Microbial Mat Communities Research Articles

Unexpected population distribution in a microbial mat community: sulfate-reducing bacteria localized to the highly oxic chemocline in contrast to a eukaryotic preference for anoxia.

The distribution and abundance of sulfate-reducing bacteria (SRB) and eukaryotes within the upper 4 mm of a hypersaline cyanobacterial mat community were characterized at high resolution with group-specific hybridization probes to quantify 16S rRNA extracted from 100-microm depth intervals. This revealed a preferential localization of SRB within the region defined by the oxygen chemocline. Among the different groups of SRB quantified, including members of the provisional families "Desulfovibrionaceae" and "Desulfobacteriaceae," Desulfonema-like populations dominated and accounted for up to 30% of total rRNA extracted from certain depth intervals of the chemocline. These data suggest that recognized genera of SRB are not necessarily restricted by high levels of oxygen in this mat community and the possibility of significant sulfur cycling within the chemocline. In marked contrast, eukaryotic populations in this community demonstrated a preference for regions of anoxia.

Environmental conditions during freezing, and response of microbial mats in ponds of the McMurdo Ice Shelf, Antarctica

Environmental conditions, both external to and within three shallow ponds of the McMurdo Ice Shelf, were measured over an annual cycle between January 1997 and January 1998. We combined this with a study of the response of the benthic microbial mat communities to the transition from summer conditions to winter freezing. Over the study period air temperature was above 0°C for a few days during summer. At this time pond temperatures were higher than air temperatures, with evidence of thermo-haline stratification. The shallow areas of ponds froze between late February and early March, with bottom waters in the deepest pond remaining unfrozen until early June. Minimum winter air temperatures were below −40°C. There was little evidence of freezing point depression due to freeze-concentration of solutes, except at the very bottom of ponds. In the most conductive pond investigated, the temperature of basal freezing was −4°C and conductivity did not exceed approximately 60 mS cm−1 immediately prior to freezing. Microbial mats remained photosynthetically active up to conductivities between 40 and 80 mS cm−1, and were able to acclimate to lowered irradiance associated with ice formation. Although photosynthesis and respiration were reduced by 11% and 40% respectively at temperatures of −2°C compared to 1°C, there was no difference in light harvesting efficiency. Results from this study suggest that light limitation of photosynthesis, or freezing, determine the growth season for the microbial communities, depending on depth.

Isotopic Inferences on Early Ecosystems

Long thought to be inaccessible to empirical inquiry, Earth's early biosphere has in recent decades become a central focus of evolutionary and paleobiological research. Knowledge of Precambrian ecosystems comes from three principal sources. The conventional fossil record consists of the compressed and permineralized remains of cyanobacteria, protists and other microorganisms (e.g., Knoll, 1996), complemented by stromatolites and oncolites, the accretionary trace fossils of microbial mat communities (Walter, 1976). Independent inferences about early evolution can be drawn from molecular phylogenies (Pace, 1997). The third principal source of information comprises biogeochemical signatures encrypted in the chemistry of ancient sedimentary rocks. Biomarker molecular fossils and distinctive isotopic compositions record the metabolic activities of organisms not necessarily preserved morphologically (Summons and Walter, 1990). In this paper, we review the inferences about early life and environments that can be drawn from the isotopic records of carbon and sulfur.

Potential salinity limitations on nitrogen fixation in sediments from Mono Lake, California

Mono Lake is a hypersaline alkaline lake in the high altitude Great Basin desert of eastern California. Algal productivity of the lake is nitrogen-limited, and a contributing source is derived from benthic nitrogen fixation. Lake level and salinity have fluctuated with natural climatic variations but have also been affected by the diversion of tributary streams. This research examines the influence of varied salinity and lake level on the potential for benthic nitrogen fixation in Mono Lake. A sediment-surface microbial mat community was exposed directly, and in acclimated cultures, to a range of Mono Lake salinities under anaerobic incubations and the activity of nitrogenase assayed by acetylene reduction. Activity was stimulated in light, but also occurred in darkness. Over an experimental salinity range from 50 to 150 g L-1 TDS, nitrogenase activity was reduced by 90 per crnt, with the activity persisting at the highest salinity being attributable to dark fixation alone. Between a salinity of 50 g L-1, occurring in Mono Lake over 50 years ago, and 100 g L-1, nitrogenase activity was reduced by nearly half. Changes in the area of the littoral zone at varied lake levels also affect the total amount of potential benthic nitrogen fixation in the lake. An accounting of yearly inputs of nitrogen to Mono Lake suggests N2-fixation could contribute as much as 76–81 percent of the total. Inhibition of nitrogen fixation rates by increased salinity could limit the long-term nutrient supply and benthic primary productivity of this ecosystem.

