Abstract
For three billion years, before the Cambrian diversification of life, laminated carbonate build-ups called stromatolites were widespread in shallow marine seas1, 2. These ancient structures are generally thought to be microbial in origin and potentially preserve evidence of the Earth's earliest biosphere1, 2, 3. Despite their evolutionary significance, little is known about stromatolite formation, especially the relative roles of microbial and environmental factors in stromatolite accretion1, 3. Here we show that growth of modern marine stromatolites represents a dynamic balance between sedimentation and intermittent lithification of cyanobacterial mats. Periods of rapid sediment accretion, during which stromatolite surfaces are dominated by pioneer communities of gliding filamentous cyanobacteria, alternate with hiatal intervals. These discontinuities in sedimentation are characterized by development of surface films of exopolymer and subsequent heterotrophic bacterial decomposition, forming thin crusts of microcrystalline carbonate. During prolonged hiatal periods, climax communities develop, which include endolithic coccoid cyanobacteria. These coccoids modify the sediment, forming thicker lithified laminae. Preservation of lithified layers at depth creates millimetre-scale lamination. This simple model of modern marine stromatolite growth may be applicable to ancient stromatolites.
The only known examples of stromatolites presently forming in open marine environments of normal seawater salinity are on the margins of Exuma Sound, Bahamas4, 5, 6. Our study focused on well-laminated build-ups at Highborne Cay (76° 49' W, 24° 43' N) as potential analogues of ancient stromatolites extending back to the Precambrian. Highborne Cay stromatolites form in the back reef zone of an algal-ridge fringing reef complex that extends 2.5 km along the eastern shore of the island, facing Exuma Sound7. Surface waters have a salinity of 36–37 parts per thousand and are saturated with respect to both aragonite and calcite. Stromatolites form as intertidal and subtidal build-ups, shoreward of the algal ridge. Results reported here pertain to the subtidal stromatolites, which grow in depths of less than 1 m at mean low tide and form ridges and columnar heads up to half a metre high (Fig. 1).
Figure 1: Shallow subtidal stromatolites, Highborne Cay, Bahamas.
a, Extensive columnar build-ups. b, Vertical section showing lamination; scale bar, 2 cm.
High resolution image and legend (48K)Surfaces of Highborne Cay stromatolites are covered with cyanobacterial mats. Examination of these mats using a variety of integrated geological and microbiological techniques reveals variations in microbial community structure and composition. Extensive field sampling over a two-year period revealed three mat types, representing a continuum of growth stages with minimal seasonal variability (Fig. 2).
Figure 2: Dominant prokaryotic communities on stromatolite surfaces.
Cycling between communities, indicated by large arrows, is a response to
intermittent sedimentation (see text). a, b, Pioneer community:
filamentous cyanobacteria (arrows) bind carbonate sand grains. c–e
, Bacterial biofilm community: a continuous sheet of amorphous exopolymer
(arrows, c, d) with abundant heterotrophic bacteria (Fig. 3)
forms uppermost surface; aragonite needles precipitate within this surface
film (e). f, g, Climax community: a surface biofilm overlies
filamentous cyanobacteria and endolith-infested grains, which appear grey
and are fused (arrow, f). Precipitation in tunnels that cross between
grains leads to welding (g). a, c, f, Petrographic
thin sections, plane polarized light; cyanobacteria are stained with methylene
blue. b, d, e, g, Scanning electron microscope
images. Scale bars: a, b, c, f, 100 
m;
d, 50 m; e, 5 m; g, 10 m.
Type 1: About 70% of all mats examined consist of a sparse population of the filamentous cyanobacterium Schizothrix sp.8. Schizothrix filaments are generally vertically orientated and are entwined around carbonate sand grains (Fig. 2a and b).
Type 2: Approximately 15% of mats show development of calcified biofilms,
which appear as thin crusts of microcrystalline carbonate (micrite) at the
uppermost surface of the mat (Fig. 2c and
d). These films are about 20–60 m thick; they drape
over and bridge interstitial spaces between sand grains. Silt-sized carbonate
particles, such as tunicate spicules, are commonly embedded in the films.
Cyanobacterial filaments are present, but are not abundant in the biofilms,
which are comprised mainly of copious amounts of amorphous exopolymer,
metabolically diverse heterotrophic microorganisms9, 10, 11
and aragonite needles. Needle-shaped aragonite crystals, approximately 1 m
in length, form spherical aggregates 2–5 m in diameter
and are embedded in the exopolymer matrix (Fig. 2e).
Bacteria are abundant and are commonly observed at the edges of the aragonite
spherules (Fig. 3). A sparse to moderately dense population
of Schizothrix underlies the exopolymer biofilm (Fig.
2c).
Figure 3: Scanning laser confocal microscope image of a surface biofilm.
Bacteria (red fluorescence) are abundant and show an intimate association
with carbonate precipitates (blue autofluorescence). Large, uncalcified filament
in upper left is Schizothrix sp. Sample is stained with propidium iodide;
scale bar, 5 m.
