Abstract
Stromatolites are the oldest evidence for life on Earth, but modern living examples are rare and predominantly occur in shallow marine or (hyper-) saline lacustrine environments, subject to exotic physico-chemical conditions. Here we report the discovery of living freshwater stromatolites in cool-temperate karstic wetlands in the Giblin River catchment of the UNESCO-listed Tasmanian Wilderness World Heritage Area, Australia. These stromatolites colonize the slopes of karstic spring mounds which create mildly alkaline (pH of 7.0-7.9) enclaves within an otherwise uniformly acidic organosol terrain. The freshwater emerging from the springs is Ca-HCO3 dominated and water temperatures show no evidence of geothermal heating. Using 16 S rRNA gene clone library analysis we revealed that the bacterial community is dominated by Cyanobacteria, Alphaproteobacteria and an unusually high proportion of Chloroflexi, followed by Armatimonadetes and Planctomycetes, and is therefore unique compared to other living examples. Macroinvertebrates are sparse and snails in particular are disadvantaged by the development of debilitating accumulations of carbonate on their shells, corroborating evidence that stromatolites flourish under conditions where predation by metazoans is suppressed. Our findings constitute a novel habitat for stromatolites because cool-temperate freshwater wetlands are not a conventional stromatolite niche, suggesting that stromatolites may be more common than previously thought.
Highlights
Stromatolites are a form of microbialite with repetitive, laminated structures of biologically mediated mineral precipitation[1,2]
Stromatolites growing in low-salinity, low-temperature freshwaters have been recognized at localities such as: Ruidera Pools Natural Park, Spain[22], Pavilion Lake, British Columbia, Canada[23], karst-water creeks in Germany and France[24,25], cenote lakes in south-eastern mainland Australia[26], and tufa depositing streams in SW Japan[27]
We refer to these features as ‘peat-bound karstic wetlands’, because they comprise ‘islands’ of peat-free ground within otherwise monotonous organosol terrain of karstic limestone and dolomite valleys
Summary
Stromatolites are a form of microbialite with repetitive, laminated structures of biologically (typically cyanobacteria) mediated mineral precipitation[1,2] These microbial accretions are the oldest evidence for life on Earth[2,3,4,5,6] and debates continue on whether they evolved first on land or in the ocean[4,7,8,9,10]. Older literature refer to these features as ‘alkaline pans’[28], because the water is neutral to mildly alkaline (pH ~7–8), creating exceptional pH gradients across the boundary between the wetlands and surrounding acidic blanket bogs (pH ~4–5) This is striking because the majority of surface waters in south-west Tasmania are strongly acidic due to the high humic content of the soil[29]. We have studied the microbial structure of the Giblin River stromatolites and characterize the physico-chemical conditions of their habitat to determine environmental parameters that control and allow modern freshwater stromatolite formation in this region
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