/anoxygenic types of cyanobacteria. Shotgun metagenomic sequencing revealed a remarkably low-diversity mat community dominated by just one genotype most closely related to the cyanobacterium Phormidium autumnale, for which an essentially complete genome was reconstructed. Also recovered were partial genomes from a second genotype of Phormidium and several Oscillatoria. Despite the taxonomic simplicity, diverse cyanobacterial genes putatively involved in sulfur oxidation were identified, suggesting a diversity of sulfide physiologies. The dominant Phormidium genome reflects versatile metabolism and physiology that is specialized for a communal lifestyle under fluctuating redox conditions and light availability. Overall, this study provides genomic and physiologic insights into low-O(2) cyanobacterial mat ecosystems that played crucial geobiological roles over long stretches of Earth history.     

UV B-Induced Vertical Migrations of Cyanobacteria in a Microbial Mat

Exposure to moderate doses of UV B (0.35 to 0.79 W m(sup-2) s(sup-1) or 0.98 to 2.2 (mu)mol of photons m(sup-2) s(sup-1) at 310 nm) caused the surface layers of microbial mats from Solar Lake, Sinai, Egypt, to become visibly lighter green. Concurrent with the color change were rapid and dramatic reductions in gross photosynthesis and in the resultant high porewater oxygen concentrations in the surface layers of the mats. The depths at which both maximum gross photosynthesis and maximum oxygen concentrations occurred were displaced downward. In contrast, gross photosynthesis in the deeper layers of the mats increased in response to UV B incident upon the surface. The cessation of exposure to UV B partially reversed all of these changes. Taken together, these responses suggest that photoautotrophic members of the mat community, most likely the dominant cyanobacterium Microcoleus chthonoplastes, were migrating in response to the added UV B. The migration phenomenon was also observed in response to increases in visible radiation and UV A, but UV B was ca. 100-fold more effective than visible radiation and ca. 20-fold more effective than UV A in provoking the response. Migrating microorganisms within this mat are apparently able to sense UV B directly and respond behaviorally to limit their exposure to UV. Because of strong vertical gradients of light and dissolved substances in microbial mats, the migration and the resultant vertical redistribution of photosynthetic activity have important consequences for both the photobiology of the cyanobacteria and the net primary productivity of the mat ecosystem.     

Mathematical simulation of the interactions among cyanobacteria, purple sulfur bacteria and chemotrophic sulfur bacteria in microbial mat communities

Abstract: A deterministic one-dimensional reaction diffusion model was constructed to simulate benthic stratification patterns and population dynamics of cyanobacteria, purple and colorless sulfur bacteria as found in marine microbial mats. The model involves the major biogeochemical processes of the sulfur cycle and includes growth metabolism and their kinetic parameters as described from laboratory experimentation. Hence, the metabolic production and consumption processes are coupled to population growth. The model is used to calculate benthic oxygen, sulfide and light profiles and to infer spatial relationships and interactions among the different populations. Furthermore, the model is used to explore the effect of different abiotic and biotic environmental parameters on the community structure. A strikingly clear pattern emerged of the interaction between purple and colorless sulfur bacteria: either colorless sulfur bacteria dominate or a coexistence is found of colorless and purple sulfur bacteria. The model predicts that purple sulfur bacteria only proliferate when the studied environmental parameters surpass well-defined threshold levels. However, once the appropriate conditions do occur, the purple sulfur bacteria are extremely successful as their biomass outweighs that of colorless sulfur bacteria by a factor of up to 17. The typical stratification pattern predicted closely resembles the often described bilayer communities which comprise a layer of purple sulfur bacteria below a cyanobacterial top-layer; colorless sulfur bacteria are predicted to sandwich in between both layers. The profiles of oxygen and sulfide shift on a diel basis similarly as observed in real systems.     

Biogeochemistry of hypersaline microbial mats illustrates the dynamics of modern microbial ecosystems and the early evolution of the biosphere

Photosynthetic microbial mats are remarkably complete self-sustaining ecosystems at the millimeter scale, yet they have substantially affected environmental processes on a planetary scale. These mats may be direct descendents of the most ancient biological communities in which even oxygenic photosynthesis might have developed. Photosynthetic mats are excellent natural laboratories to help us to learn how microbial populations associate to control dynamic biogeochemical gradients.     

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 degrees 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 fouling appears with a decrease in the temperature to 62 degrees C and in the 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 degrees 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 degrees C, which is 10 to 15 degrees 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 degrees C. 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 degrees C. The rate of methanogenesis did not exceed 1.188 micrograms 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.     

