Flat Laminated Microbial Mat Communities Earth-Science Reviews
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Earth-Science Reviews 96 (2009) 163–172 Contents lists available at ScienceDirect Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev Flat laminated microbial mat communities Jonathan Franks, John F. Stolz ⁎ Department of Biological Sciences, Duquesne University, Pittsburgh, PA 15282, United States article info abstract Article history: Flat laminated microbial mats are complex microbial ecosystems that inhabit a wide range of environments Received 28 January 2008 (e.g., caves, iron springs, thermal springs and pools, salt marshes, hypersaline ponds and lagoons, methane Accepted 24 October 2008 and petroleum seeps, sea mounts, deep sea vents, arctic dry valleys). Their community structure is defined by Available online 6 November 2008 physical (e.g., light quantity and quality, temperature, density and pressure) and chemical (e.g., oxygen, oxidation/reduction potential, salinity, pH, available electron acceptors and donors, chemical species) Keywords: microbial mat parameters as well as species interactions. The main primary producers may be photoautotrophs (e.g., oxygenic phototroph cyanobacteria, purple phototrophs, green phototrophs) or chemolithoautophs (e.g., colorless sulfur oxidizing anoxygenic phototroph bacteria). Anaerobic phototrophy may predominate in organic rich environments that support high rates of microbiolite respiration. These communities are dynamic systems exhibiting both spatial and temporal heterogeneity. They are characterized by steep gradients with microenvironments on the submillimeter scale. Diel oscillations in the physical-chemical profile (e.g., oxygen, hydrogen sulfide, pH) and species distribution are typical for phototroph-dominated communities. Flat laminated microbial mats are often sites of robust biogeochemical cycling. In addition to well-established modes of metabolism for phototrophy (oxygenic and non-oxygenic), respiration (both aerobic and anaerobic), and fermentation, novel energetic pathways have been discovered (e.g., nitrate reduction couple to the oxidation of ammonia, sulfur, or arsenite). The application of culture-independent techniques (e.g., 16S rRNA clonal libraries, metagenomics), continue to expand our understanding of species composition and metabolic functions of these complex ecosystems. © 2008 Elsevier B.V. All rights reserved. Contents 1. Introduction .............................................................. 163 2. Physical–chemical environment ..................................................... 164 2.1. Light quantity and quality .................................................... 165 2.2. Temperature .......................................................... 166 2.3. Oxygen ............................................................. 166 2.4. pH ............................................................... 166 2.5. Salinity ............................................................. 166 2.6. Electron acceptors and donors, and chemical species ....................................... 167 3. Community structure.......................................................... 167 4. Advances in techniques......................................................... 168 5. Summary ............................................................... 169 Acknowledgements ............................................................. 169 References ................................................................. 169 1. Introduction Microbial mats are communities of microorganisms that colonize surfaces. They range in complexity from simple, almost mono-species fi ⁎ Corresponding author. Tel.: +1 412 396 6333; fax: +1 412 396 5907. bio lms, to multi-layered ecosystems containing diverse populations of E-mail address: [email protected] (J.F. Stolz). prokaryotes and small eukaryotes (e.g., diatoms, unicellular algae) 0012-8252/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2008.10.004 164 J. Franks, J.F. Stolz / Earth-Science Reviews 96 (2009) 163–172 arranged into assemblages and guilds (Stolz, 2000). Typically associated with the sediment/water interface these communities interact with the sediment, trapping and binding particles and clastics, and in some cases inducing precipitation and lithification (Stolz, 2003, Dupraz and Visscher, 2005). These activities in concert with the predominant mineralogy (e.g., clay, silt, siliciclastic, evaporite, carbonate) can impact the structure and fabric yielding sediments with distinctive characteristics (e.g., micro- biolites). The structures can be preserved in the rock record and their interpretation can be enhanced by the study of modern microbiolites (Grotzinger and Knoll, 1999; Des Marais, 2003; Tice and Lowe, 2004; Noffke, 2007). Stromatolites