Shark Bay Stromatolites

The stromatolites of Hamelin Pool at Shark Bay (Australia) are well recognised as the best examples of actively lithifying marine stromatolites (Fig. 4). Hamelin Pool is the innermost basin of Shark Bay, a shallow hypersaline bay on the western coast of Australia. Across the mouth of Hamelin Pool is a sea-grass covered sandbank that restricts water flow into Hamelin Pool. Combined with the high evaporation rates, low rainfall, and the lack of freshwater input from the extremely arid land surrounding the bay, this has resulted in salinity that reaches at least twice that of normal seawater (Arp et al., 2001). As alluded to earlier, this increase in salinity may result in a reduction in the level of grazing by higher eukaryotes.

Extant stromatolites of Hamelin Pool are relatively young; the oldest were radiocarbon dated to be 1,000-1,250 years old, with very slow growth rates of around 0.4 mm/year (Chivas et al., 1990). Early studies often reported only taxo-nomic and physiological properties of the dominant type of cyanobacteria found in Shark Bay stromatolites (Logan et al., 1974), however, recent reports have

Figure 4. Intertidal stromatolites in Hamelin Pool, Shark Bay, Western Australia.

begun to reveal an incredible microbial diversity of these formations, allowing researchers to make more specific and informed inferences about stromatolite functional complexity. One of these studies was the first polyphasic examination of the microbial communities of Shark Bay stromatolites, combining culture-dependent and culture-independent nucleic acid-based methods (Burns et al., 2004). This study showed that the stromatolite community was characterised by microorganisms of the cyanobacterial genera Synechococcus, Xenococcus, Microcoleus, Leptolyngbya, Plectonema, Symploca, Cyanothece, Pleurocapsa, Prochloron and Nostoc. Several of these cyanobacteria isolated from the extant analogues in Shark Bay are filamentous, a characteristic known to aid sediment trapping in stromatolites (Reid et al., 2000). Extracellular polymeric substances, known to be produced by several of the cyanobacteria identified also contribute to stromatolite structure by providing an adhesive matrix to physically bind sediment, as well as providing nucleation sites that promote carbonate precipitation (Arp et al., 2001). Furthermore, phylotypes related to Synechococcus were observed in the stromatolite 16 S rDNA libraries (Burns et al., 2004), and the outermost cell surface of members of this genus has been shown to have a role in fine-grain mineral formation (Schultze-Lam et al., 1992). Interestingly, this formation occurs with both live and dead cells, so such a process could be important in stromatolite lithification even after cell death. A Precambrian counterpart to Synechococcus, Eosynechococcus, has been described in ancient stromatolites (Hofmann, 1976), and it has been suggested that characteristics of Shark Bay microbial mats may allow the preservation of cyanobacterial cells as microfossils (Lopez-Cortés, 1999).

In addition to the considerable salinity and desiccation stress stromatolite microbial communities must tolerate, high temperatures and the relatively thin atmospheric ozone layer contribute to a high ambient UV irradiance at Shark Bay (Palmisano et al., 1989). This low ozone also makes this location a better analogue for Early Earth conditions. The sheath pigments known to be present in many of the surface-dwelling cyanobacteria identified (Burns et al., 2004) are likely to play a photoprotective role in screening deeper members of the community from physiological damage. Of further interest was the finding that a number of 16 S rDNA clones clustered with the genus Prochloron (Burns et al., 2004). Prochloron is usually symbiotic with didemnid ascidians and to date there is no report of their existence as a free-living organism (Kühl and Larkum, 2002). There are also no reports of ascidians in Shark Bay, and thus the discovery of potentially free-living Prochloron associated with stromatolites was unexpected.

As described earlier, non-cyanobacteria microorganisms are also prominent in these systems, and another recent study on the Shark Bay stromatolites revealed that the most dominant sequences were in fact novel proteobacteria (Papineau et al., 2005). An example of the significant microbial diversity identified is shown in Fig. 5 and Table 1. It is quite interesting that in contrast to earlier notions, cyanobacteria do not appear to dominate in these stromatolites, though it is quite likely they still have major roles in primary production (Papineau et al., 2005). Both of the recent studies using molecular methods concluded that many of the stromatolite microorganisms were unique with no close relatives in the database (Burns et al., 2004; Papineau et al., 2005), and these microorganisms may also possess novel physiologies vital to the survival, integrity, and persistence of stromatolites. Examples of this are several novel Archaea related to Halococcus sp., that were recently isolated and characterized from the Shark Bay stromatolites (Goh et al., 2006; Allen et al., 2008). These organisms possess numerous novel physiologies, including an oxidase-negative phenotype, as opposed to all other Halococcus species. Characterisation of other novel microorganisms identified from extant stromatolites may reveal further unique metabolisms. Furthermore a diversity of other unculturable Archaea have been identified from living stromatolites for the first time, and although their exact roles in stromatolite biology are unknown, they are likely to be important community members involved in nutrient cycling. Some Halobacterial species have been shown to be capable of fixing CO2 (Javor, 1988), and it would be intriguing to ascertain whether halobacterial species in stromatolites are involved in the calcification process, in addition to the accepted role that cyanobacteria play in formation of these biogeological formations.

