Chapter 6 Ncsu Applied Ecology-Books Pdf

Chapter 6 NCSU Applied Ecology
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112 REID DUPRAZ VISSCHER SUMNER, An ideal model system in which to study microbe mineral interactions. and ecological principles governing these interactions is modern marine. stromatolites Modern marine stromatolites are living examples of one of. earth s oldest and most persistent widespread ecosystems Layered deposits. of calcium carbonate known as stromatolites first appeared in the geological. record at least three and a half billion years ago Grotzinger and Knoll 1999. Hofmann et al 1999 Stromatolites are neither biotic fossils nor abiotic. structures Rather they represent the complex interactions of microbes. minerals and the environment Walter 1994 Grotzinger and Knoll 1999. For almost 80 of Earth s history stromatoliteforming microbial. communities played a major role in regulating sedimentation and global. cycles of major elements via production and decomposition of organic. matter trapping and binding of sediment and precipitation of calcium. Modern stromatolites in Exuma Cays Bahamas offer a unique. opportunity to investigate interactions between microbes and minerals in the. highly successful stromatolite ecosystem In this paper we examine. microbial processes forming Exuma stromatolites We review what is. currently known and identify unanswered questions and future research. directions Our goal is to demonstrate that a thorough understanding of. microbe mineral interactions in modern marine stromatolites will have broad. implications in a wide variety of fields including ecology biogeochemistry. sedimentology and paleobiology and will improve our ability to interpret. the fossil record of ancient ecosystems, 2 STROMATOLITE GROWTH. Stromatolites in Exuma Cays Bahamas Dravis 1993 Dill et al 1986. Reid and Browne 1991 Reid et al 1995 Reid et al 1999 are the only. known examples of stromatolites presently forming in open marine. environments equivalent to those of many Precambrian platforms Our. recent research results e g Reid et al 2000 show that growth of these. stromatolites results from successive episodes of sediment accretion and. lithification of microbial mats Periods of rapid accretion during which. stromatolite surfaces are dominated by pioneer communities of motile. filamentous cyanobacteria alternate with hiatal intervals Hiatal periods are. characterized by development of surface films with abundant aerobic and. anaerobic heterotrophic bacteria which form thin crusts of microcrystalline. carbonate During prolonged hiatal periods climax communities develop. which include endolithic coccoid cyanobacteria These coccoids fuse. MICROBIAL PROCESSES FORMING STROMATOLITES 113, sediment grains forming thicker lithified laminae Preservation of lithified. layers at depth creates millimeter scale lamination. This growth scenario is based on extensive field and laboratory based. studies using a wide range of geological and microbiological techniques. These studies revealed that surfaces of Exuma stromatolites are covered with. cyanobacterial mats which show distinct variations in microbial community. structure and composition Three mat types representing a continuum of. growth stages are recognized Figure 1, 2 1 Type 1 mats. About 70 of surface mats consist of a sparse population of the. filamentous cyanobacterium Schizothrix sp within a mucilaginous. exopolymer matrix Stolz et al 2001 Schizothrix filaments are generally. vertically oriented and are entwined around carbonate sand grains Figures. 1a 1b These mats are pioneer communities Stal et al 1985 which. dominate during periods of rapid sediment accretion of up to one grain layer. per day Populations of diatoms and other eukaryotes are rarely found in. these accreting mats Golubic and Browne 1996 Pinckney and Reid 1997. and contrary to previous reports Awramik and Riding 1988 Riding 1994. eukaryotic organisms are not required for the trapping and binding of these. coarse grained sediments, 2 2 Type 2 mats, Approximately 15 of mats show development of surface films of.