and thrombolites are organosedimentary structures that commonly have a vertical profile that protrudes above the horizontal plane (Monte, 1976; Reid et al., 1995, 2000). Flat laminated microbial mats rarely form relief above the horizon (e.g., desiccation cracks, Fig. 1), but the layered sediments they produce may extend to some depth below the surface (e.g., a few centimeters to meters) (Figs. 1 and 2). They thrive in a wide range of habitats including extremes in pH (e.g., acidic sulfur caves and iron springs), temperature (e.g., thermal springs and pools, deep sea vents), and salinity (e.g., salt marshes, hypersaline ponds and lagoons), as well as ones with chemolithotrophic sources of energy (e.g., methane seeps, sea mounts). Their ecological success is a reflection of the metabolic versatility and physiological adaptability found within the Bacteria and Archaea. Flat laminated microbial mats are models for microbial ecology and have been studied extensively under the auspices of evolution and astrobiology as they represent the modern analogs to ancient life and possibly extraterrestrial ecosystems (Des Marais, 2003). Over the past twenty odd years several volumes have been published dedicated to microbial mats (e.g., Cohen et al., 1984; Cohen and Rosenberg, 1989 Stal and Caumette, 1994, Riding and Awramik, 2000, Krumbein et al., 2003, Inskeep and McDermott, 2005a). The purpose of this review is to present a synopsis of the current concepts, highlight some of the recent discoveries, and provide a glimpse at what lies ahead with application of new technologies. 2. Physical–chemical environment Species occurrence and abundance in microbial mats are strongly influenced by the physical properties and the chemical parameters of a Fig. 1. Sippewissett Marsh, Massachusetts. A) Field photo facing westward, B) Close up given environment. Important physical properties include light (both showing flat laminated mat on surface. quantity and quality), temperature, and pressure. Key chemical Fig. 2. Flat laminated mat at Laguna Figueroa, Baja California, Mexico. A) Field photo facing eastward, B) Cross section of laminated sediments. The top 3 mm is the seasonal sediment accreting community. J. Franks, J.F. Stolz / Earth-Science Reviews 96 (2009) 163–172 165 parameters include oxygen, pH, oxidation/reduction potential, salinity, and available electron acceptors and donors, as well as the presence or absence of specific chemical species. In this section, specific properties and parameters that create unique environments that support microbial mat communities are presented. 2.1. Light quantity and quality Photoautotrophic communities depend on both the appropriate amount of light (e.g., light quantity) and the particular wavelengths (e.g., light quality) that can be used by the light harvesting pigments and photosystems. The average amount of light that illuminates a surface on a sunny day is 1000 to 2000 μE/m2/s. Depending on the environment, however, light absorption and scattering can be significant. Particles and populations of organisms readily attenuate light in the water column. Sediment type can impact the depth at which light can penetrate into the subsurface. This light scattering can be significant resulting in the scalar irradiance being greater than the downward irradiance (Des Marais, 2003). Most phototrophs are adapted to low light intensities and often are photoinhibited. The optimum light intensity for cyanobacteria in the microbial mats of Mellum Island in the North Sea is 50 to 150 μE/m2/s. Purple sulfur bacteria from the same mats use from 5 to 10 μE/m2/s (Stal et al., 1985). Mat communities have a variety of strategies to obtain the appropriate amount of light. Cyanobacteria produce carotenoids and other light attenuating products (e.g., scytonemin), or may lie beneath a coating of sediment (Palmisano et al., 1989). Conversely, some photo- trophs are extremely adept at capturing rare photons in light-limited environments. Bacteriochlorophyll e containing green sulfur bacteria have been found at 100 m depth in the Black Sea (Manske et al., 2005). Green sulfur bacteria are particularly well suited for low light as they can produce large numbers of light harvesting structures (e.g., chlorosomes) and have a high ratio of accessory pigment to reaction center (Jochum et al., 2008). The Black Sea chlorobia, for example, are capable of photoautotrophy with only 0.015 μmol quanta m−2 s−1 (Manske et al., 2005). Geographic location can be important with respect to the growing season of mats. While equatorial mats see little annual change, northern and southern latitudes are subject to seasonality, while artic and Antarctic systems are subject to month long extremes of constant light or darkness. Phototrophic organisms have evolved light harvesting