Tunisian Coastal Sabkhas

4.3.1. Locality and Mats

The southern Tunisian coast from Djerba Island to the Libyan border is characterized by shallow restricted lagoons and low-gradient tidal flats which grade into wide sabkhas (Fig. 8A). Present climate is semi-arid and precipitation very irregular with episodic catastrophic floods followed by long periods of drought which may extend over several years (Medhioub and Perthuisot, 1981). On many of the tidal flats, microbial mats are developed from the upper intertidal to the suprati-dal zone and also on the adjoining sabkha if groundwater is present. Thick and mature mats, dominated by filamentous cyanobacteria (e.g. Microcoleus chthono-plastes, Lyngbya sp.), occur in the upper intertidal zone and in some shallow supratidal ponds; thin mats of dominantly coccoid cyanobacteria (e.g. Synechococcus sp.) thrive in the supratidal zone in which precipitation of gypsum also occurs. Mat-related structures observed in supratidal zones are described from Bhar Alouane and El Gourine, and those in sabkha environments from Bou Jemel (Fig. 8A).

4.3.2. Sustaining of Mats Under Extreme Conditions

Mats in the supratidal zone of the southern Tunisian coast are exposed to extreme UV irradiation, high rates of evaporation, and are easily cut off from water supply. Mats surviving under such conditions typically have on top a layer of EPS, containing pigments like scytonemin which protects the underlying cyanobacte-rial layer from damage caused by UV irradiation (Stal, 2000 and references therein). Due to the pigment-rich layer, supratidal mats in southern Tunisia exhibit a reddish to orange color during summer.

The strongly cohesive and elastic EPS layer on top of the mat, however, has additional effects: (1) it develops small 'photosynthetic domes' (PS-domes) tracing trapped bubbles of oxygen, resulting from photosynthetic processes in the underlying mat (Fig. 8B); PS-domes are soon overgrown and stabilized and contribute to enlarging the mat; (2) it prevents water loss from the underlying mat, due to its low permeability, and any damage of the layer, e.g., by shrinkage cracking, which allows groundwater to rise up and to escape, will in turn induce local bacterial growth and EPS production, and thus eventually lead to repairing of the damage.

As long as traces of water from upward leakage of groundwater are available, mats can thus survive. Drawdown of the groundwater table below the limit of

Figure 8. Location and structures related to mats developed in Tunisian supratidal zones and sab-khas. (A) Location of study sites Bhar Alouane (B-D), El Gourine (E, F), and Bou Jemel sabkha (G, H). Scale (coin) is 24 mm in B-F. (B) Photosynthetic domes (PS-domes) trapped below EPS film in thin mat of coccoidal cyanobacteria. (C) Small sigmoidal to curved shrinkage cracks. Arrows indicate cracks with microbial activity riggered by uprising groundwater. (D) Several generations of overgrown shrinkage cracks and PS-domes (PS). (E) Subcircular crack openings with curled margins in thin mat layer. (F) Several generations of overgrown circular cracks. (G) Aggregates and domes of coccoid cyanobacteria and gypsum crystals, related to desiccation cracks. (H) Domes of filamentous and coc-coid cyanobacteria arranged along desiccation cracks. Scale is 8 cm.

Figure 8. Location and structures related to mats developed in Tunisian supratidal zones and sab-khas. (A) Location of study sites Bhar Alouane (B-D), El Gourine (E, F), and Bou Jemel sabkha (G, H). Scale (coin) is 24 mm in B-F. (B) Photosynthetic domes (PS-domes) trapped below EPS film in thin mat of coccoidal cyanobacteria. (C) Small sigmoidal to curved shrinkage cracks. Arrows indicate cracks with microbial activity riggered by uprising groundwater. (D) Several generations of overgrown shrinkage cracks and PS-domes (PS). (E) Subcircular crack openings with curled margins in thin mat layer. (F) Several generations of overgrown circular cracks. (G) Aggregates and domes of coccoid cyanobacteria and gypsum crystals, related to desiccation cracks. (H) Domes of filamentous and coc-coid cyanobacteria arranged along desiccation cracks. Scale is 8 cm.

capillary rise or cessation of groundwater flow, however, will lead to fatal desiccation of the mat within a short time. But even then, mat-forming microbial communities may still remain alive in the sediment below the desiccated mat and provide a potential for mat recreation when moisture returns (G. Gerdes, 2007, personal communication).

4.3.3. Structures Related to Desiccation and Shrinkage

In thin but strongly cohesive mats of dominantly coccoidal cyanobacteria, shrinkage cracks are usually small and occur as isolated features (Fig. 8C); networks of cracks are rarely developed, due to rapid overgrowth and healing. Overgrown cracks remain as small irregular bulges on the mat surface (Fig. 8D).

Very thin mats in which the elastic EPS film undergoes shrinkage, tend to develop more circular openings with involute margins (Fig. 8E). Uprising of water in the fresh openings induces microbial growth and stimulates a process of 'self-healing'. Overgrown margins remain as circular bulges on the mat surface (Fig. 8F). Forming slightly elevated features on the mat, however, they are preferred sites for renewed cracking which, again, will induce microbial growth and overgrowth, and so on. In this way, mats in the supratidal zone may attain more and more irregular surfaces.

Cut off from capillary groundwater, mats rapidly desiccate, become detached from their substratum, and shrink into polygons separated by wide cracks. Dried-up mat fragments may then easily be transported by the wind and be deposited far away from their source, as described above for Amrum Island.

4.3.4. Structures Related to Locally Induced Growth

In the Bou Jemel sabkha, continuous cohesive mats do not occur. Instead, a reddish surface crust is developed, in which orange-pigmented coccoid cyanobacteria are embedded together with gypsum crystals. Where the crust is interrupted by faint desiccation cracks, small 'pop-corn-like' aggregates, arranged along the cracks, are observed (Fig. 8G). They are colonized by abundant biofilms dominated by EPS-enriched colonies of coccoid cyanobacteria, which agglutinate sediment grains and interspersed gypsum crystals and thus form 'clustered aggregates'. It is suspected that the aggregates formed when groundwater uprising along the cracks, locally induced microbial growth together with precipitation of gypsum.

Occasionally, larger domal structures, up to 7 cm across and 2 cm high, are developed. They are hollow and rigid, and have a nodular surface, individual nodules resembling the above aggregates. The domed layer is about 5 mm thick and consists of sediment grains of gypsum agglutinated by coccoid cyanobacte-ria. In some cases, it is observed that the domes are situated above junctions of polygonal desiccation cracks. As the domes may be described as upward protrusions resulting from local expansion of the surrounding surface crust, it is suspected that they originate from bacterial growth triggered by local water supply.

On a nearby shallow berm which separates the sabkha from an evaporating restricted sound, narrow polygonal desiccation and shrinkage cracks are well developed. Again, chains of domal structures, partly coalescing into longer bulges, are arranged along the cracks (Fig. 8H) and indicate local growth induced by local water supply. The domes have a smooth surface, are flexible (but brittle when dry), and the involved layer is less than 2 mm thick. Coccoid cyanobacteria and gypsum crystals are dominant in the top level, whereas filamentous Microcoleus has constructed a strongly felty fabric below (all microbiological data after G. Gerdes, 2007, personal communication).

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