Gypsum and Halite Deposition on Cyanobacteria and Algae in Salt‐Affected Soils.

Abstract

Soil surface communities are different from those of bulk soil due to the development of photo‐ synthetic communities comprising cyanobacteria, algae, and other bacteria which contribute to the formation of biological soil crusts. The structure of ecological niches forming biological crusts in saline soil environments is characterized by communities dominated by extremophiles organisms like archea, halophilic bacteria and cyanobacteria that can have a direct or indirect role in shaping soil properties. For example gypsum and halite were found by some authors as‐ sociated with the cyanobacterium Entophysalis (Braithwaite and Whitton, 1987); moreover the crystallization of gypsum and halite around filaments was observed in marine cyanobacterial mats of arid regions in the Persian Gulf (Golubic, 1973). In all these cases no causal relationship has been established between the crystallization of gypsum or halite and cyanobacterial or algal metabolism, and no mention is made of specific sedimentary structures. The present account describes the occurrence of gypsum and halite in a saline soil community and suggests hypothe‐ sis on how the organisms may have exerted a physical control over the distinctive structures produced. The study site is a naturally salt‐affected soil located in an area, Piana del Signore, where some ecological variables acted as strong shaping factors in above and belowground communities distribution, driven by a spatial salinity gradient. Scanning electron microscope observations and microanalysis (SEM‐EDS) coupled with X‐Ray diffraction analysis (XRD) were carried out to investigate the biogeochemical interfaces in soil, with a deeper overview on the morphology of the minerals occurring in the soil surface. Soil bulk samples and selected frag‐ ments of the salted crust of the soil surface were analyzed. Three well defined minerals were identified: halite, sulfates and gypsum. Energy Dispersive Spectroscopy confirmed the cubic mineral form to be halite. The results showed a conspicuous deposition of halite and gypsum on the very superficial layers of the soil areas characterized by a 100% of salt crust cover, and the SEM‐EDS analysis provided evidence of the presence of cyanobacteria and algae associated to halite and gypsum deposits. The loss of water from the soil surface due to the natural evapora‐ tion can easily explain the deposition of halite and the formation of characteristic sulfate crys‐ tals. The inner association and stratification of these minerals along cyanobcateria and algae bodies might be explained by a more complex phenomenon that involve the C02 depletion at the biogeochemical interface, due to the algal and bacterial photosynthetic and metabolic activity.