Community structure and physiological characterization of microbial mats in Byers Peninsula, Livingston Island (South Shetland Islands, Antarctica)

The community structure and physiological characteristics of three microbial mat communities in Byers Peninsula (Livingston Island, South Shetland Islands, Antarctica) were compared. One of the mats was located at the edge of a stream and was dominated by diatoms (with a thin basal layer of oscillatorian cyanobacteria), whereas the other two mats, located over moist soil and the bottom of a pond, respectively, were dominated by cyanobacteria throughout their vertical profiles. The predominant xanthophyll was fucoxanthin in the stream mat and myxoxanthophyll in the cyanobacteria-dominated mats. The sheath pigment scytonemin was absent in the stream mat but present in the soil and pond mats. The stream mat showed significantly lower delta13C and higher delta15N values than the other two mats. Consistent with the delta15N values, N2 fixation was negligible in the stream mat. The soil mat was the physiologically most active community. It showed rates of photosynthesis three times higher than in the other mats, and had the highest rates of ammonium uptake, nitrate uptake and N2 fixation. These observations underscore the taxonomic and physiological diversity of microbial mat communities in the maritime Antarctic region.     

Phototrophs in High-Iron-Concentration Microbial Mats: Physiological Ecology of Phototrophs in an Iron-Depositing Hot Spring

At Chocolate Pots Hot Springs in Yellowstone National Park the source waters have a pH near neutral, contain high concentrations of reduced iron, and lack sulfide. An iron formation that is associated with cyanobacterial mats is actively deposited. The uptake of [¹⁴C]bicarbonate was used to assess the impact of ferrous iron on photosynthesis in this environment. Photoautotrophy in some of the mats was stimulated by ferrous iron (1.0 mM). Microelectrodes were used to determine the impact of photosynthetic activity on the oxygen content and the pH in the mat and sediment microenvironments. Photosynthesis increased the oxygen concentration to 200% of air saturation levels in the top millimeter of the mats. The oxygen concentration decreased with depth and in the dark. Light-dependent increases in pH were observed. The penetration of light in the mats and in the sediments was determined. Visible radiation was rapidly attenuated in the top 2 mm of the iron-rich mats. Near-infrared radiation penetrated deeper. Iron was totally oxidized in the top few millimeters, but reduced iron was detected at greater depths. By increasing the pH and the oxygen concentration in the surface sediments, the cyanobacteria could potentially increase the rate of iron oxidation in situ. This high-iron-content hot spring provides a suitable model for studying the interactions of microbial photosynthesis and iron deposition and the role of photosynthesis in microbial iron cycling. This model may help clarify the potential role of photosynthesis in the deposition of Precambrian banded iron formations.     

Phototrophs in high-iron-concentration microbial mats: physiological ecology of phototrophs in an iron-depositing hot spring

At Chocolate Pots Hot Springs in Yellowstone National Park the source waters have a pH near neutral, contain high concentrations of reduced iron, and lack sulfide. An iron formation that is associated with cyanobacterial mats is actively deposited. The uptake of [(14)C]bicarbonate was used to assess the impact of ferrous iron on photosynthesis in this environment. Photoautotrophy in some of the mats was stimulated by ferrous iron (1.0 mM). Microelectrodes were used to determine the impact of photosynthetic activity on the oxygen content and the pH in the mat and sediment microenvironments. Photosynthesis increased the oxygen concentration to 200% of air saturation levels in the top millimeter of the mats. The oxygen concentration decreased with depth and in the dark. Light-dependent increases in pH were observed. The penetration of light in the mats and in the sediments was determined. Visible radiation was rapidly attenuated in the top 2 mm of the iron-rich mats. Near-infrared radiation penetrated deeper. Iron was totally oxidized in the top few millimeters, but reduced iron was detected at greater depths. By increasing the pH and the oxygen concentration in the surface sediments, the cyanobacteria could potentially increase the rate of iron oxidation in situ. This high-iron-content hot spring provides a suitable model for studying the interactions of microbial photosynthesis and iron deposition and the role of photosynthesis in microbial iron cycling. This model may help clarify the potential role of photosynthesis in the deposition of Precambrian banded iron formations.     

Late Devonian marine anoxia challenged by benthic cyanobacterial mats

Mass occurrence of benthic cyanobacterial mats in a sequence of Late Devonian black shales and bituminous limestones of the Holy Cross Mts. (central Poland), enclosing the famous Kellwasser and Hangenberg extinction horizons, is reported. The microbiota forming the mats is compared with some modern benthic chroococcalean cyanobacteria. Similarly to their extant counterparts, the Devonian cyanobacteria must had been phototrophic and oxygenic aerobes which could, however, tolerate slightly sulfidic conditions characterizing the near-bottom waters of the Late Devonian epicontinental sea. The cyanobacterial mats successfully colonized the oxygen-deficient and H(2) S-enriched seabed otherwise unfavorable for most other benthic biota. The redox state of this sluggish Late Devonian sea, ascribed previously mostly to anoxic or euxinic conditions, is reassessed as probably pulsating between anoxic, dysoxic, and weakly oxic conditions. The redox state was dependent on the rate of oxygen production by the cyanobacterial mats, the intensity of H(2) S emissions from the decaying mat biomass, and the rate of planktonic production.     