amorphous exopolymer and bacterial cells these surface films are referred to. in this paper as biofilms The biofilms are calcified and appear as thin. crusts 20 60 m thick of microcrystalline carbonate micrite at the. uppermost mat surface Figure 1c A sparse to moderately dense population. of Schizothrix lies below the surface films Figure 1c Schizothrix filaments. are also present but are not abundant in the surface films which are. comprised mainly of copious amounts of amorphous exopolymer. metabolically diverse heterotrophic microorganisms and aragonite needles. Visscher et al 1998 1999 Stolz et al 2001 Stolz this volume Spherical. aggregates of aragonite needles 2 to 5 m in diameter are embedded in the. exopolymer matrix Figure 1e Bacteria are abundant and are commonly. observed at the edges of the aragonite spherules Decho and Kawaguichi. 1999 Paerl et al 2001 This mat type represents a more mature Stal et al. 1985 Van Gemerden 1993 surface community and develops during hiatal. periods when sediment accretion ceases and mats begin to lithify Initial. 114 REID DUPRAZ VISSCHER SUMNER, mesocosm manipulations suggest that continuous surface biofilms form in a. matter of days, 2 3 Type 3 mats, The remaining 15 of mats are characterized by an abundant population. of the coccoid cyanobacterium Solentia sp and randomly oriented. Schizothrix filaments below a calcified biofilm Figures 1f 1h This. Solentia rich mat type represents the climax community of the stromatolite. system Solentia is an endolith that bores into carbonate sand grains These. bored grains appear grey when viewed in plane polarized light under a. petrographic microscope Figure 1f contrasting with the golden brown. coloration of unbored grains Figures 1a 1c In contrast to the conventional. view that microboring is a destructive process Golubic and Browne 1996. Perry 1998 the microboring and infilling process associated with Solentia. activity in these mats is an important constructive process This process. fuses grains at point contacts to create laterally cohesive carbonate crusts. welded grains Figure 1h Macintyre et al 2000 Field and laboratory. studies show that layers of fused microbored grains are formed in periods of. weeks to months Macintyre et al 2000 As Solentia is a photosynthetic. microorganism such prolonged periods of microboring activity can only be. sustained when this population remains at the surface during long hiatal. periods Longer hiatal periods can result in development of eukaryotic algal. communities which do not form laminated structures Steneck et al 1998. Golubic and Browne 1996, 2 4 Subsurface structure, The laminated stromatolitic fabric records a chronology of former surface. mats Figure 2 Although lamination is readily apparent in hand samples. Figure 2a it has a subtle expression in petrographic thin sections Detailed. observations show however that lithified layers have two distinct. petrographic appearances Figure 2b These laminae correspond to 1 thin. crusts of microcrystalline carbonate micrite 10 60 m thick Figures 2b. 2c and 2 layers of fused microbored grains infested with Solentia sp. these layers are 1 2 mm thick Figures 2b 2d and underlie micritic crusts. Light microscopy combined with scanning electron microscopy shows that. the thin crusts are identical in thickness composition and texture to the. calcified biofilms described above They are also similar in thickness to. micritic laminae in many ancient stromatolites Walter 1983 Bertrand. Sarfati 1976, MICROBIAL PROCESSES FORMING STROMATOLITES 115. Figure 1 Surface mats shown as a response to intermittent sedimentation a b Type 1 mats. filamentous cyanobacteria arrows bind carbonate sand grains c d e Type 2 mats a. continuous sheet of amorphous exopolymer with abundant heterotrophic bacteria drapes the. surface a arrow d aragonite needles precipitate within this film e f g h Type 3 mats a. surface biofilm overlies filamentous cyanobacteria and endolithinfested grains which appear. gray and are fused arrow f Banded pattern of fibrous aragonite in bore holes g indicates. progressive infilling Precipitation in tunnels that cross between grains leads to welding h. a c f Petrograhic thin sections plane polarized light b d e g h Scanning electron. microscope images, Figure 2 Subsurface distribution of lithified layers which form at 1 2 mm intervals a.
