Microbial carbonates, and stromatolites in particular, represent the earliest geological record of life on Earth, which dominated the planet as the sole biotic carbonate factory for almost 3 b.y., from the Archean to the late Proterozoic. Rare and sparsely scattered across the globe in the present day, modern “living” stromatolites are typically relegated to extreme environmental niches, remaining as vestiges of a prodigious microbial past. Here, we report the first discovery of living shallow-marine stromatolites in the Middle East, on Sheybarah Island, Al Wajh carbonate platform, on the NE Red Sea shelf (Saudi Arabia). We detail their regional distribution and describe their environmental conditions, internal structures, and microbial diversity. We also report the first discovery of reticulated filaments in a photic setting, associated with these stromatolites. The Sheybarah stromatolites occur in the intertidal to shallow subtidal zones along the seaward-facing beach in three depth-dependent growth forms. Their inner layers were formed by microbially mediated accretion and differential lithification of sediment grains. Compositional microbial analysis revealed the presence of a wide range of microbial life forms.

Stromatolites are a vestige of the first life on Earth, dominating carbonate-forming marine biota in the Archean and Proterozoic (Grotzinger and Knoll, 1999). Recent evidence dates the earliest occurrence of stromatolites to 3.48 Ga (Hickman-Lewis et al., 2023). With the exception of a few short intervals during the Phanerozoic, their importance in producing carbonates in modern times has been reduced to niche occurrences found predominantly in challenging environments, such as hypersaline marine settings and alkaline lakes (Carvalho et al., 2018; Samylina and Zaytseva, 2019; Marin-Carbonne et al., 2022). Understanding how lithified stromatolites form, which microbes contribute to growth processes, and how nutrient cycling works would provide insights into early life and ocean evolution on Earth, and perhaps on other planets, such as Mars. So far, the only occurrences of modern open-marine stromatolites are in the Exuma islands of the Bahamas (Dill et al., 1986; Visscher et al., 1998; Reid et al., 1995, 2000) and in Shark Bay, Australia (Suosaari et al., 2016, 2019). Here, we report the discovery of living stromatolites on Sheybarah Island, Red Sea, Saudi Arabia (Fig. 1). Although the presence of ancient microbialites (Perri et al., 2018; Strohmenger and Jameson, 2018) and microbial mats (Bontognali et al., 2010) has been previously recorded in the Middle East, this is the first record of modern intertidal stromatolites in this region.

The newly discovered stromatolites are located on SW Sheybarah Island of the Al Wajh carbonate platform along the NW coast of Saudi Arabia (Figs. 1A and 1B). The land-attached Al Wajh platform is located in the NE Red Sea and is almost completely enclosed by a 115 km reef-shoal belt (Petrovic et al., 2022). The platform hosts a 42-m-deep lagoon that is characterized by 92 islands and patch reefs of varying size. Sheybarah Island, with an area of 27 km2 and a maximum elevation of 2 m above mean sea level, is situated on the SW edge of the platform (Fig. 1B). The southern slope of the platform is characterized by extensive mangrove forest mainly on the lagoon-facing rim, a sandy and rocky interior, carbonate beach sand ridges, skeletal carbonate sand, and rocky reef flats facing the open sea (Chalastani et al., 2020; Petrovic et al., 2023a). The semi-enclosed and oligotrophic Red Sea is characterized by poor water exchange and slow surface-water renewal (Maillard and Soliman, 1986). In the NE Red Sea, the average sea-surface temperature ranges between 28 °C in summer and 23 ± 1 °C during winter, while sea-surface salinities can reach up to 41‰. The prevailing wind direction is NNW with an average speed of 4 m s–1 (1980–2015; Dasari et al., 2018; Petrovic et al., 2023b), while during wintertime, strong SW winds occasionally occur (Fig. 1C; Raitsos et al., 2013). In addition, strong eastward-blowing zonal winds transport Fe-rich eolian sediments to the Red Sea realm, alternating with westward-blowing zonal winds during summer (Jiang et al., 2009).

The first fieldwork that led to the discovery of the stromatolite field was conducted using a small local fishing boat during a scouting visit to Sheybarah Island in January 2021. Detailed descriptions of field visits and sampling and data acquisition methodologies are presented in Items S1–S3 in the Supplemental Material1. Field data acquisition methods included in situ temperature and salinity logging, a manual field survey, and drone surveys (see Item S1A). Laboratory analysis included X-ray micro–computed tomography (μCT), optical microscopy of thin sections, scanning electron microscopy (SEM), powder X-ray diffraction (XRD), and 14C dating. Preliminary investigation of bacterial community composition was also performed using 16S rRNA gene metabarcoding followed by Illumina sequencing.