Temperature effects on carbon and nitrogen metabolism in some Maritime Antarctic freshwater phototrophic communities

Biofilms growing on ice and benthic mats are among the most conspicuous biological communities in Antarctic landscapes and harbour a high diversity of organisms. These communities are consortia that make important contributions to carbon and nitrogen input in non-marine Antarctic ecosystems. Here, we study the effect of increasing temperatures on the carbon and nitrogen metabolism of two benthic communities on Byers Peninsula (Livingston Island, Maritime Antarctica): a biofilm dominated by green algae growing on seasonal ice, and a land-based microbial mat composed mainly of cyanobacteria. Inorganic carbon photoassimilation, urea and nitrate uptake and N₂-fixation (acetylene reduction activity) rates were determined in situ in parallel at five different temperatures (0, 5, 10, 15, 25°C) using thermostatic baths. The results for the cyanobacterial mat showed that photosynthesis and N₂-fixation responded positively to increased temperatures, but urea and NO₃⁻ uptake rates did not show a significant variation related to temperature. This microbial mat exhibits relatively low activity at 0°C whereas at higher temperatures (up to 15°C), N₂-fixation rate increased significantly. Similarly, the maximum photosynthetic activity increased in parallel with temperature and showed no saturation up to 25°C. In contrast, the ice biofilm displayed higher photosynthetic activity at 0°C than at the other temperatures assayed, and it showed elevated photoinhibition at warmer temperatures.     

Groundwater shapes sediment biogeochemistry and microbial diversity in a submerged Great Lake sinkhole

For a large part of earth's history, cyanobacterial mats thrived in low‐oxygen conditions, yet our understanding of their ecological functioning is limited. Extant cyanobacterial mats provide windows into the putative functioning of ancient ecosystems, and they continue to mediate biogeochemical transformations and nutrient transport across the sediment–water interface in modern ecosystems. The structure and function of benthic mats are shaped by biogeochemical processes in underlying sediments. A modern cyanobacterial mat system in a submerged sinkhole of Lake Huron (LH) provides a unique opportunity to explore such sediment–mat interactions. In the Middle Island Sinkhole (MIS), seeping groundwater establishes a low‐oxygen, sulfidic environment in which a microbial mat dominated by Phormidium and Planktothrix that is capable of both anoxygenic and oxygenic photosynthesis, as well as chemosynthesis, thrives. We explored the coupled microbial community composition and biogeochemical functioning of organic‐rich, sulfidic sediments underlying the surface mat. Microbial communities were diverse and vertically stratified to 12 cm sediment depth. In contrast to previous studies, which used low‐throughput or shotgun metagenomic approaches, our high‐throughput 16S rRNA gene sequencing approach revealed extensive diversity. This diversity was present within microbial groups, including putative sulfate‐reducing taxa of Deltaproteobacteria, some of which exhibited differential abundance patterns in the mats and with depth in the underlying sediments. The biological and geochemical conditions in the MIS were distinctly different from those in typical LH sediments of comparable depth. We found evidence for active cycling of sulfur, methane, and nutrients leading to high concentrations of sulfide, ammonium, and phosphorus in sediments underlying cyanobacterial mats. Indicators of nutrient availability were significantly related to MIS microbial community composition, while LH communities were also shaped by indicators of subsurface groundwater influence. These results show that interactions between the mats and sediments are crucial for sustaining this hot spot of biological diversity and biogeochemical cycling.     

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.     

Uptake and transformation of metals and metalloids by microbial mats and their use in bioremediation

Summary Constructed microbial mats, used for studies on the removal and transformation of metals and metalloids, are made by combining cyanobacteria inoculum with a sediment inoculum from a metal-contaminated site. These mats are a heterotrophic and autotrophic community dominated by cyanobacteria and held together by slimy secretions produced by various microbial groups. When contaminated water containing high concentrations of metals is passed over microbial mats immobilized on glass wool, there is rapid removal of the metals from the water. The mats are tolerant of high concentrations of toxic metals and metalloids, such as cadmium, lead, chromium, selenium and arsenic (up to 350 mg L^–1). This tolerance may be due to a number of mechanisms at the molecular, cellular and community levels. Management of toxic metals by the mats is related to deposition of metal compounds outside the cell surfaces as well as chemical modification of the aqueous environment surrounding the mats. The location of metal deposition is determined by factors such as redox gradients, cell surface micro-environments and secretion of extra-cellular bioflocculents. Metal-binding flocculents (polyanionic polysaccharides) are produced in large quantities by the cyanobacterial component of the mat. Steep gradients of redox and oxygen exist from the surface through the laminated strata of microbes. These are produced by photosynthetic oxygen production at the surface and heterotrophic consumption in the deeper regions. Additionally, sulfur-reducing bacteria colonize the lower strata, removing and utilizing the reducing H₂S, rather than water, for photosynthesis. Thus, depending on the chemical character of the microzone of the mat, the sequestered metals or metalloids can be oxidized, reduced and precipitated as sulfides or oxides. For example precipitates of red amorphous elemental selenium were identified in mats exposed to selenate (Se-VI) and insoluble precipitates of manganese, chromium, cadmium, cobalt, and lead were found in mats exposed to soluble salts of these metals. Constructed microbial mats offer several advantages for use in the bioremediation of metal-contaminated sites. These include low cost, durability, ability to function in both fresh and salt water, tolerance to high concentrations of metals and metalloids and the unique capacity of mats to form associations with new microbial species. Thus one or several desired microbial species might be integrated into mats in order to design the community for specific bioremediation applications.     

