RESEARCH INITIATIVE ON BAHAMIAN STROMATOLITES
The role of microbes in accretion, lamination
and early lithification of modern marine stromatolites
Shallow subtidal stromatolites, Highborne Cay, Bahamas: Extensive columnar build-ups (left) and vertical section showing lamination (right, picture 8 cm wide)
The only known examples of stromatolites presently forming in open marine environments of normal seawater salinity are on the margins of Exuma Sound, Bahamas. Our study focused on well-laminated buildups at Highborne Cay (76°49’W, 24°43’N) as potential analogs 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 Sound. Surface waters have a salinity of 36 to 37 psu and are saturated with respect to both aragonite and calcite. Stromatolites form as intertidal and subtidal buildups 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 meter high.
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.
TYPE 1 - Pioneer community: About 70% of all mats examined consists of a sparse population of the filamentous cyanobacterium Schizothrix sp.8 Schizothrix filaments are generally vertically oriented and are entwined around carbonate sand grains (Figure a,b).
TYPE 2 - Bacterial biofilm community: Approximately 15% of mats shows development of calcified biofilms, which appear as thin crusts of microcrystalline carbonate (micrite) at the uppermost surface of the mat (Fig. c, d). These films are ~20-60 µm thick and 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 microorganisms and aragonite needles. Needle-shaped aragonite crystals, approximately 1 µm in length, form spherical aggregates 2 to 5 µm in diameter and are embedded in the exopolymer matrix (Fig. e). Bacteria are abundant and are commonly observed at the edges of the aragonite spherules. A sparse to moderately dense population of Schizothrix underlies the exopolymer biofilm (Fig c).
TYPE 3 - Climax community: The remaining 15% of mats is characterized by an abundant population of the coccoid cyanobacterium Solentia sp. and randomly-oriented Schizothrix filaments below a calcified biofilm (Fig. f, 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. f), contrasting with the golden brown coloration of unbored grains (Figs a, c). The microbored grains are often fused at point contacts and appear "welded" together (Fig. f, 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 communities, 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 (unpublished video recordings, BB). 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 mats, indicating that, contrary to previous reports, 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 Schizothrix.
Type 2 mats represent a more mature 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 bacteria, which metabolize the low-molecular weight organic compounds and the labile fraction of the amorphous exopolymer. Sulfate reduction takes place despite the presence of oxygen at the surface and sulfate-reducing bacteria account for a significant fraction (30-40%) of the organic carbon consumption by the community. This bacterial activity promotes aragonite precipitation as evidenced by microscale observations that high rates of sulfate reduction coincide with micritic crusts18. In addition, microautoradiography of radiolabeled organic matter shows a close association between bacteria and aragonite needles (HWP 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 even 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 (see above). Excretion products of Solentia and Schizothrix support high rates of bacterial respiration. Microscopy and culture experiments have revealed an unusual process of boring and infilling associated with Solentia. Boreholes are infilled with aragonite as Solentia advances. Moreover, as Solentia crosses between grains at point contacts, infilling of the microbored tunnels obliterates grain boundaries and grains become fused together (Fig. g). Observations of organic matter in some bore holes, together with high sulfate reduction activity in these layers 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 process 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 months. 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 structures.
More information: Reid et al. (2000): The microbes in accretion lamination and early lithification of modern marine stromatolites. Nature 406