The Sheybarah stromatolite field is in the intertidal to shallow subtidal zone on a gently seaward-dipping fossil reef flat. The age of the underlying coral pavement is mid-Holocene based on 14C age dating of a coral from a shallow core drilled into the reef flat (5264 yr B.P.). This age period relates to the +2 m Holocene sea-level highstand 8000–4000 yr ago reported by Khanna et al. (2021). Eroded domal coral heads suggest that wave energy has lowered the former reef flat to the current sea level. Lithified sands underneath the stromatolites have been dated with 14C as 1640 yr B.P., while the laminations in the stromatolites date between 325 and 120 yr B.P. (see Supplemental Material for all age data). This indicates an onset of stromatolite growth some 300–400 yr ago, or more recently if grains eroded from the mid-Holocene reef flat have been included into the stromatolite fabric. The daily tidal range is 50–60 cm, reaching a maximum of 1 m. Rare flood surges can inundate lower-lying parts of the island. Sea-surface water temperatures over a year's cycle measured nearby at a water depth of 5 m vary between 21 °C and 31 °C. However, in the intertidal zone, seasonal and daily temperature ranges are more extreme, varying from 8 °C to >48 °C, and reflecting the daily changes caused by seawater inundation and exposure and winter-night/summer-day air temperatures. The average values of salinity, pH, and dissolved oxygen measured during high tide (March 2021) were 42 ppt, 7.8 ± 0.1, and 5.9 ± 0.5 mg/L, respectively. The environmental conditions of the Red Sea are still considered normal, even though salinity is elevated due to the water-circulation patterns and the lack of precipitation.

The Sheybarah stromatolites are distributed over an area exceeding 5 ha (Figs. 1C and 2A). We distinguished stromatolites in the upper intertidal zone adjacent to the beach covering ~3000 m2 and in the mid-intertidal to shallow subtidal zones (Fig. 2A). There are three main growth forms (Fig. 2A): Type 1—Light gray-green to dark brown, elongated-sinusoidal to rhomboidal structures aligned perpendicular to wave crests, with a height of <15 cm, length of 10–100 cm, and width of 5–50 cm, are found in the upper intertidal zone, often coalescing to larger elongated clusters up to 10 m in length (Fig. 2B). They are pustular on the outside and moderately well lithified (Fig. 2C). Type 2—The mid- to lower intertidal zones are composed of low-relief (height <5 cm), irregularly shaped, ovoid to tabular clusters up to 100 m2 in area, and these are often anchored by a slightly elevated core of an eroded late Holocene coral (Figs. 2D2E). Type 3—Poorly lithified, low-relief, irregularly shaped stromatolites occur in the lower intertidal to shallow subtidal zones, commonly covered by a thin coat of white carbonate sand (Figs. 2F2G).

Herein, we describe the inner structures of type 1 stromatolites (Fig. 3; Fig. S1.2). Cross-section cuts of hand samples and μCT analysis of type 1 stromatolites revealed moderately well-laminated, undulating sediment layers interrupted by vugs and clotted fabrics, suggesting that they are thrombolitic stromatolites sensu Riding (2011) (Figs. 3A and 3B; Item S2.1). Dense lithified layers stand out in relief in cut and washed cross sections and are characterized by high CT density values. Grazing and encrusting borers such as gastropods were often encased during the accretion and lithification of the stromatolite layers (Fig. S2.2). Millimeter-scale laminations are clearly visible in thin-section micrographs and are composed of micritic crusts sandwiched by 1–2-mm-thick sediment accretion layers (Figs. 3C3D). The sediment grains were micritized to the extent of complete obliteration of the original grain fabric. Layers of fused grains with indistinguishable grain boundaries are common, often underlying micritic crusts (Figs. 3D3E). The fused grain layers are often infested by microborings, particularly near the rims of the grains, suggesting ongoing micritization even after sediment accretion (Fig. 3E). Elongated acicular aragonite needle rim cements typically <10 μm long are abundant, either occurring as single rods or in meshes perpendicular to grain surfaces (Fig. 3F). The mineral components of the stromatolite layers are aragonite (85%), high-magnesian calcite (9%), and low-magnesian calcite (5%), with minor quantities of quartz and clay minerals (Fig. S2.3).