Stone Soup

The SciencesVolume 38, Issue 4 p. 34-38 Stone Soup RICARDO GUERRERO, RICARDO GUERRERO Ricardo Guerrero is a professor of microbiology at the University of Barcelona in Spain. Lynn Margulis is distinguished university professor in the organismal and evolutionary biology program at the University of Massachusetts in Amherst. Her latest books are What Is Sex? and Slanted Truths: Essays on Gaia, Symbiosis and Evolution, both coauthored with Dorion Sagan. Guerrero and Margulis jointly study microbial mat communities in various parts of the world, particularly those of the delta of the Ebro River in northeastern Spain.Search for more papers by this authorLYNN MARGULIS, LYNN MARGULIS Ricardo Guerrero is a professor of microbiology at the University of Barcelona in Spain. Lynn Margulis is distinguished university professor in the organismal and evolutionary biology program at the University of Massachusetts in Amherst. Her latest books are What Is Sex? and Slanted Truths: Essays on Gaia, Symbiosis and Evolution, both coauthored with Dorion Sagan. Guerrero and Margulis jointly study microbial mat communities in various parts of the world, particularly those of the delta of the Ebro River in northeastern Spain.Search for more papers by this author RICARDO GUERRERO, RICARDO GUERRERO Ricardo Guerrero is a professor of microbiology at the University of Barcelona in Spain. Lynn Margulis is distinguished university professor in the organismal and evolutionary biology program at the University of Massachusetts in Amherst. Her latest books are What Is Sex? and Slanted Truths: Essays on Gaia, Symbiosis and Evolution, both coauthored with Dorion Sagan. Guerrero and Margulis jointly study microbial mat communities in various parts of the world, particularly those of the delta of the Ebro River in northeastern Spain.Search for more papers by this authorLYNN MARGULIS, LYNN MARGULIS Ricardo Guerrero is a professor of microbiology at the University of Barcelona in Spain. Lynn Margulis is distinguished university professor in the organismal and evolutionary biology program at the University of Massachusetts in Amherst. Her latest books are What Is Sex? and Slanted Truths: Essays on Gaia, Symbiosis and Evolution, both coauthored with Dorion Sagan. Guerrero and Margulis jointly study microbial mat communities in various parts of the world, particularly those of the delta of the Ebro River in northeastern Spain.Search for more papers by this author First published: 31 July 2013 https://doi.org/10.1002/j.2326-1951.1998.tb03396.xCitations: 1 AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume38, Issue4July‐August 1998Pages 34-38 RelatedInformation

Diversity of deep-sea hydrothermal vent Archaea from Loihi Seamount, Hawaii

Through an examination of SSU rDNA (genes coding for SSU rRNA), the molecular phylogeny of the domain Archaea (e.g. one of the three major lineages of life) was analyzed from a microbial mat at an active, deep-sea hydrothermal vent ecosystem located at Pele’s Vents on the summit of Loihi Seamount, Hawaii. These SSU rDNAs were amplified from extracted microbial mat genomic DNA by PCR, cloned into a plasmid vector, and sequenced. The derived archaeal sequences were then used to infer the evolutionary relationships between these microbial mat community members and their closest known relatives. Of the four clones initially chosen for sequence analysis, a cluster of three phylogenetically similar PV A (Pele’s Vents Archaea) clones all contained in the archaeal group I lineage of the marine Crenarchaeota were detected. A single PV A clone was contained in the archaeal group II lineage of the marine Euryarchaeota. All four of the PV A clones are novel and constitute the discovery of new archaeal taxa. From further rarefaction results of 75 archaeal SSU rDNA clones, we estimate the organismal diversity of this domain from the microbial mats located at Pele’s Vents to be significantly greater than that of the bacterial domain from this same ecosystem. Analyses of archaeal diversity at both the organismal (i.e. rarefaction) and phylogenetic level suggest that hydrothermal vents, such as Pele’s Vents, are intimately linked with marine archaeoplankton (a recently discovered component of marine picoplankton) detected from oceans around the world.

