The Role of Cyanobacteria in Stromatolite Morphogenesis, Highborne Cay Bahamas: An Integrated Field and Laboratory Simulation Approach

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Introduction

Cyanobacteria are the primary producers in the Highborne Cay stromatolites providing energy, directly or indirectly, for the entire microbial community. They are physically the largest members of the microbial community and many of the filamentous types play key roles in the trapping and binding of sediments. We are therefore focusing on cyanobacteria due to their dual roles as both primary producers supporting the flow of carbon into the entire microbial community, and due to their primary structural function in constructing the 3-dimensional architecture of the microbial mats found at the stromatolite surfaces. Cyanobacteria are highly motile, and their localization in a given space indicates a clear physiological preference for environmental conditions of that particular area or depth. Different cyanobacteria have different preferential placement and an understanding of the environmental conditions which these various cyanobacteria are adapted to and selecting for will allow us far more certainty in back-interpreting environmental conditions from the microbial structures (modern or fossilized) which thrive in a given location. Increasingly our observations suggest that the activities and locations of various cyanobacterial species also contribute greatly to the localization of new mineral precipitation through a variety of processes.
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Community
We are investigating these contributions using an integrated approach combining detailed observations of field samples with manipulative experiments using both field samples and cultures of specific organisms isolated from these stromatolites. Isolates include five cyanobacteria with thin, 1 to 2 ?m trichomes, several large filament size genera including Oscillatoria, Microcoleus, Symploca and unicellular cyanobacteria of the genera Synechococcus and Solentia (Prufert-Bebout, in prep.). Previous studies had reported that a single species of thin filament cyanobacterium (Schizothrix gebeleinii) was responsible for sediment trapping and binding in Bahamian sediments at Lee Stocking Island (Golubic and Brown, 1996). Our initial field observations suggested that the main sediment trapping and binding in the Highborne Cay stromatolites also resulted from the activities of pale, thin (1 to 2 ?m wide) filamentous cyanobacteria. Due to the small size and low pigment content of the trichomes, identification of the thin filamentous cyanobacteria by optical microscopy was not possible although some subtle size variations of trichomes within the 1 to 2 ?m size range occurred (Stolz and Prufert-Bebout pers. obs.). Laboratory isolations of cyanobacteria from Highborne Cay mats have yielded five distinct 1 to 2 ?m size filaments. Subsequent comparative analyses of the 16S rRNA coding region sequences indicate that, though appearing quite similar in morphology, the group of small cyanobacterial filaments culturable from Highborne Cay stromatolites shows enormous diversity with cultivated populations exhibiting from 98 to as little as 84% homology (Prufert-Bebout et al., in prep.). With such high diversity, we infer that there may be great variability in binding properties and metabolic capacity amongst the different small filamentous “Schizothrix-like” cyanobacteria at Highborne Cay. Competition experiments between these different small filaments will help us to elucidate the role of this important group in stabilizing sediments.
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Approach

We begin with detailed field and microscopic observations and documentation of samples taken from stromatolites in the field. We then isolate as many of the cyanobacterial species as possible. Isolated cultures are then characterized with respect to their motility and sediment binding capabilities. Lastly specific cultures are chosen and introduced to sediments and then subjected to various controlled flow, sedimentation, nutrient, and irradiance regimes. Resultant texture, mineral formation, and cyanobacterial distribution changes from these simulation trials are then compared to those features in the original field collected samples. These comparisons in some cases have illuminated the importance of microbial processes not previously recognized. In other cases our simulated textures or mineralization patterns differ from those observed in the field. These observations refine our hypotheses, as well as our characterization and examination of the original samples allowing for further iterations of the field-to simulation process.

Objective: To increase our understanding of the roles played by cyanobacteria in determination of microscale layering, macroscale morphology and lithification of Bahamian stromatolites.

Challenge: To understand how cyanobacterial competition and/or responses to environmental perturbations (sedimentation, flow, irradiance) by means of motility, pigment / metabolic changes, or formation of structured assemblages (biofilms, ridges etc) contribute to stromatolite formation in complex systems.

Approach: Begin with detailed observations of the natural system, and then recreate natural sunlight laboratory environments conducive to examination of cyanobacterial motility and sediment binding in simplified controlled systems.