Filamentous cyanobacteria represent the most abundant bacterial structures as observed through the SEM technique (Figs. 4A4D). They envelop the sediment grains as single strings or bunches covered by mucous sheaths and biological matrixes consistent with extracellular polymeric substance (EPS) (Figs. 4A4D; Westall et al., 2000; Dohnalkova et al., 2011). Sub-micron-size equant Ca-Mg–carbonate crystals are abundantly present on the cyanobacterial filaments and EPS (Fig. 4B). Besides filamentous cyanobacteria, other microbial features can be observed, including copious amounts of biofilm-like structures comprising bacterial cells and matrixes compatible with EPS (Fig. 4C), Navicula-like diatoms (Fig. 4D), and Chroococcus-like structures. Among the most remarkable microbial features are reticulated filaments (Melim et al., 2015), which are ubiquitous in the upper and lower surfaces of the topmost microbial mat layers (Fig. 4E). At the phylum level, the bacterial communities inhabiting the stromatolite structures are dominated by Proteobacteria (49%), with Alphaproteobacteria (30%), Gammaproteobacteria (12%), and Deltaproteobacteria (7%) as dominant classes, Cyanobacteria (16%), and Bacteroidetes (11%) (Fig. 4F).

The occurrence of a living stromatolite field under open-marine conditions on Sheybarah Island, NE Red Sea, is likely driven by environmental factors. The dominantly intertidal position of the stromatolites exposes them not only to regular wetting and drying conditions, but also to an extreme temperature range between 8 °C and >48 °C. Currents in the shallow intertidal environment of SW Sheybarah Island are light and remain limited to high tides or occasional storm events. Considering that similar environmental conditions prevail in other islands of the Al Wajh carbonate platform, we predict that other stromatolite fields may occur in the region. The overall intertidal to shallow subtidal setting and the oligotrophic conditions are analogous to the microtidal setting in the Bahamas. However, the height of the Sheybarah stromatolites is limited up to 15 cm, potentially due to lower accommodation space attributed to the microtidal conditions. Their lateral extent appears to be limited by competition with red algae and corals, which dominate the backreef subtidal environment.

The growth of Sheybarah stromatolites is primarily attributed to microbially mediated accretion and differential lithification of sediment grains. The diverse range of sediment-microbial microtextural features observed in the Sheybarah stromatolites indicates cycles of grain entrapment by filamentous microbial structures and fused grains, and cementation similar to the intertidal stromatolites in the Bahamas (Reid and Browne, 1991; Browne, 2011; Dupraz et al., 2013; Frantz et al., 2015). Consistently, relative abundances of bacterial operational taxonomic units composing the microbial communities of Sheybarah stromatolites include a combination of photoautotrophic (Cyanobacteria, 16%) and heterotrophic (e.g., taxa capable of sulfate reduction, including Gammaproteobacteria [12%], Desulfobacteriota [0.8%], and Campylobacteriota [0.02%]; Madigan et al., 2014; Florentino et al., 2016) taxa, which can promote different metabolic processes, contributing to the stromatolite formation.

An intriguing aspect of the Sheybarah stromatolites is the presence of reticulated filaments (Fig. 4E), which have so far only been reported from aphotic environments in caves (Melim et al., 2015). In the Sheybarah stromatolites, these enigmatic features are ubiquitously present in the surface microbial mat and are characterized by variable morphologies, including horizontal ridges supported by vertical columnar structures (Fig. 4E). Their biogeochemical nature and their role in the formation and morphological structure of these stromatolites are unclear and will be subjects of ongoing research.

To the best of our knowledge, the discovery of the Sheybarah stromatolites is the first of its kind in the Middle East, presenting an unprecedented opportunity to study their geobiology in this unique geographic region. Modern open shallow-marine stromatolites are sparsely present on the planet, leading to a lack of suitable analogues for their ancient counterparts. The only previously known modern analogue to the open shallow-marine settings, where most Proterozoic stromatolites developed, has so far been recorded in the Bahamian Archipelago (Dill et al., 1986; Reid and Browne, 1991; Reid et al., 1995, 2000). The discovery of the Sheybarah stromatolite field holds important implications, not only in the scientific perspective, but also in terms of ecosystem services and environmental heritage awareness in line with the ongoing projects for sustainability and ecotourism development promoted by Saudi Arabia. The site is currently under consideration for being a dedicated conservation zone.

We acknowledge funding from King Abdullah University of Science and Technology (KAUST) baseline supports to V. Vahrenkamp and D. Daffonchio. We thank Red Sea Global for their support in accessing the field locations. We thank L. Melim and H. Westphal for fruitful discussions regarding reticulated filaments. We thank the reviewers for their constructive comments and feedback to improve the manuscript.

1Supplemental Material. Detailed methodology description for field and laboratory data acquisition. Please visit https://doi.org/10.1130/GEOL.S.25180655 to access the supplemental material; contact editing@geosociety.org with any questions.
Gold Open Access: This paper is published under the terms of the CC-BY license.