Elsevier

Earth-Science Reviews

Volume 210, November 2020, 103364
Earth-Science Reviews

Review Article
Modern supratidal microbialites fed by groundwater: functional drivers, value and trajectories

https://doi.org/10.1016/j.earscirev.2020.103364Get rights and content

Abstract

Microbial mats were the dominant habitat type in shallow marine environments between the Palaeoarchean and Phanerozoic. Many of these (termed ‘microbialites’) calcified as they grew but such lithified mats are rare along modern coasts for reasons such as unsuitable water chemistry, destructive metazoan influences and competition with other reef-builders such as corals or macroalgae. Nonetheless, extant microbialites occur in unique coastal ecosystems such as the Exuma Cays, Bahamas or Lake Clifton and Hamelin Pool, Australia, where limitations such as calcium carbonate availability or destructive bioturbation are diminished. Along the coast of South Africa, extensive distributions of living microbialites (including layered stromatolites) have been discovered and described since the early 2000s. Unlike the Bahamian and Australian ecosystems, the South African microbialites form exclusively in the supratidal coastal zone at the convergence of emergent groundwater seepage. Similar systems were documented subsequently in southwestern Australia, Northern Ireland and the Scottish Hebrides, as recently as 2018, revealing that supratidal microbialites have a global distribution. This review uses the best-studied formations to contextualise formative drivers and processes of these supratidal ecosystems and highlight their geological, ecological and societal relevance. Dynamic interchanges between salinity states both exclude many destructive metazoans and competitors and provides optimal nutrient conditions for benthic microbial and microalgal growth. The outflowing groundwater seeps are alkaline and rich in calcium carbonate, which reflects local catchment geological processes. These habitats support a diverse microbial community dominated by Cyanobacteria as well as some metazoan species previously unknown to science, or unknown for the region. Several taxa (from invertebrates to fish) utilise this environment as refugia. Supratidal microbialites are important coastal features because of the organisms they support and the ecological processes that they facilitate, such as habitat connectivity. Culturally and socially, the value of these habitats is increasingly being appreciated, for example as traditional freshwater supply points or as an unrealised geotourism opportunity. This review also frames new information about threats, opportunities for future research and conservation trajectories for these unique geobiological habitats.

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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