Bacteriohopanepolyols across environmental gradients in Lake Vanda,Antarctica

Bacteriohopanepolyols (BHPs) are bacterial membrane lipids that may be used as biological or environmental biomarkers. Previous studies have described the diversity, distribution, and abundance of BHPs in a variety of modern environments. However, the regulation of BHP production in polar settings is not well understood. Benthic microbial mats from ice‐covered lakes of the McMurdo Dry Valleys, Antarctica provide an opportunity to investigate the sources, physiological roles, and preservation of BHPs in high‐latitude environments. Lake Vanda is one of the most stable lakes on Earth, with microbial communities occupying specific niches along environmental gradients. We describe the influence of mat morphology and local environmental conditions on the diversity and distribution of BHPs and their biological sources in benthic microbial mats from Lake Vanda. The abundance and diversity of C‐2 methylated hopanoids (2‐MeBHP) are of particular interest, given that their stable degradation products, 2‐methylhopanes, are among the oldest and most prevalent taxonomically informative biomarkers preserved in sedimentary rocks. Furthermore, the interpretation of sedimentary 2‐methylhopanes is of great interest to the geobiology community. We identify cyanobacteria as the sole source of 2‐MeBHP in benthic microbial mats from Lake Vanda and assess the hypothesis that 2‐MeBHP are regulated in response to a particular environmental variable, namely solar irradiance.     

Competition for sulfide among colorless and purple sulfur bacteria in cyanobacterial mats

Abstract The vertical zonation of light, O₂, H₂S, pH, and sulfur bacteria was studied in two benthic cyanobacterial mats from hypersaline ponds at Guerrero Negro, baja California, Mexico. The physical-chemical gradients were analyzed in the upper few mm at ≥ 100 μm spatial resolution by microelectrodes and by a fiber optic microprobe. In mats, where oxygen produced by photosynthesis diffused far below the depth of the photic zone, colorless sulfur bacteria ( Beggiatoa sp.) were the dominant sulfide oxidizing organisms. In a mat, where the O₂–H₂S interface was close to the photic zone, but yet received no significant visible light, purple sulfur bacteria ( Chromatium sp.) were the dominant sulfide oxidizers. Analysis of the spectral light distribution heare showed that the penetration of only 1% of the incident near-IR light (800–900 nm) into the sulfide zone was sufficient for the development of Chromatium in a narrow band of 300 μm thickness. The balance betweem O₂ and light penetration down into the sulfide zone thus deterined in mcro-scale which type of sulfur bacteria becamed dominant.     

Diel Vertical Movements of the Cyanobacterium Oscillatoria terebriformis in a Sulfide-Rich Hot Spring Microbial Mat

Oscillatoria terebriformis, a thermophilic cyanobacterium, carried out a diel vertical movement pattern in Hunter's Hot Springs, Oreg. Throughout most daylight hours, populations of O. terebriformis covered the surface of microbial mats in the hot spring outflows below an upper temperature limit of 54°C. Upon darkness trichomes moved downward by gliding motility into the substrate to a depth of 0.5 to 1.0 mm, where the population remained until dawn. At dawn the population rapidly returned to the top of the mats. Field studies with microelectrodes showed that the dense population of O. terebriformis moved each night across an oxygen-sulfide interface, entering a microenvironment which was anaerobic and reducing, a dramatic contrast to the daytime environment at the mat surface where oxygenic photosynthesis resulted in supersaturated O₂. Laboratory experiments on motility with the use of sulfide gradients produced in agar revealed a negative response to sulfide at concentrations similar to those found in the natural mats. The motility response may help explain the presence of O. terebriformis below the mat surface at night. The movement back to the surface at dawn appears to be due to a combination of phototaxis, photokinesis, and the onset of oxygenic photosynthesis which consumes sulfide.