Type 3: The remaining 15% of mats are characterized by an abundant population of the coccoid cyanobacterium Solentia sp. and randomly-orientated Schizothrix filaments below a calcified biofilm (Fig. 2f and g). Solentia is an endolith, which bores into carbonate sand grains. These bored grains appear grey when viewed in plane polarized light in a petrographic microscope (Fig. 2f), contrasting with the golden-brown colour of unbored grains (Figs 2a and c). The microbored grains are often fused at point contacts and appear 'welded' together (Fig. 2f and g).
The variations in surface mats described above represent changes in microbial community structure and activity in response to intermittent sedimentation. Type 1 mats, characterized by a sparse population of Schizothrix filaments, resemble pioneer communities12, which dominate during periods of sediment accretion. Formation of these mats during intervals of rapid sedimentation is documented by field observations showing that accretion rates of one grain-layer per day produce mats with Type 1 fabrics. The activities of Schizothrix , in particular, photosynthetic production of exopolymer, are crucial in the accretion process. Flume studies show that sand grains, which settle from suspension when flow rate is low, adhere to mucous-like exopolymer (B.M.B., unpublished video recordings). These 'trapped' grains are subsequently bound by filaments and exopolymer as Schizothrix moves upward to the sediment surface. Populations of diatoms and other eukaryotes are minor to absent in these accreting mats8, 13, 14 indicating that, contrary to previous reports15, 16, eukaryotic organisms are not required for the trapping and binding of coarse-grained sediment. Aragonite precipitation is inhibited during this stage through calcium ion binding by exopolymer and low-molecular-weight organic acids excreted by Schizothrix11.
Type 2 mats represent a more mature12, 17 surface community characterized by development of a continuous surface film of exopolymer. This mat type develops during quiescent periods when sedimentation ceases and mats begin to lithify. Formation during calm periods is indicated by carbonate silt, such as tunicate spicules, which is commonly entrapped in the surface films but is characteristically lacking in Type 1 mats. Mesocosm manipulations suggest that continuous surface biofilms form in a matter of days. These surface films support heterotrophic activity of both aerobic and anaerobic bacteria9, 11, which metabolize the low-molecular-weight organic compounds and the labile fraction of the amorphous exopolymer10, 11. Sulphate reduction takes place despite the presence of oxygen at the surface and sulphate-reducing bacteria account for a significant fraction (30–40%) of the organic carbon consumption by the community9, 10. This bacterial activity promotes aragonite precipitation as evidenced by microscale observations that high rates of sulphate reduction coincide with micritic crusts18. In addition, microautoradiography of radiolabelled organic matter shows a close association between bacteria and aragonite needles (H.W.P., unpublished data). The net result of these processes is calcification of the biofilm and formation of a thin micritic crust. When additional carbonate sand is accreted onto the stromatolite, this surface-coating film persists into the subsurface as a nearly continuous thin sheet of micritic cement.
Longer hiatal periods allow formation of Type 3 mats, which are more fully developed than Type 2 mats and include an abundant population of the coccoid cyanobacterium Solentia sp. These Solentia-rich mats represent the 'climax' community of the stromatolite system ( Fig. 2). Excretion products of Solentia and Schizothrix support high rates of bacterial respiration10, 12. Microscopy and culture experiments have revealed an unusual process of boring and infilling associated with Solentia19, 20. Boreholes are filled in with aragonite as Solentia advances19, 20. Moreover, as Solentia crosses between grains at point contacts, infilling of the microbored tunnels obliterates grain boundaries and grains become fused together (Fig. 2g). Observations of organic matter in some boreholes, together with high sulphate reduction activity in these layers18 indicates that, as in Type 2 mats, heterotrophic activity may be important in the precipitation process. In contrast to the conventional view that microboring is principally a destructive process8, 21, the microboring and infilling processes associated with Solentia activity in these mats is a constructive process. This process fuses grains together to create laterally cohesive carbonate crusts. These crusts persist into the subsurface and provide structural support for the growth and long-term preservation of the stromatolite. Field and laboratory studies show that layers of fused microbored grains are formed in periods of weeks to months19. 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. Even longer hiatal periods result in a community succession to eukaryotic algal communities, which do not form laminated structures8, 14.
Laminations in the fossilized part of the stromatolites represent a chronology
of former surface mats (Fig. 4). Stromatolite laminae
are most easily observed on water-washed, cut surfaces where lithified layers
stand out in relief (Fig. 4a). Although lamination is
readily apparent in hand samples, it has a subtle expression in petrographic
thin sections. Detailed microstructural analyses show, however, that the lithified
layers have two distinct petrographic appearances (Fig. 4b
). These laminae correspond to (1) thin crusts of micrite, 10–60 m
thick (blue lines in Fig. 4b and c),
and (2) layers of fused, microbored grains infested with Solentia sp.;
these layers are 1–2 mm thick (orange lines in
Fig. 4b and d) 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 stromatolites3, 22. In addition, microstructural
features of the layers of fused, microbored grains are identical to those
formed by the climax community described above.