 

Focal Points:
We are beginning with a focus on 3 components of stromatolite structure

A) The role of Cyanobacteria in binding sediments in biofilm formation

B) The role of hydrodynamic flow regime in determining cyanobacterial distribution, development of laminated textures and overall structural morphology

C) The role of endolithic cyanobacteria (Solentia spp., and others) in the creation of fused ooid crusts

Experiments are conducted both in standard laboratory conditions and in running seawater flumes either in the laboratory, or increasingly exposed to natural sunlight. A variety of standard techniques; SEM, petrographic analyses, TEM, are utilized either at NASA Ames or in collaboration with RIBS co-investigators to compare cyanobacterial distribution and mineralization processes in field samples with those generated in laboratory-flume simulations.
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Preliminary Results (Binding)

Figures show motility of a Phormidium from a Bahamian stromatolite through different depths of sediments after one week. Cells were inoculated at the base of the container. Sediments were added and the sides were covered to insure that irradiance was only received via the surface incident levels. Results show new biofilm formation at the sediment water interface. With only 1mm cover, new biofilm formation was uneven. With 5mm cover biofilm formation created a smooth cohesive structure. With 1cm sediment cover Phormidium could only migrate through sediments at the corners of the container and no new biofilm was created. SEM images contrast the smooth laboratory biofilm with a similarly colored biofilm from the field showing differences in fine structure.

 

Preliminary Results (Flow studies)

Cyanobacteria from four cultured populations were re-introduced to sterile ooids and the mix was formed into hemispheres using ultraclean agarose as a gelling agent. This approach provided for homogeneous distribution of all cultures throughout the matrix. After one week under laminated flow, different populations can be seen inhabiting different areas of the hemispheres. A model of idealized flow over hemispheres indicates areas of turbulent flow or stagnant conditions. Results indicate Schizothrix sp. type species dominating in areas experiencing turbulent flow or thin boundary layers. Red phormidium type dominated in the areas experiencing thicker boundary layer conditions. Oscillatoria are generally widely distributed and do not form films.

An experimental run of hemisphere shapes under three flow regimes: stagnant, 1cm/s laminar and 3 cm/s laminar to turbulent. Under stagnant conditions most extensive growth was in a small area at the top of the hemispheres, under the highest flow, turbulent regime there was extensive biofilm formation on all major surfaces, Dye injection illustrates flow regimes.

A cross sectional view of an experimental hemisphere from turbulent flow conditions shows vertical stratification of “schizothrix” type cyanobacteria at the surface (golden film) and Oscillatoria sp. at depth (darker subsurface layer).

 

Preliminary Results (fused crust)

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Endolithic cyanobacteria have previously been well documented as destructive agents of biological erosion (Golubic and Brown 1996; Perry, 1998; Golubic et al, 2000). In contrast, our studies have demonstrated that the endolithic Solentia sp., population at Highborne Cay plays a net constructional role in stromatolite growth and stabilization by forming lithified layers of welded grains.
OBSERVATIONS: Geology: (fig l) Fused horizons shown at depth in this petrographic thin section show extensive re-crystallization of ooids. Grains are fused at point contacts and upper surfaces of ooids are frequently truncated. These can be contrasted with normal ooids shown at the top of the micrograph Biology: Stromatolites at Highborne Cay frequently show the presence of a dark grey pigmented layers dominated by cyanobacteria of the genus Solentia (fig m) Geobiology: Isolated Solentia spp., cells grow and travel within ooids on a laboratory petri plate (fig n). Solentia are able to move between ooids facilitating the fusion of individual ooids into a coherent crust. A crust generated in the laboratory by the activity of Solentia on loose ooids is shown in cross section with a dime for scale (fig o).

Summary

The process of observing, then attempting simulation of stromatolite structure formation processes in the laboratory through manipulation of field derived cultures has proven to be a useful tool. Findings of our focal point studies indicate that this approach will be useful in:

A) Further sediment binding studies in conjunction with flume tanks. More experimentation with flow and irradiance regimes will be needed to better simulate the binding properties seen in the field. We have not yet simulated the same type of fine micrite formation in biofilms as seen at Highborne Cay and further experiments will focus to improve this aspect of our simulations.

B) Further lamination and morphology experiments. Different cyanobacterial cultures show distinct location preferences. The interplay of flow and structure morphology are instrumental in determining the microflow regimes experienced by microbes, which in turn can effect their distribution

C) Further exploration of endolith contribution to fused crust formation. Our simulations to date were essential in elucidating the process that occurs in the field, which had previously been misinterpreted as a purely chemical re-crystillazation Process (see MacIntyre et al.m 2000). Co-culture experiments growing Solentia with and without filamentous cyanobacteria can control the amount and placement of matrix cements (data not shown)

D) Additional experiments will prove useful information on microbial activity effects on sediment permeability.

E) Where simulations do not match field processes, they still prove effective in helping us to refine our observations and hypotheses.

This research will greatly enhance our ability to interpret the early record of microbial community evolution on Earth and potentially other planets where water flow influences on biofilm, microbial mat or stromatolite structures could impact the evolution of life.

 

 

Design by Paola L.Gutierrez - paogutie@yahoo.com - Duquesne University. June 2003