Water washed vertical section showing lithified laminae which stand out in relief b Low. magnification thin section photomicrograph of boxed area in a showing the distribution of. lithified layers c Micritic crust equivalent to the blue lines in b d Layer of microbored. fused grains equivalent to the orange lines in b underlying a micritic crust black dashed. 116 REID DUPRAZ VISSCHER SUMNER, In addition the microstructure of the layers of fused microbored grains is. identical to that formed by the climax community described above Analyses. of the distribution of these layers indicate that micritic crusts representing. mature biofilm communities occur at 1 to 2 mm intervals whereas layers of. fused grains representing climax communities occur at 2 to 3 mm and 3 to. 4 mm intervals Figure 2 Reid et al 2000, 3 CONTROLS OF STROMATOLITE. MORPHOGENESIS, Our recent research results as summarized above are the first to. document a set of microbemineral interactions resulting in the growth of. lithified laminated carbonate buildups in a modern environment Theses. studies indicate that Exuma stromatolites are the net result of interactions. between microbes and minerals in three distinct mat types Figure 3 These. mat types represent a spectrum of community development and include 1 a. pioneer stage of motile cyanobacteria 2 more mature mats having. continuous surface biofilms with a more substantial heterotrophic. population and 3 mats developed to a stage where surface carbonate grains. have been invaded by endolithic cyanobacteria and grains are fused. Processes of trapping and binding of sediment by the pioneer communities. and carbonate precipitation in biofilm and endolithic communities result in. stromatolite accretion Cycling between microbial communities with. accompanying changes in accretion style leads to lamination a fundamental. feature of stromatolites through time Iteration of lamination is expressed as. stromatolite morphology Figure 3, Currently the dynamics of the microbial system and the factors that. regulate community succession and stromatolite morphogenesis are. unconstrained Figure 3 On first consideration it may seem likely that. successions between pioneer communities which trap and bind sediment. and biofilm and climax communities which precipitate calcium carbonate. are controlled primarily by sediment supply Initial trapping of sediment is. however dependent on adhesion of grains to a microbial mat Thus. cessation of sediment accretion could result for two different reasons 1 a. lack of sediment influx an environmental control or 2 sediment adhesion. could be inhibited by accumulation of exopolymer that is not sticky enough. to trap grains a biological control In other words hiatuses in sediment. accretion could conceivably reflect conditions of mat biology Microbial. communities may therefore be actively controlling stromatolite. morphogenesis, MICROBIAL PROCESSES FORMING STROMATOLITES 117.
Our recent results provide critical insight into microbial functional. groups and major processes that are involved in stromatolite growth. Specific mechanisms and rates of these processes however remain largely. unknown and at present we can only speculate on intrinsic and extrinsic. factors that control stromatolite morphogenesis Future advances in. understanding stromatolite growth will require detailed knowledge of. accretion mechanisms microbial and environmental factors that regulate. accretion and quantitative models that link accretion to morphology. 3 1 Stromatolite accretion, 3 1 1 Sediment Trapping and Binding. Exact mechanisms of trapping and binding by pioneer communities of. filamentous cyanobacteria are not known Initial observations suggest. however that trapping occurs when grains adhere to sticky exopolymer at. the surface of a mat unpublished video recordings These grains are. subsequently bound when they are entwined by filamentous sheaths as. cyanobacteria move upward to the mat surface Water turbidity fluctuates. but sediment is typically abundant due to tidal cycles frequent high winds. and frequent burial events The factor that is considered most likely to. impact initial trapping is stickiness of exopolymer Decho this volume. additional factors that may be important in the binding of sediment are. cyanobacterial growth rates motility and response to light substrate. availability or chemical cues In addition erosive events such as storms. may have a negative impact on accretion erodibility will reflect the cohesive. properties of the bound sediment, 118 REID DUPRAZ VISSCHER SUMNER. Figure 3 Microbial environmental interactions leading to stromatolite morphogenesis. MICROBIAL PROCESSES FORMING STROMATOLITES 119, 3 1 2 Carbonate Precipitation. Carbonate precipitation in Exuma stromatolites is primarily associated. with biofilm and climax communities In biofilm communities precipitation. occurs within surface biofilms forming thin micritic crusts In climax. communities additional precipitation occurs in endolithic borings resulting. Chapter 6 MICROBIAL PROCESSES FORMING MARINE STROMATOLITES Microbe Mineral Interactions with a Three Billion Year Rock Record R P REID C D DUPRAZ Rosenstiel School of Marine and Atmospheric Science University of Miami 4600 Rickenbacker Causeway Miami FL 33149 USA P T VISSCHER University of Connecticut 1084 Shennecosset Road Groton CT 06340 USA D Y SUMNER University of California One

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