Review ArticleModern supratidal microbialites fed by groundwater: functional drivers, value and trajectories
Section snippets
Context
Lithified deposits of microbial activity (microbialites) constitute the most uninterrupted and long-standing record of life on Earth, appearing continuously as fossils since at least 3.45 Ga (Allwood et al., 2006; Riding, 2011; Zawaski et al., 2020). Modern microbialites are not ubiquitous in aquatic environments and are instead confined to locations where competitors and destructors are excluded, or suitable biogeochemical conditions enable regular microbialite growth. Such habitats include
Past and extant microbialites
Microbial organisms comprise the dominant proportion of life's evolutionary history (Knoll et al., 2016), and evidence for their existence is primarily found in the form of deposited mineral structures or altered local isotopic and geochemical signatures (Grotzinger and Knoll, 1999; Bosak et al., 2013). During their metabolism and growth, microbial organisms precipitated calcium carbonate (CaCO3) under alkaline conditions, which was abundant in Precambrian oceans (Grotzinger, 1990), and trapped
Groundwater-fed supratidal microbialites
Despite being briefly reported on in the early 20th century (Mountain, 1937), extant microbialite systems along the South African coast have only recently been recognised and described in detail by Smith and Uken (2003). These systems were identified in the area of the Kei Estuary mouth and Cape Morgan (Fig. 2, Fig. 3). Smith et al. (2005) suggested that these microbialites were a partial analogue for some early Archaean peritidal stromatolites because of shared properties of mineral
Geology
The South African supratidal microbialites are more numerous and spatially extensive than those recorded on the rocky shores of Australia, Northern Ireland and the Scottish Hebrides (Forbes et al., 2010; Cooper et al., 2013; Perissinotto et al., 2014; Smith et al., 2018), occurring in all four coastal provinces of South Africa, from the southwest to the east coast (Fig. 2). Of the 18 documented clusters, 17 are associated with microbialite growth forming primarily on the competent strata of
Microbiology
Microbial assemblages of South African supratidal microbialites (see Table 2) broadly resemble the communities of many other microbialite systems worldwide (Myshrall et al., 2010; Mobberley et al., 2015; Suosaari et al., 2016; Yanez-Montalvo et al., 2020). However, the South African systems display remarkable intra- and inter-site variability with respect to community structure and taxonomic diversity (Fig. 5H). In general, they are dominated by taxa classified in the Cyanobacteria,
Precambrian analogues
The value of contemporary microbialite deposits, in general, has been ascribed to the potential they provide to probe into ancient systems of early Earth and the origin of life (e.g. Bosak et al., 2013). However, several differences between modern and ancient systems exist, of which perhaps the most apparent, after distribution and abundance, are the coarser-grained fabrics of the former, as discussed in Suarez-Gonzalez et al. (2019) and Suosaari et al. (2019b).
The contemporary groundwater-fed
Groundwater
All microbialite systems discussed in this review are reliant on groundwater seepage in the supratidal zone (e.g. Smith et al., 2005; Perissinotto et al., 2014). Microbialite deposition may occur with minimal seawater contribution, for example, the waterfall deposits at Schoenmakerskop (Edwards et al., 2017). No active microbialite growth however occurs under purely marine conditions in these locations (Dodd et al., 2018). The reason for this could be the lower carbonate mineral saturation
Future trajectories and concluding remarks
Modern supratidal microbialites are driven by interlinked geobiological influences (Fig. 4) which are both abiotic components of the local environment (i.e. stable bedrock, carbonate-rich groundwater seepage, periodic tidal inundation) and conducive biological features (i.e. lithifying biofilm community, non-destructive metazoan bioturbation). A disruption to any of these by threats acting as possible disturbances has direct consequences for the persistence of this ecosystem type (e.g. Fig. 6
Declaration of Competing Interest
The authors have no conflicts of interest to declare.
Acknowledgements
We thank the Editor and two anonymous reviewers for their constructive comments, which helped improve the focus and global context of this review. This research was funded by grants awarded by the South African National Research Foundation (NRF) to RP/JBA (UID: 84375) and RAD (UID: 87583; 109680; 110612). Within these are the Department of Science and Innovation (DSI)/NRF South African Research Chairs Initiative (SARChI) for SARChI: Shallow Water Ecosystems and SARChI: Marine Natural Products
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