Figure 4: Lamination and microstructure in stromatolite subsurface.
Lithified layers representing former surface mats 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. Blue
lines represent micritic crusts (c); orange lines represent welded,
micritized grains (d). c, Thin-section photomicrograph of a
micritic crust; these crusts represent calcified biofilms (Type 2 mats).
d, Thin-section photomicrograph of a layer of microbored, fused grains,
which underlie a micritic crust; these layers represent former climax communities
(Type 3 mats). c, d, Plane polarized light. Scale bars:
b, 10 mm; c, d, 100 m.
Lithified layers, which represent former surfaces of mats, show a millimetre-scale distribution. This is indicated by petrographic analyses of the upper several centimetres of 37 stromatolites containing 453 micritic crusts and 174 microbored layers. The micritic crusts form at intervals averaging 1–2 mm. Distances between tops of layers of fused, microbored grains show two modes, one at 2–3 mm and a second at 4–5 mm. Typical marine cements, such as acicular fringes of aragonite, are notably absent during these early stages of growth. Thus, the lithified mats provide initial structural support for the development of laminated build-ups with topographic relief.
To our knowledge, this is the first study to define a specific set of mechanisms that link lamination in marine stromatolites to a dynamic balance between sedimentation, a succession of prokaryotic communities and early lithification. Integration of detailed geological and microbiological analyses of stromatolites in a modern marine system has shown that the structure and composition of surface mats alter in response to intermittent sedimentation and that mats lithify during hiatal periods. Lithification depends on two fundamentally important microbial processes: photosynthetic production by cyanobacteria and heterotrophic respiration by bacteria. A laminated microstructure is formed by precipitation of laterally continuous sheets of micrite in surface biofilms, which are formed during frequent discontinuties in sedimentation. In some cases, thicker layers of fused grains form below these biofilms in response to microboring activities and precipitation, probably resulting from polymer degradation in boreholes.
These findings provide insight into the role of microbes in stromatolite accretion, lamination and lithification. Although most researchers agree that, "microbial mats and their associated sediments must be lithified early in order to be preserved in the record as stromatolites"1, the proposed mechanisms and precise timing of early lithification have been "vigorously debated"1. Historically, early lithification was attributed to abiotic processes of submarine cementation23, 5 or to calcification of cyanobacterial sheaths24 related to photosynthetic activity. More recently, attention has shifted to heterotrophic bacterial decomposition of cyanobacterial sheaths in subsurface, aphotic zones25, 26. Although field studies have documented bacterial precipitation of micrite on the sheaths of dead cyanobacteria in the subsurface of laminated microbial mats in tidal flats25, 27, these mats do not form fully lithified laminae and stromatolitic build-ups. We argue that growth of laminated microbial structures with topographic relief, such as those that dominated the fossil record for three billion years, depends on penecontemporaneous lithification of surface mats. This lithification process occurs by decomposition of an amorphous matrix of bacterial exopolymer (not sheath material) in the photic zone across the stromatolite surface. Similar processes of precipitation within the amorphous exopolymer matrix of biofilms, rather than on cyanobacterial sheaths, offer an additional mechanism to account for the paucity of preserved microfossils in Precambrian stromatolites, which is typically ascribed to recrystallization and/or rapid degradation of sheaths1, 26, 28. The potential role that climax microbial communities, functionally equivalent to the endolithic coccoid cyanobacterial communities in modern marine stromatolites, may have played in the growth and lithification of ancient stromatolites remains to be evaluated.
Methods
This study combined a range of geological, microbial and chemical analyses. An extensive field program was conducted during January and June 1997, and March and August 1998. Physicochemical indices of stromatolite mats were determined in situ, primarily with O2, sulphide, pH needle electrodes (0.8 mm outer diameter)9, whereas microstructural, chemical and microbial analyses and incubations were done in the laboratory at the field site and in home institutions. Mat communities and microstructural features were identified using a variety of microscope techniques (light, scanning electron, transmission electron, and scanning laser confocal29) and microbial populations were enumerated using epifluorescence microscopy counts9, most-probable number enumerations9, 10 and molecular phylogenetic techniques. Microbial activities were assessed using depth profiles measured with microelectrodes9 and radioisotope incubations using 3H, 14C and 35S (refs 9, 10 and 30). Heterotrophic activity was also studied with microautoradiography of labelled organic matter uptake30. Microscale distribution of sulphate reduction was assessed using Ag foil coated with 35SO 2-4 (ref. 18). Exopolymer distribution and production were evaluated by physical and chemical extractions and 14C-bicarbonate experiments, respectively11. Other methods used are described elsewhere9, 10, 11, 13, 29, 30.