Population structure and physiological changes within a hot spring microbial mat community following disturbance.

The influence of disturbance on a hot spring cyanobacterial mat community was investigated by physically removing the top 3.0 mm, which included the entire cyanobacterial layer. Changes in 16S rRNA-defined populations were monitored by denaturing gradient gel electrophoresis analysis of PCR-amplified 16S rRNA gene segments. Some previously absent cyanobacterial populations colonized the disturbed areas, while some populations which were present before the disturbance remained absent for up to 40 days. Changes in physiological activity were measured by oxygen microelectrode analyses and by 14CO2 incorporation into cyanobacterial molecular components. These investigations indicated substantial differences between the disturbed and undisturbed mats, including an unexplained light-induced oxygen consumption in the freshly exposed mat, increased carbon partitioning by phototrophs into growth-related macromolecules, bimodal vertical photosynthesis profiles, and delayed recovery of respiration relative to photosynthesis.

Anoxygenic photosynthesis and nitrogen fixation by a microbial mat community in a bahamian hypersaline lagoon

Simultaneous measurements of photosynthesis (both oxygenic and anoxygenic) and N(inf2) fixation were conducted to discern the relationships between photosynthesis, N(inf2) fixation, and environmental factors potentially regulating these processes in microbial mats in a tropical hypersaline lagoon (Salt Pond, San Salvador Island, Bahamas). Major photoautotrophs included cyanobacteria, purple phototrophic bacteria, and diatoms. Chemosystematic photopigments were used as indicators of the relative abundance of mat phototrophs. Experimental manipulations consisted of light and dark incubations of intact mat samples exposed to the photosystem II inhibitor DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea], a dissolved organic carbon source (D-glucose), and normal seawater (37(permil)). Photosynthetic rates were measured by both O(inf2) and (sup14)C methods, and nitrogenase activity (NA) was estimated by the acetylene reduction assay. Moderate reductions in salinity (from 74 to 37(permil)) had no measurable effect on photosynthesis, O(inf2) consumption, or NA. CO(inf2) fixation in DCMU-amended samples was (symbl)25% of that in the control (nonamended) samples and demonstrated photosynthetic activity by anoxygenic phototrophs. NA in DCMU-amended samples, which was consistently higher (by a factor of 2 to 3) than the other (light and dark) treatments, was also attributed to purple phototrophic bacteria. The ecological implication is that N(inf2) fixation by anoxygenic phototrophs (purple phototrophic bacteria and possibly cyanobacteria) may be regulated by the activity of oxygenic phototrophs (cyanobacteria and diatoms). Consortial interactions that enhance the physiological plasticity of the mat community may be a key for optimizing production, N(inf2) fixation, and persistence in these extreme environments.

Anoxygenic Photosynthesis and Nitrogen Fixation by a Microbial Mat Community in a Bahamian Hypersaline Lagoon

Anoxygenic Photosynthesis and Nitrogen Fixation by a Microbial Mat Community in a Bahamian Hypersaline Lagoon

Biodiversity within hot spring microbial mat communities: molecular monitoring of enrichment cultures.