An immense diversity of prokaryotic life associated with modern stromatolites has been revealed, and combined with our knowledge on the prevailing environmental conditions reveals an intimate association between biotic and abiotic factors in stromatolite formation. Most studies on both the Shark Bay and Bahaman stromatolites also revealed that eukaroytes were scarce in these extant formations (Reid et al., 2000; Burns et al., 2004; Papineau et al., 2005), although one study has documented various flagellates in Shark Bay (Al-Qassab et al., 2002). This supports the

Lyngbya Hieronymusii

Figure 5. Light microscopy of cyanobacterial isolates from Shark Bay stromatolites. (A) Euhalothece (B) Microcoleus, (C) Pleurocapsa, (D) Pleurocapsa, (E) Stanieria, (F) Chroococcidiopsis, (G) Xenoccocus, (H) Halomicronema, (I) Halothece, (J) Chroococcus, (K) Spirulina, (L) Lyngbya. Scale bar is 10 |m in each image. (Photo courtesy of Michelle Allen.)

Figure 5. Light microscopy of cyanobacterial isolates from Shark Bay stromatolites. (A) Euhalothece (B) Microcoleus, (C) Pleurocapsa, (D) Pleurocapsa, (E) Stanieria, (F) Chroococcidiopsis, (G) Xenoccocus, (H) Halomicronema, (I) Halothece, (J) Chroococcus, (K) Spirulina, (L) Lyngbya. Scale bar is 10 |m in each image. (Photo courtesy of Michelle Allen.)

original theories that modern stromatolites appear to thrive in environments that exclude most higher organisms. In addition, the differences observed between the microbial community composition of extant stromatolites in different locations (Reid et al., 2000; Burns et al., 2004; Papineau et al., 2005), suggests that different stromatolite morphotypes will depend on the community present and therefore will be determined by it. For example, specific microorganisms trap sediments differently (Papineau et al., 2005), resulting in different stromatolite morphologies. The numerous novel or unknown microbes yet to be identified may also play pivotal roles in stromatolite systems that we do not yet understand.

Table 1. Summary of microorganisms identified from stromatolites in Shark Bay (Burns et al., 2004).

Sequence analysis of 16 S rRNA gene

Prokaryotic group Isolate ID

Nearest relative in GenBank database

% Similarity Accession no.

Bacteria HPB25 Bacillus sp. SD-18 98

HPB26 Bacillus sp. BA-54 95

HPB28 Bacillus hwajinpoensis 99

HPB29 Bacillus marismortui strain 123 99

HPB9 Halomonadeaceae LA44 99

HPB55 Bacterium K2-13 99

HPB30 Bacillus sp. PL30 98

HPB31 Halobacillus sp. D-8 95

HPB32 Halobacillus trueperi GSP38 99

HPB13 Marinobacter sp. MED104 98

HPB14 Salinivibrio costicola GSP14 97

HPB33 Bacillus sp. SG-1 98

HPB34 Bacillus litoralis 99

HPB8 Halomonas alimentaria GSP27 98

HPB10 Idiomarina sp. NT N118 99

HPB35 Bacillus megaterium 99 SAFB-011

HPB36 Bacillus sp. KMM 3737 97

HPB6 Porphyrobacter tepidarius 97 DSM10

HPB37 Bacillus firmus 97

HPB56 Bacterium K34 97

HSC29 Euhalothece strain 96 MPI95AH13

HSC25 Cyanothece sp. ATCC 51142 92

HSC31 Gloeocapsa sp. PCC 73106 94

HSC36 LPP-group MBIC10087 97

HSC37 Geitlerinema sp. PCC 7105 92

HSC22 Xenococcus PCC 7305 92

HSC20 Xenococcus sp. Cyano35 96

HSC17 Lyngbya hieronymusii 94

HSC19 Chroococcidiopsis sp. PCC 6712 94

HSC3 Dermocarpella incrassata 98

HSC34 Gloeothece sp KO11DG 92

HSC24 Uncultured Chroococcus sp. 94

Archaea HSA8 Uncultured archeon, 92

clone HW25

HSA16 Halococcus sp. 92

HSA21 Halobacterium NCIMB 734 94

HSA22 Haloarchaeon 14AHG 95

HSA23 Uncultured archeon 93 clone HW54

HSA24 Uncultured archeon IMT315 95

HSA25 Uncultured archeon 92 clone HW11

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