We have begun to examine the basis for incongruence between hot spring microbial mat populations detected by cultivation or by 16S rRNA methods. We used denaturing gradient gel electrophoresis (DGGE) to monitor enrichments and isolates plated therefrom. At near extincting inoculum dilutions we observed Chloroflexus-like and cyanobacterial populations whose 16S rRNA sequences have been detected in the 'New Pit' Spring Chloroflexus mat and the Octopus Spring cyanobacterial mat. Cyanobacterial populations enriched from 44 to 54 degrees C and 56 to 63 degrees C samples at near habitat temperatures were similar to those previously detected in mat samples of comparable temperatures. However, a lower temperature enrichment from the higher temperature sample selected for the populations found in the lower temperature sample. Three Thermus populations detected by both DGGE and isolation exemplify even more how enrichment may bias our view of community structure. The most abundant population was adapted to the habitat temperature (50 degrees C), while populations adapted to 65 degrees C and 70 degrees C were 10(2)- and 10(4)-fold less abundant, respectively. However, enrichment at 70 degrees C favored the least abundant strain. Inoculum dilution and incubation at the habitat temperature favored the more numerically relevant populations. We enriched many other aerobic chemoorganotrophic populations at various inoculum dilutions and substrate concentrations, most of whose 16S rRNA sequences have not been detected in mats. A common feature of numerically relevant cyanobacterial, Chloroflexus-like and aerobic chemorganotrophic populations, is that they grow poorly and resist cultivation on solidified medium, suggesting plating bias, and that the medium composition and incubation conditions may not reflect the natural microenvironments these populations inhabit.

Wrinkle structures: Microbially mediated sedimentary structures common in subtidal siliciclastic settings at the Proterozoic-Phanerozoic transition

Research Article| November 01, 1997 Wrinkle structures: Microbially mediated sedimentary structures common in subtidal siliciclastic settings at the Proterozoic-Phanerozoic transition James W. Hagadorn; James W. Hagadorn 1Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740 Search for other works by this author on: GSW Google Scholar David J. Bottjer David J. Bottjer 1Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740 Search for other works by this author on: GSW Google Scholar Geology (1997) 25 (11): 1047–1050. https://doi.org/10.1130/0091-7613(1997)025<1047:WSMMSS>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation James W. Hagadorn, David J. Bottjer; Wrinkle structures: Microbially mediated sedimentary structures common in subtidal siliciclastic settings at the Proterozoic-Phanerozoic transition. Geology 1997;; 25 (11): 1047–1050. doi: https://doi.org/10.1130/0091-7613(1997)025<1047:WSMMSS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Sedimentary structures produced by microbial activity are well known from ancient carbonate facies, but little is known about equivalent microbial structures in ancient siliciclastic facies. Wrinkle marks and Kinneyia ripples are closely related sedimentary structures that are commonly found in ancient siliciclastics, and they may represent microbial activity. To evaluate this possibility, these structures were studied in Vendian–Cambrian strata of North America and compared to structures formed by modern microbial mat communities in Redfish Bay, Texas. Striking similarities in sedimentologic, petrographic, and morphologic characteristics of modern and ancient occurrences suggest that the enigmatic but widespread ancient structures could have been formed by microbial processes. The relative abundance of these structures in siliciclastic facies of this interval further suggests that prior to the onset of relatively intense bioturbation in the Ordovician, microbial mats could have played a significant role in the evolution and diversification of early life in a broad variety of sea-floor environments. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Community Metabolism in Microbial Mats: The Occurrence of Biologically-mediated Iron and Manganese Reduction

Community metabolism and nutrient, iron (Fe) and manganese (Mn) cycling were examined in two intertidal, marine, microbial mat communities during short (4–5 days) incubations in closed, flow-through microcosms. Sediment microcosms were incubated under either light (light–dark cycles) or dark (continuous darkness) conditions to assess the effect(s) of photosynthetic oxygen production and microalgal activity on nutrient, Fe and Mn cycling. The effects of chemical redox reactions between reduced sulphur (S), Fe and Mn cycling were examined by blocking sulphate reduction, and reduced S production, with 25 mM molybdate while incubating under dark conditions. In light-incubated microcosms, negligible fluxes of nutrients (nitrogen and phosphorus) and trace metals were observed. A substantial sediment–water flux of reduced Fe (Fe 2+) and Mn (Mn 2+) was observed in microcosms incubated under continuous darkness; highest fluxes were observed in molybdate-amended microcosms. At both sites, biologically-mediated redox reactions accounted for a substantial (>50%) portion of the Fe 2+and Mn 2+flux. Both microbial mat communities exhibited similar rates of gross photosynthetic oxygen (O 2) production, but dramatically different rates of net benthic O 2flux. Distinct patterns of net O 2production and trace metal cycling arose from differences in either trace metal oxide availability or reactivity (mineralogy), organic carbon mineralization rates, or sediment characteristics (porosity). Variations in the microbial community responsible for trace metal cycling could have also contributed to the pattern. The present data illustrate that chemically-mediated redox reactions between metal oxides and reduced S complicate interpretation of Fe and Mn fluxes, underscoring the need to separate chemical and biological reactions when attempting to determine the role of biological trace metal reduction in organic carbon oxidation.

Photosynthate partitioning and fermentation in hot spring microbial mat communities

Patterns of (sup14)CO(inf2) incorporation into molecular components of the thermophilic cyanobacterial mat communities inhabiting hot springs located in Yellowstone National Park and Synechococcus sp. strain C1 were investigated. Exponentially growing Synechococcus sp. strain C1 partitioned the majority of incorporated (sup14)CO(inf2) into protein, low-molecular-weight metabolites, and lipid fractions (45, 22, and 18% of total incorporated carbon, respectively). In contrast, mat cores from various hot springs predominantly accumulated polyglucose during periods of illumination (between 77 and 85% of total incorporated (sup14)CO(inf2)). Although photosynthetically active, mat photoautotrophs do not appear to be rapidly growing, since we also detected only limited synthesis of macromolecules associated with growth (i.e., protein and rRNA). To test the hypothesis that polysaccharide reserves are fermented in situ under the dark anaerobic conditions cyanobacterial mats experience at night, mat cores were prelabeled with (sup14)CO(inf2) under illuminated conditions and then transferred to dark anaerobic conditions. Radiolabel in the polysaccharide fraction decreased by 74.7% after 12 h, of which 58.5% was recovered as radiolabeled acetate, CO(inf2), and propionate. These results indicate tightly coupled carbon fixation and fermentative processes and the potential for significant transfer of carbon from primary producers to heterotrophic members of these cyanobacterial mat communities.

Seasonal nitrogen fixation dynamics in a marine microbial mat: Potential roles of cyanobacteria and microheterotrophs

Diel rates of nitrogen (N2) fixation (acetylene reduction) and primary production (14CO2 fixation) were examined seasonally on a North Carolina Atlantic coastal, intertidal, benthic microbial mat community dominated by the filamentous, nonheterocystous cyanobacterial genera Microcoleus and Lyngbya. Highest hourly and daily rates of N2 and CO2 fixation were observed during spring through fall. During this period, an inverse temporal relationship was noted between these processes, with CO2 fixation closely tracking irradiance and N2 fixation rates remaining low during daylight and becoming maximal at night. Under the influence of the photosynthetic (PS 2) inhibitor 3‐(3,4 dichlorophenyl)‐1,1 dimethylurea (DCMU), daytime N2 fixation was enhanced, indicating in situ O2 inhibition of N2 fixation. The most pronounced DCMU stimulation of daytime N2 fixation was in spring‐fall. Both N2 and CO2 fixation rates were lower in winter. Winter patterns of diel N2 fixation were the reverse of those in summer, with maximum rates at midday. The reversal was related to seasonal changes in daily and hourly photosynthetic rates, leading to differential O2 suppression of N2 fixation. Seasonal changes in cyanobacterial community composition and bacterial diazotrophy may have played additional roles in determining diel rates and patterns of N2 fixation and mat production. Results indicate a more important role for bacteria in the dynamics of mat N2 fixation than has been previously recognized.

Denaturing gradient gel electrophoresis profiles of 16S rRNA-defined populations inhabiting a hot spring microbial mat community.

Denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA gene segments was used to profile microbial populations inhabiting different temperature regions in the microbial mat community of Octopus Spring, Yellowstone National Park. DGGE allowed a rapid evaluation of the distributions of amplifiable sequence types. Profiles were essentially identical within regions of the mat defined by one temperature range but varied between sites with different temperature ranges. Individual DGGE bands were sequenced, and the sequences were compared with those previously obtained from the mat by cloning and from cultivated Octopus Spring isolates. Two known cyanobacterial populations and one known green nonsulfur bacterium-like population were detected by DGGE, as were many new cyanobacterial and green nonsulfur and green sulfur bacterium-like populations and a novel bacterial population of uncertain phylogenetic affiliation. The distributions of several cyanobacterial populations compared favorably with results obtained previously by oligonucleotide probe analyses and suggest that adaptation to temperature has occurred among cyanobacteria which are phylogenetically very similar.

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

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.

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

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.

Salinity control of benthic microbial mat community production in a Bahamian hypersaline lagoon

The purpose of this study was to determine the production and N 2 fixation responses of a hypersaline mat community following a reduction in salinity and nutrient enrichment. Cyanobateria-dominated microbial mat samples were collected from hypersaline Storr's Lake and normal seawater salinity Pigeon Creek and preincubated at ambient (90%.) and reduced (45%.) salinities following no nutrient as well as inorganic nutrient (NO 3 −, PO 4 −, trace metals) and dissolved organic carbon (DOC, as mannitol) enrichment. CO 2 and N 2 fixation rates were determined 2 and 4 days later. In addition, DOC (trace concentrations of 3H-labeled glucose and amino acids) uptake was measured in mats under normal and hypersaline conditions. A reduction in salinity from 90 to 45%. significantly enhanced CO 2 and N 2 fixation rates, but inorganic nutrient and DOC additions did not significantly enhance rates compared with the controls. Dissolved organic carbon/dissolved organic nitrogen (DON) uptake was not influenced over the entire range of salinities (45–90%.) used in this study. When salinity-induced osmotic stress was relieved, mats underwent enhanced primary production and nitrogen fixation. Abiotic stress, induced by hypersaline conditions in Bahamian lagoons, results in lower productivity of the microbial mat communities and this stress may outweigh the typical limiting factors regulating phototrophic community primary production.

Impacts of seasonality and nutrients on microbial mat community structure and function

Impacts of seasonality and nutrients on microbial mat community structure and function

Impacts of seasonality and nutrients on microbial mat community structure and function

To understand the mechanisms responsible for seasonal fluctuations in growth and N2 fixation in intertidal microbial mat communties, we quantified seasonal changes in mat community composition, related these changes to die1 and seasonal N2 fixation rates, and evaluated community responses (growth, N2 fixation, composition) to long-term (22 d ) nutrient addition bioassays. A temperate intertidal cyanobacterial mat community, located in coastal North Carolina, USA, was sampled at monthly intervals for 1 yr (1993-94) to determine changes in community composition. The abundances of major phototrophic groups were quantified based on the relative concentrations of taxaspecific photopigments (chlorophylls and carotenoids). The most abundant phototrophs were cyanobacteria, diatoms, and photosynthetic bacteria. Mat blomass and community composition underwent marked changes on both monthly and seasonal scales and corresponded with seasonal shifts in the dlel patterns of N2 fixation. Diatom biomass increased during periods of low N2 fixation. Nutnent (nitrate and phosphate) addition bioassays indicated that both cyanobacterial and diatom growth were N limited. Cyanobacteria were able to circumvent N llmltation by N2 fixation. The addition of high concentrations of N (100 ,pM NaNO,) in combination with P (100 ~ J M NaH2P04) resulted In an Increase (163'Y) in the relative abundance of diatoms The addition of P alone more than doubled N2 fixation rates and cyanobacterial abundance increased (+34 'h) relative to diatoms. However, N and NP additions significantly lowered (by more than 75%) N2 fixation rates. Here we show that manipulative experiments, together with quantitative assessments of community composition based on chemotaxonomic pigments, can provide useful insights into the mechanisms that relate mat community structure and function to environmental constraints, including nutrient limitation and seasonal climatic changes